IndexShard.java

/*
 * SPDX-License-Identifier: Apache-2.0
 *
 * The OpenSearch Contributors require contributions made to
 * this file be licensed under the Apache-2.0 license or a
 * compatible open source license.
 */

/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch licenses this file to you under
 * the Apache License, Version 2.0 (the "License"); you may
 * not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

/*
 * Modifications Copyright OpenSearch Contributors. See
 * GitHub history for details.
 */

package org.opensearch.index.shard;

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.message.ParameterizedMessage;
import org.apache.lucene.codecs.CodecUtil;
import org.apache.lucene.index.CheckIndex;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.FilterDirectoryReader;
import org.apache.lucene.index.IndexCommit;
import org.apache.lucene.index.IndexFileNames;
import org.apache.lucene.index.LeafReader;
import org.apache.lucene.index.SegmentInfos;
import org.apache.lucene.index.Term;
import org.apache.lucene.search.Query;
import org.apache.lucene.search.QueryCachingPolicy;
import org.apache.lucene.search.ReferenceManager;
import org.apache.lucene.search.Sort;
import org.apache.lucene.search.UsageTrackingQueryCachingPolicy;
import org.apache.lucene.store.AlreadyClosedException;
import org.apache.lucene.store.BufferedChecksumIndexInput;
import org.apache.lucene.store.ChecksumIndexInput;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FilterDirectory;
import org.apache.lucene.store.IOContext;
import org.apache.lucene.store.IndexInput;
import org.apache.lucene.util.ThreadInterruptedException;
import org.opensearch.ExceptionsHelper;
import org.opensearch.OpenSearchException;
import org.opensearch.action.ActionRunnable;
import org.opensearch.action.admin.indices.flush.FlushRequest;
import org.opensearch.action.admin.indices.forcemerge.ForceMergeRequest;
import org.opensearch.action.admin.indices.upgrade.post.UpgradeRequest;
import org.opensearch.action.support.replication.PendingReplicationActions;
import org.opensearch.action.support.replication.ReplicationResponse;
import org.opensearch.cluster.metadata.DataStream;
import org.opensearch.cluster.metadata.IndexMetadata;
import org.opensearch.cluster.metadata.MappingMetadata;
import org.opensearch.cluster.routing.IndexShardRoutingTable;
import org.opensearch.cluster.routing.RecoverySource;
import org.opensearch.cluster.routing.RecoverySource.SnapshotRecoverySource;
import org.opensearch.cluster.routing.ShardRouting;
import org.opensearch.cluster.routing.ShardRoutingState;
import org.opensearch.common.Booleans;
import org.opensearch.common.CheckedConsumer;
import org.opensearch.common.CheckedFunction;
import org.opensearch.common.CheckedRunnable;
import org.opensearch.common.Nullable;
import org.opensearch.common.SetOnce;
import org.opensearch.common.annotation.PublicApi;
import org.opensearch.common.collect.Tuple;
import org.opensearch.common.concurrent.GatedCloseable;
import org.opensearch.common.io.stream.BytesStreamOutput;
import org.opensearch.common.lease.Releasable;
import org.opensearch.common.lease.Releasables;
import org.opensearch.common.lucene.Lucene;
import org.opensearch.common.lucene.index.OpenSearchDirectoryReader;
import org.opensearch.common.metrics.CounterMetric;
import org.opensearch.common.metrics.MeanMetric;
import org.opensearch.common.settings.Settings;
import org.opensearch.common.unit.TimeValue;
import org.opensearch.common.util.BigArrays;
import org.opensearch.common.util.concurrent.AbstractRunnable;
import org.opensearch.common.util.concurrent.AsyncIOProcessor;
import org.opensearch.common.util.concurrent.BufferedAsyncIOProcessor;
import org.opensearch.common.util.concurrent.RunOnce;
import org.opensearch.common.util.concurrent.ThreadContext;
import org.opensearch.common.util.io.IOUtils;
import org.opensearch.core.Assertions;
import org.opensearch.core.action.ActionListener;
import org.opensearch.core.common.unit.ByteSizeValue;
import org.opensearch.core.index.Index;
import org.opensearch.core.index.shard.ShardId;
import org.opensearch.core.indices.breaker.CircuitBreakerService;
import org.opensearch.core.rest.RestStatus;
import org.opensearch.core.xcontent.MediaTypeRegistry;
import org.opensearch.gateway.WriteStateException;
import org.opensearch.index.IndexModule;
import org.opensearch.index.IndexNotFoundException;
import org.opensearch.index.IndexService;
import org.opensearch.index.IndexSettings;
import org.opensearch.index.ReplicationStats;
import org.opensearch.index.SegmentReplicationShardStats;
import org.opensearch.index.VersionType;
import org.opensearch.index.cache.IndexCache;
import org.opensearch.index.cache.bitset.ShardBitsetFilterCache;
import org.opensearch.index.cache.request.ShardRequestCache;
import org.opensearch.index.codec.CodecService;
import org.opensearch.index.engine.CommitStats;
import org.opensearch.index.engine.Engine;
import org.opensearch.index.engine.Engine.GetResult;
import org.opensearch.index.engine.EngineConfig;
import org.opensearch.index.engine.EngineConfigFactory;
import org.opensearch.index.engine.EngineException;
import org.opensearch.index.engine.EngineFactory;
import org.opensearch.index.engine.NRTReplicationEngine;
import org.opensearch.index.engine.ReadOnlyEngine;
import org.opensearch.index.engine.RefreshFailedEngineException;
import org.opensearch.index.engine.SafeCommitInfo;
import org.opensearch.index.engine.Segment;
import org.opensearch.index.engine.SegmentsStats;
import org.opensearch.index.fielddata.FieldDataStats;
import org.opensearch.index.fielddata.ShardFieldData;
import org.opensearch.index.flush.FlushStats;
import org.opensearch.index.get.GetStats;
import org.opensearch.index.get.ShardGetService;
import org.opensearch.index.mapper.DocumentMapper;
import org.opensearch.index.mapper.DocumentMapperForType;
import org.opensearch.index.mapper.IdFieldMapper;
import org.opensearch.index.mapper.MapperService;
import org.opensearch.index.mapper.Mapping;
import org.opensearch.index.mapper.ParsedDocument;
import org.opensearch.index.mapper.RootObjectMapper;
import org.opensearch.index.mapper.SourceToParse;
import org.opensearch.index.mapper.Uid;
import org.opensearch.index.merge.MergeStats;
import org.opensearch.index.recovery.RecoveryStats;
import org.opensearch.index.refresh.RefreshStats;
import org.opensearch.index.remote.RemoteSegmentStats;
import org.opensearch.index.remote.RemoteStoreStatsTrackerFactory;
import org.opensearch.index.search.stats.SearchStats;
import org.opensearch.index.search.stats.ShardSearchStats;
import org.opensearch.index.seqno.ReplicationTracker;
import org.opensearch.index.seqno.RetentionLease;
import org.opensearch.index.seqno.RetentionLeaseStats;
import org.opensearch.index.seqno.RetentionLeaseSyncer;
import org.opensearch.index.seqno.RetentionLeases;
import org.opensearch.index.seqno.SeqNoStats;
import org.opensearch.index.seqno.SequenceNumbers;
import org.opensearch.index.shard.PrimaryReplicaSyncer.ResyncTask;
import org.opensearch.index.similarity.SimilarityService;
import org.opensearch.index.store.RemoteSegmentStoreDirectory;
import org.opensearch.index.store.RemoteStoreFileDownloader;
import org.opensearch.index.store.Store;
import org.opensearch.index.store.Store.MetadataSnapshot;
import org.opensearch.index.store.StoreFileMetadata;
import org.opensearch.index.store.StoreStats;
import org.opensearch.index.store.remote.metadata.RemoteSegmentMetadata;
import org.opensearch.index.translog.RemoteBlobStoreInternalTranslogFactory;
import org.opensearch.index.translog.RemoteFsTranslog;
import org.opensearch.index.translog.RemoteTranslogStats;
import org.opensearch.index.translog.Translog;
import org.opensearch.index.translog.TranslogConfig;
import org.opensearch.index.translog.TranslogFactory;
import org.opensearch.index.translog.TranslogRecoveryRunner;
import org.opensearch.index.translog.TranslogStats;
import org.opensearch.index.warmer.ShardIndexWarmerService;
import org.opensearch.index.warmer.WarmerStats;
import org.opensearch.indices.IndexingMemoryController;
import org.opensearch.indices.IndicesService;
import org.opensearch.indices.cluster.IndicesClusterStateService;
import org.opensearch.indices.recovery.PeerRecoveryTargetService;
import org.opensearch.indices.recovery.RecoveryFailedException;
import org.opensearch.indices.recovery.RecoveryListener;
import org.opensearch.indices.recovery.RecoverySettings;
import org.opensearch.indices.recovery.RecoveryState;
import org.opensearch.indices.recovery.RecoveryTarget;
import org.opensearch.indices.replication.checkpoint.ReplicationCheckpoint;
import org.opensearch.indices.replication.checkpoint.SegmentReplicationCheckpointPublisher;
import org.opensearch.indices.replication.common.ReplicationTimer;
import org.opensearch.repositories.RepositoriesService;
import org.opensearch.repositories.Repository;
import org.opensearch.search.suggest.completion.CompletionStats;
import org.opensearch.threadpool.ThreadPool;

import java.io.Closeable;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.PrintStream;
import java.nio.channels.ClosedByInterruptException;
import java.nio.channels.FileChannel;
import java.nio.charset.StandardCharsets;
import java.nio.file.NoSuchFileException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.EnumSet;
import java.util.HashSet;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;
import java.util.Set;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.LongSupplier;
import java.util.function.Supplier;
import java.util.stream.Collectors;
import java.util.stream.StreamSupport;

import static org.opensearch.index.seqno.RetentionLeaseActions.RETAIN_ALL;
import static org.opensearch.index.seqno.SequenceNumbers.LOCAL_CHECKPOINT_KEY;
import static org.opensearch.index.seqno.SequenceNumbers.MAX_SEQ_NO;
import static org.opensearch.index.seqno.SequenceNumbers.UNASSIGNED_SEQ_NO;
import static org.opensearch.index.translog.Translog.Durability;
import static org.opensearch.index.translog.Translog.TRANSLOG_UUID_KEY;

/**
 * An OpenSearch index shard
 *
 * @opensearch.api
 */
@PublicApi(since = "1.0.0")
public class IndexShard extends AbstractIndexShardComponent implements IndicesClusterStateService.Shard {

    private final ThreadPool threadPool;
    private final MapperService mapperService;
    private final IndexCache indexCache;
    private final Store store;
    private final InternalIndexingStats internalIndexingStats;
    private final ShardSearchStats searchStats = new ShardSearchStats();
    private final ShardGetService getService;
    private final ShardIndexWarmerService shardWarmerService;
    private final ShardRequestCache requestCacheStats;
    private final ShardFieldData shardFieldData;
    private final ShardBitsetFilterCache shardBitsetFilterCache;
    private final Object mutex = new Object();
    private final String checkIndexOnStartup;
    private final CodecService codecService;
    private final Engine.Warmer warmer;
    private final SimilarityService similarityService;
    private final TranslogConfig translogConfig;
    private final IndexEventListener indexEventListener;
    private final QueryCachingPolicy cachingPolicy;
    private final Supplier<Sort> indexSortSupplier;
    // Package visible for testing
    final CircuitBreakerService circuitBreakerService;

    private final SearchOperationListener searchOperationListener;

    private final GlobalCheckpointListeners globalCheckpointListeners;
    private final PendingReplicationActions pendingReplicationActions;
    private final ReplicationTracker replicationTracker;
    private final SegmentReplicationCheckpointPublisher checkpointPublisher;

    protected volatile ShardRouting shardRouting;
    protected volatile IndexShardState state;
    // ensure happens-before relation between addRefreshListener() and postRecovery()
    private final Object postRecoveryMutex = new Object();
    private volatile long pendingPrimaryTerm; // see JavaDocs for getPendingPrimaryTerm
    private final Object engineMutex = new Object(); // lock ordering: engineMutex -> mutex
    private final AtomicReference<Engine> currentEngineReference = new AtomicReference<>();
    final EngineFactory engineFactory;
    final EngineConfigFactory engineConfigFactory;

    private final IndexingOperationListener indexingOperationListeners;
    private final Runnable globalCheckpointSyncer;

    Runnable getGlobalCheckpointSyncer() {
        return globalCheckpointSyncer;
    }

    private final RetentionLeaseSyncer retentionLeaseSyncer;

    @Nullable
    private volatile RecoveryState recoveryState;

    private final RecoveryStats recoveryStats = new RecoveryStats();
    private final MeanMetric refreshMetric = new MeanMetric();
    private final MeanMetric externalRefreshMetric = new MeanMetric();
    private final MeanMetric flushMetric = new MeanMetric();
    private final CounterMetric periodicFlushMetric = new CounterMetric();

    private final ShardEventListener shardEventListener = new ShardEventListener();

    private final ShardPath path;

    private final IndexShardOperationPermits indexShardOperationPermits;

    private static final EnumSet<IndexShardState> readAllowedStates = EnumSet.of(IndexShardState.STARTED, IndexShardState.POST_RECOVERY);
    // for primaries, we only allow to write when actually started (so the cluster has decided we started)
    // in case we have a relocation of a primary, we also allow to write after phase 2 completed, where the shard may be
    // in state RECOVERING or POST_RECOVERY.
    // for replicas, replication is also allowed while recovering, since we index also during recovery to replicas and rely on
    // version checks to make sure its consistent a relocated shard can also be target of a replication if the relocation target has not
    // been marked as active yet and is syncing it's changes back to the relocation source
    private static final EnumSet<IndexShardState> writeAllowedStates = EnumSet.of(
        IndexShardState.RECOVERING,
        IndexShardState.POST_RECOVERY,
        IndexShardState.STARTED
    );

    private final CheckedFunction<DirectoryReader, DirectoryReader, IOException> readerWrapper;

    /**
     * True if this shard is still indexing (recently) and false if we've been idle for long enough (as periodically checked by {@link
     * IndexingMemoryController}).
     */
    private final AtomicBoolean active = new AtomicBoolean();
    /**
     * Allows for the registration of listeners that are called when a change becomes visible for search.
     */
    private final RefreshListeners refreshListeners;

    private final AtomicLong lastSearcherAccess = new AtomicLong();
    private final AtomicReference<Translog.Location> pendingRefreshLocation = new AtomicReference<>();
    private final RefreshPendingLocationListener refreshPendingLocationListener;
    private volatile boolean useRetentionLeasesInPeerRecovery;
    private final Store remoteStore;
    private final BiFunction<IndexSettings, ShardRouting, TranslogFactory> translogFactorySupplier;
    private final boolean isTimeSeriesIndex;
    private final RemoteStoreStatsTrackerFactory remoteStoreStatsTrackerFactory;

    private final List<ReferenceManager.RefreshListener> internalRefreshListener = new ArrayList<>();
    private final RemoteStoreFileDownloader fileDownloader;
    private final RecoverySettings recoverySettings;

    public IndexShard(
        final ShardRouting shardRouting,
        final IndexSettings indexSettings,
        final ShardPath path,
        final Store store,
        final Supplier<Sort> indexSortSupplier,
        final IndexCache indexCache,
        final MapperService mapperService,
        final SimilarityService similarityService,
        final EngineFactory engineFactory,
        final EngineConfigFactory engineConfigFactory,
        final IndexEventListener indexEventListener,
        final CheckedFunction<DirectoryReader, DirectoryReader, IOException> indexReaderWrapper,
        final ThreadPool threadPool,
        final BigArrays bigArrays,
        final Engine.Warmer warmer,
        final List<SearchOperationListener> searchOperationListener,
        final List<IndexingOperationListener> listeners,
        final Runnable globalCheckpointSyncer,
        final RetentionLeaseSyncer retentionLeaseSyncer,
        final CircuitBreakerService circuitBreakerService,
        final BiFunction<IndexSettings, ShardRouting, TranslogFactory> translogFactorySupplier,
        @Nullable final SegmentReplicationCheckpointPublisher checkpointPublisher,
        @Nullable final Store remoteStore,
        final RemoteStoreStatsTrackerFactory remoteStoreStatsTrackerFactory,
        final Supplier<TimeValue> clusterRemoteTranslogBufferIntervalSupplier,
        final String nodeId,
        final RecoverySettings recoverySettings
    ) throws IOException {
        super(shardRouting.shardId(), indexSettings);
        assert shardRouting.initializing();
        this.shardRouting = shardRouting;
        final Settings settings = indexSettings.getSettings();
        this.codecService = new CodecService(mapperService, indexSettings, logger);
        this.warmer = warmer;
        this.similarityService = similarityService;
        Objects.requireNonNull(store, "Store must be provided to the index shard");
        this.engineFactory = Objects.requireNonNull(engineFactory);
        this.engineConfigFactory = Objects.requireNonNull(engineConfigFactory);
        this.store = store;
        this.indexSortSupplier = indexSortSupplier;
        this.indexEventListener = indexEventListener;
        this.threadPool = threadPool;
        this.translogSyncProcessor = createTranslogSyncProcessor(
            logger,
            threadPool,
            this::getEngine,
            indexSettings.isRemoteTranslogStoreEnabled(),
            () -> getRemoteTranslogUploadBufferInterval(clusterRemoteTranslogBufferIntervalSupplier)
        );
        this.mapperService = mapperService;
        this.indexCache = indexCache;
        this.internalIndexingStats = new InternalIndexingStats();
        final List<IndexingOperationListener> listenersList = new ArrayList<>(listeners);
        listenersList.add(internalIndexingStats);
        this.indexingOperationListeners = new IndexingOperationListener.CompositeListener(listenersList, logger);
        this.globalCheckpointSyncer = globalCheckpointSyncer;
        this.retentionLeaseSyncer = Objects.requireNonNull(retentionLeaseSyncer);
        final List<SearchOperationListener> searchListenersList = new ArrayList<>(searchOperationListener);
        searchListenersList.add(searchStats);
        this.searchOperationListener = new SearchOperationListener.CompositeListener(searchListenersList, logger);
        this.getService = new ShardGetService(indexSettings, this, mapperService);
        this.shardWarmerService = new ShardIndexWarmerService(shardId, indexSettings);
        this.requestCacheStats = new ShardRequestCache();
        this.shardFieldData = new ShardFieldData();
        this.shardBitsetFilterCache = new ShardBitsetFilterCache(shardId, indexSettings);
        state = IndexShardState.CREATED;
        this.path = path;
        this.circuitBreakerService = circuitBreakerService;
        /* create engine config */
        logger.debug("state: [CREATED]");

        this.checkIndexOnStartup = indexSettings.getValue(IndexSettings.INDEX_CHECK_ON_STARTUP);
        this.translogConfig = new TranslogConfig(shardId, shardPath().resolveTranslog(), indexSettings, bigArrays, nodeId);
        final String aId = shardRouting.allocationId().getId();
        final long primaryTerm = indexSettings.getIndexMetadata().primaryTerm(shardId.id());
        this.pendingPrimaryTerm = primaryTerm;
        this.globalCheckpointListeners = new GlobalCheckpointListeners(shardId, threadPool.scheduler(), logger);
        this.pendingReplicationActions = new PendingReplicationActions(shardId, threadPool);
        this.replicationTracker = new ReplicationTracker(
            shardId,
            aId,
            indexSettings,
            primaryTerm,
            UNASSIGNED_SEQ_NO,
            globalCheckpointListeners::globalCheckpointUpdated,
            threadPool::absoluteTimeInMillis,
            (retentionLeases, listener) -> retentionLeaseSyncer.sync(shardId, aId, getPendingPrimaryTerm(), retentionLeases, listener),
            this::getSafeCommitInfo,
            pendingReplicationActions
        );

        // the query cache is a node-level thing, however we want the most popular filters
        // to be computed on a per-shard basis
        if (IndexModule.INDEX_QUERY_CACHE_EVERYTHING_SETTING.get(settings)) {
            cachingPolicy = new QueryCachingPolicy() {
                @Override
                public void onUse(Query query) {

                }

                @Override
                public boolean shouldCache(Query query) {
                    return true;
                }
            };
        } else {
            cachingPolicy = new UsageTrackingQueryCachingPolicy();
        }
        indexShardOperationPermits = new IndexShardOperationPermits(shardId, threadPool);
        readerWrapper = indexReaderWrapper;
        refreshListeners = buildRefreshListeners();
        lastSearcherAccess.set(threadPool.relativeTimeInMillis());
        persistMetadata(path, indexSettings, shardRouting, null, logger);
        this.useRetentionLeasesInPeerRecovery = replicationTracker.hasAllPeerRecoveryRetentionLeases();
        this.refreshPendingLocationListener = new RefreshPendingLocationListener();
        this.checkpointPublisher = checkpointPublisher;
        this.remoteStore = remoteStore;
        this.translogFactorySupplier = translogFactorySupplier;
        this.isTimeSeriesIndex = (mapperService == null || mapperService.documentMapper() == null)
            ? false
            : mapperService.documentMapper().mappers().containsTimeStampField();
        this.remoteStoreStatsTrackerFactory = remoteStoreStatsTrackerFactory;
        this.recoverySettings = recoverySettings;
        this.fileDownloader = new RemoteStoreFileDownloader(shardRouting.shardId(), threadPool, recoverySettings);
    }

    public ThreadPool getThreadPool() {
        return this.threadPool;
    }

    public Store store() {
        return this.store;
    }

    public Store remoteStore() {
        return this.remoteStore;
    }

    /**
     * Return the sort order of this index, or null if the index has no sort.
     */
    public Sort getIndexSort() {
        return indexSortSupplier.get();
    }

    public ShardGetService getService() {
        return this.getService;
    }

    public ShardBitsetFilterCache shardBitsetFilterCache() {
        return shardBitsetFilterCache;
    }

    public MapperService mapperService() {
        return mapperService;
    }

    public SearchOperationListener getSearchOperationListener() {
        return this.searchOperationListener;
    }

    public ShardIndexWarmerService warmerService() {
        return this.shardWarmerService;
    }

    public ShardRequestCache requestCache() {
        return this.requestCacheStats;
    }

    public ShardFieldData fieldData() {
        return this.shardFieldData;
    }

    public boolean isSystem() {
        return indexSettings.getIndexMetadata().isSystem();
    }

    /**
     * Returns the name of the default codec in codecService
     */
    public String getDefaultCodecName() {
        return codecService.codec(CodecService.DEFAULT_CODEC).getName();
    }

    /**
     * USE THIS METHOD WITH CARE!
     * Returns the primary term the index shard is supposed to be on. In case of primary promotion or when a replica learns about
     * a new term due to a new primary, the term that's exposed here will not be the term that the shard internally uses to assign
     * to operations. The shard will auto-correct its internal operation term, but this might take time.
     * See {@link org.opensearch.cluster.metadata.IndexMetadata#primaryTerm(int)}
     */
    public long getPendingPrimaryTerm() {
        return this.pendingPrimaryTerm;
    }

    /** Returns the primary term that is currently being used to assign to operations */
    public long getOperationPrimaryTerm() {
        return replicationTracker.getOperationPrimaryTerm();
    }

    /**
     * Returns the latest cluster routing entry received with this shard.
     */
    @Override
    public ShardRouting routingEntry() {
        return this.shardRouting;
    }

    public QueryCachingPolicy getQueryCachingPolicy() {
        return cachingPolicy;
    }

    /** Only used for testing **/
    protected RemoteStoreStatsTrackerFactory getRemoteStoreStatsTrackerFactory() {
        return remoteStoreStatsTrackerFactory;
    }

    public String getNodeId() {
        return translogConfig.getNodeId();
    }

    public RecoverySettings getRecoverySettings() {
        return recoverySettings;
    }

    public RemoteStoreFileDownloader getFileDownloader() {
        return fileDownloader;
    }

    @Override
    public void updateShardState(
        final ShardRouting newRouting,
        final long newPrimaryTerm,
        final BiConsumer<IndexShard, ActionListener<ResyncTask>> primaryReplicaSyncer,
        final long applyingClusterStateVersion,
        final Set<String> inSyncAllocationIds,
        final IndexShardRoutingTable routingTable
    ) throws IOException {
        final ShardRouting currentRouting;
        synchronized (mutex) {
            currentRouting = this.shardRouting;
            assert currentRouting != null;

            if (!newRouting.shardId().equals(shardId())) {
                throw new IllegalArgumentException(
                    "Trying to set a routing entry with shardId " + newRouting.shardId() + " on a shard with shardId " + shardId()
                );
            }
            if (newRouting.isSameAllocation(currentRouting) == false) {
                throw new IllegalArgumentException(
                    "Trying to set a routing entry with a different allocation. Current " + currentRouting + ", new " + newRouting
                );
            }
            if (currentRouting.primary() && newRouting.primary() == false) {
                throw new IllegalArgumentException(
                    "illegal state: trying to move shard from primary mode to replica mode. Current "
                        + currentRouting
                        + ", new "
                        + newRouting
                );
            }

            if (newRouting.primary()) {
                replicationTracker.updateFromClusterManager(applyingClusterStateVersion, inSyncAllocationIds, routingTable);
            }

            if (state == IndexShardState.POST_RECOVERY && newRouting.active()) {
                assert currentRouting.active() == false : "we are in POST_RECOVERY, but our shard routing is active " + currentRouting;
                assert currentRouting.isRelocationTarget() == false
                    || currentRouting.primary() == false
                    || replicationTracker.isPrimaryMode()
                    : "a primary relocation is completed by the cluster-managerr, but primary mode is not active " + currentRouting;

                changeState(IndexShardState.STARTED, "global state is [" + newRouting.state() + "]");

                // Flush here after relocation of primary, so that replica get all changes from new primary rather than waiting for more
                // docs to get indexed.
                if (indexSettings.isSegRepEnabled()) {
                    flush(new FlushRequest().waitIfOngoing(true).force(true));
                }
            } else if (currentRouting.primary()
                && currentRouting.relocating()
                && replicationTracker.isRelocated()
                && (newRouting.relocating() == false || newRouting.equalsIgnoringMetadata(currentRouting) == false)) {
                    // if the shard is not in primary mode anymore (after primary relocation) we have to fail when any changes in shard
                    // routing occur (e.g. due to recovery failure / cancellation). The reason is that at the moment we cannot safely
                    // reactivate primary mode without risking two active primaries.
                    throw new IndexShardRelocatedException(
                        shardId(),
                        "Shard is marked as relocated, cannot safely move to state " + newRouting.state()
                    );
                }
            assert newRouting.active() == false || state == IndexShardState.STARTED || state == IndexShardState.CLOSED
                : "routing is active, but local shard state isn't. routing: " + newRouting + ", local state: " + state;
            persistMetadata(path, indexSettings, newRouting, currentRouting, logger);
            final CountDownLatch shardStateUpdated = new CountDownLatch(1);

            if (newRouting.primary()) {
                if (newPrimaryTerm == pendingPrimaryTerm) {
                    if (currentRouting.initializing() && currentRouting.isRelocationTarget() == false && newRouting.active()) {
                        // the cluster-manager started a recovering primary, activate primary mode.
                        replicationTracker.activatePrimaryMode(getLocalCheckpoint());
                        postActivatePrimaryMode();
                    }
                } else {
                    assert currentRouting.primary() == false : "term is only increased as part of primary promotion";
                    /* Note that due to cluster state batching an initializing primary shard term can failed and re-assigned
                     * in one state causing it's term to be incremented. Note that if both current shard state and new
                     * shard state are initializing, we could replace the current shard and reinitialize it. It is however
                     * possible that this shard is being started. This can happen if:
                     * 1) Shard is post recovery and sends shard started to the cluster-manager
                     * 2) Node gets disconnected and rejoins
                     * 3) Cluster-manager assigns the shard back to the node
                     * 4) Cluster-manager processes the shard started and starts the shard
                     * 5) The node process the cluster state where the shard is both started and primary term is incremented.
                     *
                     * We could fail the shard in that case, but this will cause it to be removed from the insync allocations list
                     * potentially preventing re-allocation.
                     */
                    assert newRouting.initializing() == false : "a started primary shard should never update its term; "
                        + "shard "
                        + newRouting
                        + ", "
                        + "current term ["
                        + pendingPrimaryTerm
                        + "], "
                        + "new term ["
                        + newPrimaryTerm
                        + "]";
                    assert newPrimaryTerm > pendingPrimaryTerm : "primary terms can only go up; current term ["
                        + pendingPrimaryTerm
                        + "], new term ["
                        + newPrimaryTerm
                        + "]";
                    /*
                     * Before this call returns, we are guaranteed that all future operations are delayed and so this happens before we
                     * increment the primary term. The latch is needed to ensure that we do not unblock operations before the primary
                     * term is incremented.
                     */
                    // to prevent primary relocation handoff while resync is not completed
                    boolean resyncStarted = primaryReplicaResyncInProgress.compareAndSet(false, true);
                    if (resyncStarted == false) {
                        throw new IllegalStateException("cannot start resync while it's already in progress");
                    }
                    bumpPrimaryTerm(newPrimaryTerm, () -> {
                        shardStateUpdated.await();
                        assert pendingPrimaryTerm == newPrimaryTerm : "shard term changed on primary. expected ["
                            + newPrimaryTerm
                            + "] but was ["
                            + pendingPrimaryTerm
                            + "]"
                            + ", current routing: "
                            + currentRouting
                            + ", new routing: "
                            + newRouting;
                        assert getOperationPrimaryTerm() == newPrimaryTerm;
                        try {
                            if (indexSettings.isSegRepEnabled()) {
                                // this Shard's engine was read only, we need to update its engine before restoring local history from xlog.
                                assert newRouting.primary() && currentRouting.primary() == false;
                                ReplicationTimer timer = new ReplicationTimer();
                                timer.start();
                                logger.debug(
                                    "Resetting engine on promotion of shard [{}] to primary, startTime {}\n",
                                    shardId,
                                    timer.startTime()
                                );
                                resetEngineToGlobalCheckpoint();
                                timer.stop();
                                logger.info("Completed engine failover for shard [{}] in: {} ms", shardId, timer.time());
                                // It is possible an engine can open with a SegmentInfos on a higher gen but the reader does not refresh to
                                // trigger our refresh listener.
                                // Force update the checkpoint post engine reset.
                                updateReplicationCheckpoint();
                            }

                            replicationTracker.activatePrimaryMode(getLocalCheckpoint());
                            if (indexSettings.isSegRepEnabled()) {
                                // force publish a checkpoint once in primary mode so that replicas not caught up to previous primary
                                // are brought up to date.
                                checkpointPublisher.publish(this, getLatestReplicationCheckpoint());
                            }
                            postActivatePrimaryMode();
                            /*
                             * If this shard was serving as a replica shard when another shard was promoted to primary then
                             * its Lucene index was reset during the primary term transition. In particular, the Lucene index
                             * on this shard was reset to the global checkpoint and the operations above the local checkpoint
                             * were reverted. If the other shard that was promoted to primary subsequently fails before the
                             * primary/replica re-sync completes successfully and we are now being promoted, we have to restore
                             * the reverted operations on this shard by replaying the translog to avoid losing acknowledged writes.
                             */
                            final Engine engine = getEngine();
                            engine.translogManager()
                                .restoreLocalHistoryFromTranslog(
                                    engine.getProcessedLocalCheckpoint(),
                                    (snapshot) -> runTranslogRecovery(engine, snapshot, Engine.Operation.Origin.LOCAL_RESET, () -> {})
                                );
                            /* Rolling the translog generation is not strictly needed here (as we will never have collisions between
                             * sequence numbers in a translog generation in a new primary as it takes the last known sequence number
                             * as a starting point), but it simplifies reasoning about the relationship between primary terms and
                             * translog generations.
                             */
                            engine.translogManager().rollTranslogGeneration();
                            engine.fillSeqNoGaps(newPrimaryTerm);
                            replicationTracker.updateLocalCheckpoint(currentRouting.allocationId().getId(), getLocalCheckpoint());
                            primaryReplicaSyncer.accept(this, new ActionListener<ResyncTask>() {
                                @Override
                                public void onResponse(ResyncTask resyncTask) {
                                    logger.info("primary-replica resync completed with {} operations", resyncTask.getResyncedOperations());
                                    boolean resyncCompleted = primaryReplicaResyncInProgress.compareAndSet(true, false);
                                    assert resyncCompleted : "primary-replica resync finished but was not started";
                                }

                                @Override
                                public void onFailure(Exception e) {
                                    boolean resyncCompleted = primaryReplicaResyncInProgress.compareAndSet(true, false);
                                    assert resyncCompleted : "primary-replica resync finished but was not started";
                                    if (state == IndexShardState.CLOSED) {
                                        // ignore, shutting down
                                    } else {
                                        failShard("exception during primary-replica resync", e);
                                    }
                                }
                            });
                        } catch (final AlreadyClosedException e) {
                            // okay, the index was deleted
                        }
                    }, null);
                }
            }
            // set this last, once we finished updating all internal state.
            this.shardRouting = newRouting;

            assert this.shardRouting.primary() == false || this.shardRouting.started() == false || // note that we use started and not
            // active to avoid relocating shards
                this.indexShardOperationPermits.isBlocked() || // if permits are blocked, we are still transitioning
                this.replicationTracker.isPrimaryMode() : "a started primary with non-pending operation term must be in primary mode "
                    + this.shardRouting;
            shardStateUpdated.countDown();
        }
        if (currentRouting.active() == false && newRouting.active()) {
            indexEventListener.afterIndexShardStarted(this);
        }
        if (newRouting.equals(currentRouting) == false) {
            indexEventListener.shardRoutingChanged(this, currentRouting, newRouting);
        }

        if (indexSettings.isSoftDeleteEnabled() && useRetentionLeasesInPeerRecovery == false && state() == IndexShardState.STARTED) {
            final RetentionLeases retentionLeases = replicationTracker.getRetentionLeases();
            final Set<ShardRouting> shardRoutings = new HashSet<>(routingTable.getShards());
            shardRoutings.addAll(routingTable.assignedShards()); // include relocation targets
            if (shardRoutings.stream()
                .allMatch(
                    shr -> shr.assignedToNode() && retentionLeases.contains(ReplicationTracker.getPeerRecoveryRetentionLeaseId(shr))
                )) {
                useRetentionLeasesInPeerRecovery = true;
                turnOffTranslogRetention();
            }
        }
    }

    /**
     * Marks the shard as recovering based on a recovery state, fails with exception is recovering is not allowed to be set.
     */
    public IndexShardState markAsRecovering(String reason, RecoveryState recoveryState) throws IndexShardStartedException,
        IndexShardRelocatedException, IndexShardRecoveringException, IndexShardClosedException {
        synchronized (mutex) {
            if (state == IndexShardState.CLOSED) {
                throw new IndexShardClosedException(shardId);
            }
            if (state == IndexShardState.STARTED) {
                throw new IndexShardStartedException(shardId);
            }
            if (state == IndexShardState.RECOVERING) {
                throw new IndexShardRecoveringException(shardId);
            }
            if (state == IndexShardState.POST_RECOVERY) {
                throw new IndexShardRecoveringException(shardId);
            }
            this.recoveryState = recoveryState;
            return changeState(IndexShardState.RECOVERING, reason);
        }
    }

    private final AtomicBoolean primaryReplicaResyncInProgress = new AtomicBoolean();

    /**
     * Completes the relocation. Operations are blocked and current operations are drained before changing state to
     * relocated. After all operations are successfully blocked, performSegRep is executed followed by target relocation
     * handoff.
     *
     * @param performSegRep a {@link Runnable} that is executed after operations are blocked
     * @param consumer a {@link Runnable} that is executed after performSegRep
     * @throws IllegalIndexShardStateException if the shard is not relocating due to concurrent cancellation
     * @throws IllegalStateException           if the relocation target is no longer part of the replication group
     * @throws InterruptedException            if blocking operations is interrupted
     */
    public void relocated(
        final String targetAllocationId,
        final Consumer<ReplicationTracker.PrimaryContext> consumer,
        final Runnable performSegRep
    ) throws IllegalIndexShardStateException, IllegalStateException, InterruptedException {
        assert shardRouting.primary() : "only primaries can be marked as relocated: " + shardRouting;
        // The below list of releasable ensures that if the relocation does not happen, we undo the activity of close and
        // acquire all permits. This will ensure that the remote store uploads can still be done by the existing primary shard.
        List<Releasable> releasablesOnHandoffFailures = new ArrayList<>(2);
        try (Releasable forceRefreshes = refreshListeners.forceRefreshes()) {
            indexShardOperationPermits.blockOperations(30, TimeUnit.MINUTES, () -> {
                forceRefreshes.close();

                boolean syncTranslog = isRemoteTranslogEnabled() && Durability.ASYNC == indexSettings.getTranslogDurability();
                // Since all the index permits are acquired at this point, the translog buffer will not change.
                // It is safe to perform sync of translogs now as this will ensure for remote-backed indexes, the
                // translogs has been uploaded to the remote store.
                if (syncTranslog) {
                    maybeSync();
                }

                // Ensures all in-flight remote store refreshes drain, before we perform the performSegRep.
                for (ReferenceManager.RefreshListener refreshListener : internalRefreshListener) {
                    if (refreshListener instanceof ReleasableRetryableRefreshListener) {
                        releasablesOnHandoffFailures.add(((ReleasableRetryableRefreshListener) refreshListener).drainRefreshes());
                    }
                }

                // Ensure all in-flight remote store translog upload drains, before we perform the performSegRep.
                releasablesOnHandoffFailures.add(getEngine().translogManager().drainSync());

                // no shard operation permits are being held here, move state from started to relocated
                assert indexShardOperationPermits.getActiveOperationsCount() == OPERATIONS_BLOCKED
                    : "in-flight operations in progress while moving shard state to relocated";

                performSegRep.run();
                /*
                 * We should not invoke the runnable under the mutex as the expected implementation is to handoff the primary context via a
                 * network operation. Doing this under the mutex can implicitly block the cluster state update thread on network operations.
                 */
                verifyRelocatingState();
                final ReplicationTracker.PrimaryContext primaryContext = replicationTracker.startRelocationHandoff(targetAllocationId);
                try {
                    consumer.accept(primaryContext);
                    synchronized (mutex) {
                        verifyRelocatingState();
                        replicationTracker.completeRelocationHandoff(); // make changes to primaryMode and relocated flag only under
                        // mutex
                    }
                } catch (final Exception e) {
                    try {
                        replicationTracker.abortRelocationHandoff();
                    } catch (final Exception inner) {
                        e.addSuppressed(inner);
                    }
                    throw e;
                }
            });
        } catch (TimeoutException e) {
            logger.warn("timed out waiting for relocation hand-off to complete");
            // This is really bad as ongoing replication operations are preventing this shard from completing relocation hand-off.
            // Fail primary relocation source and target shards.
            failShard("timed out waiting for relocation hand-off to complete", null);
            throw new IndexShardClosedException(shardId(), "timed out waiting for relocation hand-off to complete");
        } catch (Exception ex) {
            assert replicationTracker.isPrimaryMode();
            // If the primary mode is still true after the end of handoff attempt, it basically means that the relocation
            // failed. The existing primary will continue to be the primary, so we need to allow the segments and translog
            // upload to resume.
            Releasables.close(releasablesOnHandoffFailures);
            throw ex;
        }
    }

    private void maybeSync() {
        try {
            if (isSyncNeeded()) {
                sync();
            }
        } catch (IOException e) {
            logger.warn("failed to sync translog", e);
        }
    }

    private void verifyRelocatingState() {
        if (state != IndexShardState.STARTED) {
            throw new IndexShardNotStartedException(shardId, state);
        }
        /*
         * If the cluster-manager cancelled recovery, the target will be removed and the recovery will be cancelled. However, it is still possible
         * that we concurrently end up here and therefore have to protect that we do not mark the shard as relocated when its shard routing
         * says otherwise.
         */

        if (shardRouting.relocating() == false) {
            throw new IllegalIndexShardStateException(shardId, IndexShardState.STARTED, ": shard is no longer relocating " + shardRouting);
        }

        if (primaryReplicaResyncInProgress.get()) {
            throw new IllegalIndexShardStateException(
                shardId,
                IndexShardState.STARTED,
                ": primary relocation is forbidden while primary-replica resync is in progress " + shardRouting
            );
        }
    }

    @Override
    public IndexShardState state() {
        return state;
    }

    /**
     * Changes the state of the current shard
     *
     * @param newState the new shard state
     * @param reason   the reason for the state change
     * @return the previous shard state
     */
    private IndexShardState changeState(IndexShardState newState, String reason) {
        assert Thread.holdsLock(mutex);
        logger.debug("state: [{}]->[{}], reason [{}]", state, newState, reason);
        IndexShardState previousState = state;
        state = newState;
        this.indexEventListener.indexShardStateChanged(this, previousState, newState, reason);
        return previousState;
    }

    public Engine.IndexResult applyIndexOperationOnPrimary(
        long version,
        VersionType versionType,
        SourceToParse sourceToParse,
        long ifSeqNo,
        long ifPrimaryTerm,
        long autoGeneratedTimestamp,
        boolean isRetry
    ) throws IOException {
        assert versionType.validateVersionForWrites(version);
        return applyIndexOperation(
            getEngine(),
            UNASSIGNED_SEQ_NO,
            getOperationPrimaryTerm(),
            version,
            versionType,
            ifSeqNo,
            ifPrimaryTerm,
            autoGeneratedTimestamp,
            isRetry,
            Engine.Operation.Origin.PRIMARY,
            sourceToParse,
            null
        );
    }

    public Engine.IndexResult applyIndexOperationOnReplica(
        String id,
        long seqNo,
        long opPrimaryTerm,
        long version,
        long autoGeneratedTimeStamp,
        boolean isRetry,
        SourceToParse sourceToParse
    ) throws IOException {
        return applyIndexOperation(
            getEngine(),
            seqNo,
            opPrimaryTerm,
            version,
            null,
            UNASSIGNED_SEQ_NO,
            0,
            autoGeneratedTimeStamp,
            isRetry,
            Engine.Operation.Origin.REPLICA,
            sourceToParse,
            id
        );
    }

    private Engine.IndexResult applyIndexOperation(
        Engine engine,
        long seqNo,
        long opPrimaryTerm,
        long version,
        @Nullable VersionType versionType,
        long ifSeqNo,
        long ifPrimaryTerm,
        long autoGeneratedTimeStamp,
        boolean isRetry,
        Engine.Operation.Origin origin,
        SourceToParse sourceToParse,
        String id
    ) throws IOException {

        // For Segment Replication enabled replica shards we can be skip parsing the documents as we directly copy segments from primary
        // shard.
        if (indexSettings.isSegRepEnabled() && routingEntry().primary() == false) {
            Engine.Index index = new Engine.Index(
                new Term(IdFieldMapper.NAME, Uid.encodeId(id)),
                new ParsedDocument(null, null, id, null, null, sourceToParse.source(), sourceToParse.getMediaType(), null),
                seqNo,
                opPrimaryTerm,
                version,
                null,
                Engine.Operation.Origin.REPLICA,
                System.nanoTime(),
                autoGeneratedTimeStamp,
                isRetry,
                UNASSIGNED_SEQ_NO,
                0
            );
            return getEngine().index(index);
        }
        assert opPrimaryTerm <= getOperationPrimaryTerm() : "op term [ "
            + opPrimaryTerm
            + " ] > shard term ["
            + getOperationPrimaryTerm()
            + "]";
        ensureWriteAllowed(origin);
        Engine.Index operation;
        try {
            operation = prepareIndex(
                docMapper(),
                sourceToParse,
                seqNo,
                opPrimaryTerm,
                version,
                versionType,
                origin,
                autoGeneratedTimeStamp,
                isRetry,
                ifSeqNo,
                ifPrimaryTerm
            );
            Mapping update = operation.parsedDoc().dynamicMappingsUpdate();
            if (update != null) {
                return new Engine.IndexResult(update);
            }
        } catch (Exception e) {
            // We treat any exception during parsing and or mapping update as a document level failure
            // with the exception side effects of closing the shard. Since we don't have the shard, we
            // can not raise an exception that may block any replication of previous operations to the
            // replicas
            verifyNotClosed(e);
            return new Engine.IndexResult(e, version, opPrimaryTerm, seqNo);
        }

        return index(engine, operation);
    }

    public static Engine.Index prepareIndex(
        DocumentMapperForType docMapper,
        SourceToParse source,
        long seqNo,
        long primaryTerm,
        long version,
        VersionType versionType,
        Engine.Operation.Origin origin,
        long autoGeneratedIdTimestamp,
        boolean isRetry,
        long ifSeqNo,
        long ifPrimaryTerm
    ) {
        long startTime = System.nanoTime();
        ParsedDocument doc = docMapper.getDocumentMapper().parse(source);
        if (docMapper.getMapping() != null) {
            doc.addDynamicMappingsUpdate(docMapper.getMapping());
        }
        Term uid = new Term(IdFieldMapper.NAME, Uid.encodeId(doc.id()));
        return new Engine.Index(
            uid,
            doc,
            seqNo,
            primaryTerm,
            version,
            versionType,
            origin,
            startTime,
            autoGeneratedIdTimestamp,
            isRetry,
            ifSeqNo,
            ifPrimaryTerm
        );
    }

    private Engine.IndexResult index(Engine engine, Engine.Index index) throws IOException {
        active.set(true);
        final Engine.IndexResult result;
        index = indexingOperationListeners.preIndex(shardId, index);
        try {
            if (logger.isTraceEnabled()) {
                // don't use index.source().utf8ToString() here source might not be valid UTF-8
                logger.trace(
                    "index [{}] seq# [{}] allocation-id [{}] primaryTerm [{}] operationPrimaryTerm [{}] origin [{}]",
                    index.id(),
                    index.seqNo(),
                    routingEntry().allocationId(),
                    index.primaryTerm(),
                    getOperationPrimaryTerm(),
                    index.origin()
                );
            }
            result = engine.index(index);
            if (logger.isTraceEnabled()) {
                logger.trace(
                    "index-done [{}] seq# [{}] allocation-id [{}] primaryTerm [{}] operationPrimaryTerm [{}] origin [{}] "
                        + "result-seq# [{}] result-term [{}] failure [{}]",
                    index.id(),
                    index.seqNo(),
                    routingEntry().allocationId(),
                    index.primaryTerm(),
                    getOperationPrimaryTerm(),
                    index.origin(),
                    result.getSeqNo(),
                    result.getTerm(),
                    result.getFailure()
                );
            }
        } catch (Exception e) {
            if (logger.isTraceEnabled()) {
                logger.trace(
                    new ParameterizedMessage(
                        "index-fail [{}] seq# [{}] allocation-id [{}] primaryTerm [{}] operationPrimaryTerm [{}] origin [{}]",
                        index.id(),
                        index.seqNo(),
                        routingEntry().allocationId(),
                        index.primaryTerm(),
                        getOperationPrimaryTerm(),
                        index.origin()
                    ),
                    e
                );
            }
            indexingOperationListeners.postIndex(shardId, index, e);
            throw e;
        }
        indexingOperationListeners.postIndex(shardId, index, result);
        return result;
    }

    public Engine.NoOpResult markSeqNoAsNoop(long seqNo, long opPrimaryTerm, String reason) throws IOException {
        return markSeqNoAsNoop(getEngine(), seqNo, opPrimaryTerm, reason, Engine.Operation.Origin.REPLICA);
    }

    private Engine.NoOpResult markSeqNoAsNoop(Engine engine, long seqNo, long opPrimaryTerm, String reason, Engine.Operation.Origin origin)
        throws IOException {
        assert opPrimaryTerm <= getOperationPrimaryTerm() : "op term [ "
            + opPrimaryTerm
            + " ] > shard term ["
            + getOperationPrimaryTerm()
            + "]";
        long startTime = System.nanoTime();
        ensureWriteAllowed(origin);
        final Engine.NoOp noOp = new Engine.NoOp(seqNo, opPrimaryTerm, origin, startTime, reason);
        return noOp(engine, noOp);
    }

    private Engine.NoOpResult noOp(Engine engine, Engine.NoOp noOp) throws IOException {
        active.set(true);
        if (logger.isTraceEnabled()) {
            logger.trace("noop (seq# [{}])", noOp.seqNo());
        }
        return engine.noOp(noOp);
    }

    public Engine.IndexResult getFailedIndexResult(Exception e, long version) {
        return new Engine.IndexResult(e, version);
    }

    public Engine.DeleteResult getFailedDeleteResult(Exception e, long version) {
        return new Engine.DeleteResult(e, version, getOperationPrimaryTerm());
    }

    public Engine.DeleteResult applyDeleteOperationOnPrimary(
        long version,
        String id,
        VersionType versionType,
        long ifSeqNo,
        long ifPrimaryTerm
    ) throws IOException {
        assert versionType.validateVersionForWrites(version);
        return applyDeleteOperation(
            getEngine(),
            UNASSIGNED_SEQ_NO,
            getOperationPrimaryTerm(),
            version,
            id,
            versionType,
            ifSeqNo,
            ifPrimaryTerm,
            Engine.Operation.Origin.PRIMARY
        );
    }

    public Engine.DeleteResult applyDeleteOperationOnReplica(long seqNo, long opPrimaryTerm, long version, String id) throws IOException {
        if (indexSettings.isSegRepEnabled()) {
            final Engine.Delete delete = new Engine.Delete(
                id,
                new Term(IdFieldMapper.NAME, Uid.encodeId(id)),
                seqNo,
                opPrimaryTerm,
                version,
                null,
                Engine.Operation.Origin.REPLICA,
                System.nanoTime(),
                UNASSIGNED_SEQ_NO,
                0
            );
            return getEngine().delete(delete);
        }
        return applyDeleteOperation(
            getEngine(),
            seqNo,
            opPrimaryTerm,
            version,
            id,
            null,
            UNASSIGNED_SEQ_NO,
            0,
            Engine.Operation.Origin.REPLICA
        );
    }

    private Engine.DeleteResult applyDeleteOperation(
        Engine engine,
        long seqNo,
        long opPrimaryTerm,
        long version,
        String id,
        @Nullable VersionType versionType,
        long ifSeqNo,
        long ifPrimaryTerm,
        Engine.Operation.Origin origin
    ) throws IOException {
        assert opPrimaryTerm <= getOperationPrimaryTerm() : "op term [ "
            + opPrimaryTerm
            + " ] > shard term ["
            + getOperationPrimaryTerm()
            + "]";
        ensureWriteAllowed(origin);
        final Engine.Delete delete = prepareDelete(id, seqNo, opPrimaryTerm, version, versionType, origin, ifSeqNo, ifPrimaryTerm);
        return delete(engine, delete);
    }

    public static Engine.Delete prepareDelete(
        String id,
        long seqNo,
        long primaryTerm,
        long version,
        VersionType versionType,
        Engine.Operation.Origin origin,
        long ifSeqNo,
        long ifPrimaryTerm
    ) {
        long startTime = System.nanoTime();
        final Term uid = new Term(IdFieldMapper.NAME, Uid.encodeId(id));
        return new Engine.Delete(id, uid, seqNo, primaryTerm, version, versionType, origin, startTime, ifSeqNo, ifPrimaryTerm);
    }

    private Engine.DeleteResult delete(Engine engine, Engine.Delete delete) throws IOException {
        active.set(true);
        final Engine.DeleteResult result;
        delete = indexingOperationListeners.preDelete(shardId, delete);
        try {
            if (logger.isTraceEnabled()) {
                logger.trace("delete [{}] (seq no [{}])", delete.uid().text(), delete.seqNo());
            }
            result = engine.delete(delete);
        } catch (Exception e) {
            indexingOperationListeners.postDelete(shardId, delete, e);
            throw e;
        }
        indexingOperationListeners.postDelete(shardId, delete, result);
        return result;
    }

    public Engine.GetResult get(Engine.Get get) {
        readAllowed();
        DocumentMapper mapper = mapperService.documentMapper();
        if (mapper == null) {
            return GetResult.NOT_EXISTS;
        }
        return getEngine().get(get, this::acquireSearcher);
    }

    /**
     * Writes all indexing changes to disk and opens a new searcher reflecting all changes.  This can throw {@link AlreadyClosedException}.
     */
    public void refresh(String source) {
        verifyNotClosed();
        if (logger.isTraceEnabled()) {
            logger.trace("refresh with source [{}]", source);
        }
        getEngine().refresh(source);
    }

    /**
     * Returns how many bytes we are currently moving from heap to disk
     */
    public long getWritingBytes() {
        Engine engine = getEngineOrNull();
        if (engine == null) {
            return 0;
        }
        return engine.getWritingBytes();
    }

    public RefreshStats refreshStats() {
        int listeners = refreshListeners.pendingCount();
        return new RefreshStats(
            refreshMetric.count(),
            TimeUnit.NANOSECONDS.toMillis(refreshMetric.sum()),
            externalRefreshMetric.count(),
            TimeUnit.NANOSECONDS.toMillis(externalRefreshMetric.sum()),
            listeners
        );
    }

    public FlushStats flushStats() {
        return new FlushStats(flushMetric.count(), periodicFlushMetric.count(), TimeUnit.NANOSECONDS.toMillis(flushMetric.sum()));
    }

    public DocsStats docStats() {
        readAllowed();
        return getEngine().docStats();
    }

    /**
     * @return {@link CommitStats}
     * @throws AlreadyClosedException if shard is closed
     */
    public CommitStats commitStats() {
        return getEngine().commitStats();
    }

    /**
     * @return {@link SeqNoStats}
     * @throws AlreadyClosedException if shard is closed
     */
    public SeqNoStats seqNoStats() {
        return getEngine().getSeqNoStats(replicationTracker.getGlobalCheckpoint());
    }

    public IndexingStats indexingStats() {
        Engine engine = getEngineOrNull();
        final boolean throttled;
        final long throttleTimeInMillis;
        if (engine == null) {
            throttled = false;
            throttleTimeInMillis = 0;
        } else {
            throttled = engine.isThrottled();
            throttleTimeInMillis = engine.getIndexThrottleTimeInMillis();
        }
        return internalIndexingStats.stats(throttled, throttleTimeInMillis);
    }

    public SearchStats searchStats(String... groups) {
        return searchStats.stats(groups);
    }

    public GetStats getStats() {
        return getService.stats();
    }

    public StoreStats storeStats() {
        try {
            final RecoveryState recoveryState = this.recoveryState;
            final long bytesStillToRecover = recoveryState == null ? -1L : recoveryState.getIndex().bytesStillToRecover();
            return store.stats(bytesStillToRecover == -1 ? StoreStats.UNKNOWN_RESERVED_BYTES : bytesStillToRecover);
        } catch (IOException e) {
            failShard("Failing shard because of exception during storeStats", e);
            throw new OpenSearchException("io exception while building 'store stats'", e);
        }
    }

    public MergeStats mergeStats() {
        final Engine engine = getEngineOrNull();
        if (engine == null) {
            return new MergeStats();
        }
        final MergeStats mergeStats = engine.getMergeStats();
        mergeStats.addUnreferencedFileCleanUpStats(engine.unreferencedFileCleanUpsPerformed());
        return mergeStats;
    }

    public SegmentsStats segmentStats(boolean includeSegmentFileSizes, boolean includeUnloadedSegments) {
        SegmentsStats segmentsStats = getEngine().segmentsStats(includeSegmentFileSizes, includeUnloadedSegments);
        segmentsStats.addBitsetMemoryInBytes(shardBitsetFilterCache.getMemorySizeInBytes());
        // Populate remote_store stats only if the index is remote store backed
        if (indexSettings.isRemoteStoreEnabled()) {
            segmentsStats.addRemoteSegmentStats(
                new RemoteSegmentStats(remoteStoreStatsTrackerFactory.getRemoteSegmentTransferTracker(shardId).stats())
            );
        }
        if (indexSettings.isSegRepEnabled()) {
            segmentsStats.addReplicationStats(getReplicationStats());
        }
        return segmentsStats;
    }

    public WarmerStats warmerStats() {
        return shardWarmerService.stats();
    }

    public FieldDataStats fieldDataStats(String... fields) {
        return shardFieldData.stats(fields);
    }

    public TranslogStats translogStats() {
        TranslogStats translogStats = getEngine().translogManager().getTranslogStats();
        // Populate remote_store stats only if the index is remote store backed
        if (indexSettings.isRemoteStoreEnabled()) {
            translogStats.addRemoteTranslogStats(
                new RemoteTranslogStats(remoteStoreStatsTrackerFactory.getRemoteTranslogTransferTracker(shardId).stats())
            );
        }

        return translogStats;
    }

    public CompletionStats completionStats(String... fields) {
        readAllowed();
        return getEngine().completionStats(fields);
    }

    /**
     * Executes the given flush request against the engine.
     *
     * @param request the flush request
     */
    public void flush(FlushRequest request) {
        final boolean waitIfOngoing = request.waitIfOngoing();
        final boolean force = request.force();
        logger.trace("flush with {}", request);
        /*
         * We allow flushes while recovery since we allow operations to happen while recovering and we want to keep the translog under
         * control (up to deletes, which we do not GC). Yet, we do not use flush internally to clear deletes and flush the index writer
         * since we use Engine#writeIndexingBuffer for this now.
         */
        verifyNotClosed();
        final long time = System.nanoTime();
        getEngine().flush(force, waitIfOngoing);
        flushMetric.inc(System.nanoTime() - time);
    }

    /**
     * checks and removes translog files that no longer need to be retained. See
     * {@link org.opensearch.index.translog.TranslogDeletionPolicy} for details
     */
    public void trimTranslog() {
        if (isRemoteTranslogEnabled()) {
            return;
        }
        verifyNotClosed();
        final Engine engine = getEngine();
        engine.translogManager().trimUnreferencedTranslogFiles();
    }

    /**
     * Rolls the tranlog generation and cleans unneeded.
     */
    public void rollTranslogGeneration() throws IOException {
        final Engine engine = getEngine();
        engine.translogManager().rollTranslogGeneration();
    }

    public void forceMerge(ForceMergeRequest forceMerge) throws IOException {
        verifyActive();
        if (logger.isTraceEnabled()) {
            logger.trace("force merge with {}", forceMerge);
        }
        Engine engine = getEngine();
        engine.forceMerge(
            forceMerge.flush(),
            forceMerge.maxNumSegments(),
            forceMerge.onlyExpungeDeletes(),
            false,
            false,
            forceMerge.forceMergeUUID()
        );
    }

    /**
     * Upgrades the shard to the current version of Lucene and returns the minimum segment version
     */
    public org.apache.lucene.util.Version upgrade(UpgradeRequest upgrade) throws IOException {
        verifyActive();
        if (logger.isTraceEnabled()) {
            logger.trace("upgrade with {}", upgrade);
        }
        org.apache.lucene.util.Version previousVersion = minimumCompatibleVersion();
        // we just want to upgrade the segments, not actually forge merge to a single segment
        final Engine engine = getEngine();
        engine.forceMerge(
            true,  // we need to flush at the end to make sure the upgrade is durable
            Integer.MAX_VALUE, // we just want to upgrade the segments, not actually optimize to a single segment
            false,
            true,
            upgrade.upgradeOnlyAncientSegments(),
            null
        );
        org.apache.lucene.util.Version version = minimumCompatibleVersion();
        if (logger.isTraceEnabled()) {
            logger.trace("upgraded segments for {} from version {} to version {}", shardId, previousVersion, version);
        }

        return version;
    }

    public org.apache.lucene.util.Version minimumCompatibleVersion() {
        org.apache.lucene.util.Version luceneVersion = null;
        for (Segment segment : getEngine().segments(false)) {
            if (luceneVersion == null || luceneVersion.onOrAfter(segment.getVersion())) {
                luceneVersion = segment.getVersion();
            }
        }
        return luceneVersion == null ? indexSettings.getIndexVersionCreated().luceneVersion : luceneVersion;
    }

    /**
     * Creates a new {@link IndexCommit} snapshot from the currently running engine. All resources referenced by this
     * commit won't be freed until the commit / snapshot is closed.
     *
     * @param flushFirst <code>true</code> if the index should first be flushed to disk / a low level lucene commit should be executed
     */
    public GatedCloseable<IndexCommit> acquireLastIndexCommit(boolean flushFirst) throws EngineException {
        final IndexShardState state = this.state; // one time volatile read
        // we allow snapshot on closed index shard, since we want to do one after we close the shard and before we close the engine
        if (state == IndexShardState.STARTED || state == IndexShardState.CLOSED) {
            return getEngine().acquireLastIndexCommit(flushFirst);
        } else {
            throw new IllegalIndexShardStateException(shardId, state, "snapshot is not allowed");
        }
    }

    public GatedCloseable<IndexCommit> acquireLastIndexCommitAndRefresh(boolean flushFirst) throws EngineException {
        GatedCloseable<IndexCommit> indexCommit = acquireLastIndexCommit(flushFirst);
        getEngine().refresh("Snapshot for Remote Store based Shard");
        return indexCommit;
    }

    /**
     *
     * @param snapshotId Snapshot UUID.
     * @param primaryTerm current primary term.
     * @param generation Snapshot Commit Generation.
     * @throws IOException if there is some failure in acquiring lock in remote store.
     */
    public void acquireLockOnCommitData(String snapshotId, long primaryTerm, long generation) throws IOException {
        RemoteSegmentStoreDirectory remoteSegmentStoreDirectory = getRemoteDirectory();
        remoteSegmentStoreDirectory.acquireLock(primaryTerm, generation, snapshotId);
    }

    /**
     *
     * @param snapshotId Snapshot UUID.
     * @param primaryTerm current primary term.
     * @param generation Snapshot Commit Generation.
     * @throws IOException if there is some failure in releasing lock in remote store.
     */
    public void releaseLockOnCommitData(String snapshotId, long primaryTerm, long generation) throws IOException {
        RemoteSegmentStoreDirectory remoteSegmentStoreDirectory = getRemoteDirectory();
        remoteSegmentStoreDirectory.releaseLock(primaryTerm, generation, snapshotId);
    }

    public Optional<NRTReplicationEngine> getReplicationEngine() {
        if (getEngine() instanceof NRTReplicationEngine) {
            return Optional.of((NRTReplicationEngine) getEngine());
        } else {
            return Optional.empty();
        }
    }

    public void finalizeReplication(SegmentInfos infos) throws IOException {
        if (getReplicationEngine().isPresent()) {
            getReplicationEngine().get().updateSegments(infos);
        }
    }

    /**
     * Snapshots the most recent safe index commit from the currently running engine.
     * All index files referenced by this index commit won't be freed until the commit/snapshot is closed.
     */
    public GatedCloseable<IndexCommit> acquireSafeIndexCommit() throws EngineException {
        final IndexShardState state = this.state; // one time volatile read
        // we allow snapshot on closed index shard, since we want to do one after we close the shard and before we close the engine
        if (state == IndexShardState.STARTED || state == IndexShardState.CLOSED) {
            return getEngine().acquireSafeIndexCommit();
        } else {
            throw new IllegalIndexShardStateException(shardId, state, "snapshot is not allowed");
        }
    }

    /**
     * return the most recently computed ReplicationCheckpoint for a particular shard.
     * The checkpoint is updated inside a refresh listener and may lag behind the SegmentInfos on the reader.
     * To guarantee the checkpoint is upto date with the latest on-reader infos, use `getLatestSegmentInfosAndCheckpoint` instead.
     *
     * @return {@link ReplicationCheckpoint} - The most recently computed ReplicationCheckpoint.
     */
    public ReplicationCheckpoint getLatestReplicationCheckpoint() {
        return replicationTracker.getLatestReplicationCheckpoint();
    }

    /**
     * Compute and return the latest ReplicationCheckpoint for a shard and a GatedCloseable containing the corresponding SegmentInfos.
     * The segments referenced by the SegmentInfos will remain on disk until the GatedCloseable is closed.
     * <p>
     * Primary shards compute the seqNo used in the replication checkpoint from the fetched SegmentInfos.
     * Replica shards compute the seqNo from its latest processed checkpoint, which only increases when refreshing on new segments.
     *
     * @return A {@link Tuple} containing SegmentInfos wrapped in a {@link GatedCloseable} and the {@link ReplicationCheckpoint} computed from the infos.
     *
     */
    public Tuple<GatedCloseable<SegmentInfos>, ReplicationCheckpoint> getLatestSegmentInfosAndCheckpoint() {
        assert indexSettings.isSegRepEnabled();

        // do not close the snapshot - caller will close it.
        GatedCloseable<SegmentInfos> snapshot = null;
        try {
            snapshot = getSegmentInfosSnapshot();
            final SegmentInfos segmentInfos = snapshot.get();
            return new Tuple<>(snapshot, computeReplicationCheckpoint(segmentInfos));
        } catch (IOException | AlreadyClosedException e) {
            logger.error("Error Fetching SegmentInfos and latest checkpoint", e);
            if (snapshot != null) {
                try {
                    snapshot.close();
                } catch (IOException ex) {
                    throw new OpenSearchException("Error Closing SegmentInfos Snapshot", e);
                }
            }
        }
        return new Tuple<>(new GatedCloseable<>(null, () -> {}), getLatestReplicationCheckpoint());
    }

    /**
     * Compute the latest {@link ReplicationCheckpoint} from a SegmentInfos.
     * This function fetches a metadata snapshot from the store that comes with an IO cost.
     * We will reuse the existing stored checkpoint if it is at the same SI version.
     *
     * @param segmentInfos {@link SegmentInfos} infos to use to compute.
     * @return {@link ReplicationCheckpoint} Checkpoint computed from the infos.
     * @throws IOException When there is an error computing segment metadata from the store.
     */
    ReplicationCheckpoint computeReplicationCheckpoint(SegmentInfos segmentInfos) throws IOException {
        if (segmentInfos == null) {
            return ReplicationCheckpoint.empty(shardId);
        }
        final ReplicationCheckpoint latestReplicationCheckpoint = getLatestReplicationCheckpoint();
        if (latestReplicationCheckpoint.getSegmentInfosVersion() == segmentInfos.getVersion()
            && latestReplicationCheckpoint.getSegmentsGen() == segmentInfos.getGeneration()) {
            return latestReplicationCheckpoint;
        }
        final Map<String, StoreFileMetadata> metadataMap = store.getSegmentMetadataMap(segmentInfos);
        final ReplicationCheckpoint checkpoint = new ReplicationCheckpoint(
            this.shardId,
            getOperationPrimaryTerm(),
            segmentInfos.getGeneration(),
            segmentInfos.getVersion(),
            metadataMap.values().stream().mapToLong(StoreFileMetadata::length).sum(),
            getEngine().config().getCodec().getName(),
            metadataMap
        );
        logger.trace("Recomputed ReplicationCheckpoint for shard {}", checkpoint);
        return checkpoint;
    }

    /**
     * Checks if this target shard should start a round of segment replication.
     * @return - True if the shard is able to perform segment replication.
     */
    public boolean isSegmentReplicationAllowed() {
        if (indexSettings.isSegRepEnabled() == false) {
            logger.trace("Attempting to perform segment replication when it is not enabled on the index");
            return false;
        }
        if (getReplicationTracker().isPrimaryMode()) {
            logger.trace("Shard is in primary mode and cannot perform segment replication as a replica.");
            return false;
        }
        if (this.routingEntry().primary()) {
            logger.trace("Shard routing is marked primary thus cannot perform segment replication as replica");
            return false;
        }
        if (state().equals(IndexShardState.STARTED) == false
            && (state() == IndexShardState.POST_RECOVERY && shardRouting.state() == ShardRoutingState.INITIALIZING) == false) {
            logger.trace(
                () -> new ParameterizedMessage(
                    "Shard is not started or recovering {} {} and cannot perform segment replication as a replica",
                    state(),
                    shardRouting.state()
                )
            );
            return false;
        }
        if (getReplicationEngine().isEmpty()) {
            logger.trace(
                () -> new ParameterizedMessage(
                    "Shard does not have the correct engine type to perform segment replication {}.",
                    getEngine().getClass()
                )
            );
            return false;
        }
        return true;
    }

    /**
     * Checks if checkpoint should be processed
     *
     * @param requestCheckpoint       received checkpoint that is checked for processing
     * @return true if checkpoint should be processed
     */
    public final boolean shouldProcessCheckpoint(ReplicationCheckpoint requestCheckpoint) {
        if (isSegmentReplicationAllowed() == false) {
            return false;
        }
        final ReplicationCheckpoint localCheckpoint = getLatestReplicationCheckpoint();
        if (requestCheckpoint.isAheadOf(localCheckpoint) == false) {
            logger.trace(
                () -> new ParameterizedMessage(
                    "Ignoring new replication checkpoint - Shard is already on checkpoint {} that is ahead of {}",
                    localCheckpoint,
                    requestCheckpoint
                )
            );
            return false;
        }
        return true;
    }

    /**
     * gets a {@link Store.MetadataSnapshot} for the current directory. This method is safe to call in all lifecycle of the index shard,
     * without having to worry about the current state of the engine and concurrent flushes.
     *
     * @throws org.apache.lucene.index.IndexNotFoundException     if no index is found in the current directory
     * @throws org.apache.lucene.index.CorruptIndexException      if the lucene index is corrupted. This can be caused by a checksum
     *                                                            mismatch or an unexpected exception when opening the index reading the
     *                                                            segments file.
     * @throws org.apache.lucene.index.IndexFormatTooOldException if the lucene index is too old to be opened.
     * @throws org.apache.lucene.index.IndexFormatTooNewException if the lucene index is too new to be opened.
     * @throws java.io.FileNotFoundException                      if one or more files referenced by a commit are not present.
     * @throws java.nio.file.NoSuchFileException                  if one or more files referenced by a commit are not present.
     */
    public Store.MetadataSnapshot snapshotStoreMetadata() throws IOException {
        assert Thread.holdsLock(mutex) == false : "snapshotting store metadata under mutex";
        GatedCloseable<IndexCommit> wrappedIndexCommit = null;
        store.incRef();
        try {
            synchronized (engineMutex) {
                // if the engine is not running, we can access the store directly, but we need to make sure no one starts
                // the engine on us. If the engine is running, we can get a snapshot via the deletion policy of the engine.
                final Engine engine = getEngineOrNull();
                if (engine != null) {
                    wrappedIndexCommit = engine.acquireLastIndexCommit(false);
                }
                if (wrappedIndexCommit == null) {
                    return store.getMetadata(null, true);
                }
            }
            return store.getMetadata(wrappedIndexCommit.get());
        } finally {
            store.decRef();
            IOUtils.close(wrappedIndexCommit);
        }
    }

    /**
     * Fetch a map of StoreFileMetadata for each segment from the latest SegmentInfos.
     * This is used to compute diffs for segment replication.
     *
     * @return - Map of Segment Filename to its {@link StoreFileMetadata}
     * @throws IOException - When there is an error loading metadata from the store.
     */
    public Map<String, StoreFileMetadata> getSegmentMetadataMap() throws IOException {
        try (final GatedCloseable<SegmentInfos> snapshot = getSegmentInfosSnapshot()) {
            return store.getSegmentMetadataMap(snapshot.get());
        }
    }

    /**
     * Fails the shard and marks the shard store as corrupted if
     * <code>e</code> is caused by index corruption
     */
    public void failShard(String reason, @Nullable Exception e) {
        // fail the engine. This will cause this shard to also be removed from the node's index service.
        getEngine().failEngine(reason, e);
    }

    /**
     * Acquires a point-in-time reader that can be used to create {@link Engine.Searcher}s on demand.
     */
    public Engine.SearcherSupplier acquireSearcherSupplier() {
        return acquireSearcherSupplier(Engine.SearcherScope.EXTERNAL);
    }

    /**
     * Acquires a point-in-time reader that can be used to create {@link Engine.Searcher}s on demand.
     */
    public Engine.SearcherSupplier acquireSearcherSupplier(Engine.SearcherScope scope) {
        readAllowed();
        markSearcherAccessed();
        final Engine engine = getEngine();
        return engine.acquireSearcherSupplier(this::wrapSearcher, scope);
    }

    public Engine.Searcher acquireSearcher(String source) {
        return acquireSearcher(source, Engine.SearcherScope.EXTERNAL);
    }

    private void markSearcherAccessed() {
        lastSearcherAccess.lazySet(threadPool.relativeTimeInMillis());
    }

    private Engine.Searcher acquireSearcher(String source, Engine.SearcherScope scope) {
        readAllowed();
        markSearcherAccessed();
        final Engine engine = getEngine();
        return engine.acquireSearcher(source, scope, this::wrapSearcher);
    }

    private Engine.Searcher wrapSearcher(Engine.Searcher searcher) {
        assert OpenSearchDirectoryReader.unwrap(searcher.getDirectoryReader()) != null
            : "DirectoryReader must be an instance or OpenSearchDirectoryReader";
        boolean success = false;
        try {
            final Engine.Searcher newSearcher = readerWrapper == null ? searcher : wrapSearcher(searcher, readerWrapper);
            assert newSearcher != null;
            success = true;
            return newSearcher;
        } catch (IOException ex) {
            throw new OpenSearchException("failed to wrap searcher", ex);
        } finally {
            if (success == false) {
                Releasables.close(success, searcher);
            }
        }
    }

    static Engine.Searcher wrapSearcher(
        Engine.Searcher engineSearcher,
        CheckedFunction<DirectoryReader, DirectoryReader, IOException> readerWrapper
    ) throws IOException {
        assert readerWrapper != null;
        final OpenSearchDirectoryReader openSearchDirectoryReader = OpenSearchDirectoryReader.getOpenSearchDirectoryReader(
            engineSearcher.getDirectoryReader()
        );
        if (openSearchDirectoryReader == null) {
            throw new IllegalStateException("Can't wrap non opensearch directory reader");
        }
        NonClosingReaderWrapper nonClosingReaderWrapper = new NonClosingReaderWrapper(engineSearcher.getDirectoryReader());
        DirectoryReader reader = readerWrapper.apply(nonClosingReaderWrapper);
        if (reader != nonClosingReaderWrapper) {
            if (reader.getReaderCacheHelper() != openSearchDirectoryReader.getReaderCacheHelper()) {
                throw new IllegalStateException(
                    "wrapped directory reader doesn't delegate IndexReader#getCoreCacheKey,"
                        + " wrappers must override this method and delegate to the original readers core cache key. Wrapped readers can't be "
                        + "used as cache keys since their are used only per request which would lead to subtle bugs"
                );
            }
            if (OpenSearchDirectoryReader.getOpenSearchDirectoryReader(reader) != openSearchDirectoryReader) {
                // prevent that somebody wraps with a non-filter reader
                throw new IllegalStateException("wrapped directory reader hides actual OpenSearchDirectoryReader but shouldn't");
            }
        }

        if (reader == nonClosingReaderWrapper) {
            return engineSearcher;
        } else {
            // we close the reader to make sure wrappers can release resources if needed....
            // our NonClosingReaderWrapper makes sure that our reader is not closed
            return new Engine.Searcher(
                engineSearcher.source(),
                reader,
                engineSearcher.getSimilarity(),
                engineSearcher.getQueryCache(),
                engineSearcher.getQueryCachingPolicy(),
                () -> IOUtils.close(
                    reader, // this will close the wrappers excluding the NonClosingReaderWrapper
                    engineSearcher
                )
            ); // this will run the closeable on the wrapped engine reader
        }
    }

    public void onCheckpointPublished(ReplicationCheckpoint checkpoint) {
        replicationTracker.startReplicationLagTimers(checkpoint);
    }

    /**
     * Used with segment replication during relocation handoff, this method updates current read only engine to global
     * checkpoint followed by changing to writeable engine
     *
     * @throws IOException if communication failed
     * @throws InterruptedException if calling thread is interrupted
     * @throws TimeoutException if timed out waiting for in-flight operations to finish
     *
     * @opensearch.internal
     */
    public void resetToWriteableEngine() throws IOException, InterruptedException, TimeoutException {
        indexShardOperationPermits.blockOperations(30, TimeUnit.MINUTES, () -> { resetEngineToGlobalCheckpoint(); });
    }

    /**
     * Wrapper for a non-closing reader
     *
     * @opensearch.internal
     */
    private static final class NonClosingReaderWrapper extends FilterDirectoryReader {

        private NonClosingReaderWrapper(DirectoryReader in) throws IOException {
            super(in, new SubReaderWrapper() {
                @Override
                public LeafReader wrap(LeafReader reader) {
                    return reader;
                }
            });
        }

        @Override
        protected DirectoryReader doWrapDirectoryReader(DirectoryReader in) throws IOException {
            return new NonClosingReaderWrapper(in);
        }

        @Override
        protected void doClose() throws IOException {
            // don't close here - mimic the MultiReader#doClose = false behavior that FilterDirectoryReader doesn't have
        }

        @Override
        public CacheHelper getReaderCacheHelper() {
            return in.getReaderCacheHelper();
        }

    }

    public void close(String reason, boolean flushEngine, boolean deleted) throws IOException {
        synchronized (engineMutex) {
            try {
                synchronized (mutex) {
                    changeState(IndexShardState.CLOSED, reason);
                }
            } finally {
                final Engine engine = this.currentEngineReference.getAndSet(null);
                try {
                    if (engine != null && flushEngine) {
                        engine.flushAndClose();
                    }
                } finally {
                    // playing safe here and close the engine even if the above succeeds - close can be called multiple times
                    // Also closing refreshListeners to prevent us from accumulating any more listeners
                    IOUtils.close(engine, globalCheckpointListeners, refreshListeners, pendingReplicationActions);

                    if (deleted && engine != null && isPrimaryMode()) {
                        // Translog Clean up
                        engine.translogManager().onDelete();
                    }

                    indexShardOperationPermits.close();
                }
            }
        }
    }

    /*
    ToDo : Fix this https://github.com/opensearch-project/OpenSearch/issues/8003
     */
    private RemoteSegmentStoreDirectory getRemoteDirectory() {
        assert indexSettings.isRemoteStoreEnabled();
        assert remoteStore.directory() instanceof FilterDirectory : "Store.directory is not an instance of FilterDirectory";
        FilterDirectory remoteStoreDirectory = (FilterDirectory) remoteStore.directory();
        FilterDirectory byteSizeCachingStoreDirectory = (FilterDirectory) remoteStoreDirectory.getDelegate();
        final Directory remoteDirectory = byteSizeCachingStoreDirectory.getDelegate();
        return ((RemoteSegmentStoreDirectory) remoteDirectory);
    }

    /**
    Returns true iff it is able to verify that remote segment store
    is in sync with local
     */
    boolean isRemoteSegmentStoreInSync() {
        assert indexSettings.isRemoteStoreEnabled();
        try {
            RemoteSegmentStoreDirectory directory = getRemoteDirectory();
            if (directory.readLatestMetadataFile() != null) {
                // verifying that all files except EXCLUDE_FILES are uploaded to the remote
                Collection<String> uploadFiles = directory.getSegmentsUploadedToRemoteStore().keySet();
                SegmentInfos segmentInfos = store.readLastCommittedSegmentsInfo();
                Collection<String> localFiles = segmentInfos.files(true);
                if (uploadFiles.containsAll(localFiles)) {
                    return true;
                }
            }
        } catch (IOException e) {
            logger.error("Exception while reading latest metadata", e);
        }
        return false;
    }

    public void preRecovery() {
        final IndexShardState currentState = this.state; // single volatile read
        if (currentState == IndexShardState.CLOSED) {
            throw new IndexShardNotRecoveringException(shardId, currentState);
        }
        assert currentState == IndexShardState.RECOVERING : "expected a recovering shard " + shardId + " but got " + currentState;
        indexEventListener.beforeIndexShardRecovery(this, indexSettings);
    }

    public void postRecovery(String reason) throws IndexShardStartedException, IndexShardRelocatedException, IndexShardClosedException {
        synchronized (postRecoveryMutex) {
            // we need to refresh again to expose all operations that were index until now. Otherwise
            // we may not expose operations that were indexed with a refresh listener that was immediately
            // responded to in addRefreshListener. The refresh must happen under the same mutex used in addRefreshListener
            // and before moving this shard to POST_RECOVERY state (i.e., allow to read from this shard).
            getEngine().refresh("post_recovery");
            synchronized (mutex) {
                if (state == IndexShardState.CLOSED) {
                    throw new IndexShardClosedException(shardId);
                }
                if (state == IndexShardState.STARTED) {
                    throw new IndexShardStartedException(shardId);
                }
                recoveryState.setStage(RecoveryState.Stage.DONE);
                changeState(IndexShardState.POST_RECOVERY, reason);
            }
        }
    }

    /**
     * called before starting to copy index files over
     */
    public void prepareForIndexRecovery() {
        if (state != IndexShardState.RECOVERING) {
            throw new IndexShardNotRecoveringException(shardId, state);
        }
        recoveryState.setStage(RecoveryState.Stage.INDEX);
        assert currentEngineReference.get() == null;
    }

    /**
     * A best effort to bring up this shard to the global checkpoint using the local translog before performing a peer recovery.
     *
     * @return a sequence number that an operation-based peer recovery can start with.
     * This is the first operation after the local checkpoint of the safe commit if exists.
     */
    private long recoverLocallyUpToGlobalCheckpoint() {
        validateLocalRecoveryState();
        final Optional<SequenceNumbers.CommitInfo> safeCommit;
        final long globalCheckpoint;
        try {
            final String translogUUID = store.readLastCommittedSegmentsInfo().getUserData().get(TRANSLOG_UUID_KEY);
            globalCheckpoint = Translog.readGlobalCheckpoint(translogConfig.getTranslogPath(), translogUUID);
            safeCommit = store.findSafeIndexCommit(globalCheckpoint);
        } catch (org.apache.lucene.index.IndexNotFoundException e) {
            logger.trace("skip local recovery as no index commit found");
            return UNASSIGNED_SEQ_NO;
        } catch (Exception e) {
            logger.debug("skip local recovery as failed to find the safe commit", e);
            return UNASSIGNED_SEQ_NO;
        }
        try {
            maybeCheckIndex(); // check index here and won't do it again if ops-based recovery occurs
            recoveryState.setStage(RecoveryState.Stage.TRANSLOG);
            if (safeCommit.isPresent() == false) {
                logger.trace("skip local recovery as no safe commit found");
                return UNASSIGNED_SEQ_NO;
            }
            assert safeCommit.get().localCheckpoint <= globalCheckpoint : safeCommit.get().localCheckpoint + " > " + globalCheckpoint;
            if (safeCommit.get().localCheckpoint == globalCheckpoint) {
                logger.trace(
                    "skip local recovery as the safe commit is up to date; safe commit {} global checkpoint {}",
                    safeCommit.get(),
                    globalCheckpoint
                );
                recoveryState.getTranslog().totalLocal(0);
                return globalCheckpoint + 1;
            }
            if (indexSettings.getIndexMetadata().getState() == IndexMetadata.State.CLOSE
                || IndexMetadata.INDEX_BLOCKS_WRITE_SETTING.get(indexSettings.getSettings())) {
                logger.trace(
                    "skip local recovery as the index was closed or not allowed to write; safe commit {} global checkpoint {}",
                    safeCommit.get(),
                    globalCheckpoint
                );
                recoveryState.getTranslog().totalLocal(0);
                return safeCommit.get().localCheckpoint + 1;
            }
            try {
                final TranslogRecoveryRunner translogRecoveryRunner = (snapshot) -> {
                    recoveryState.getTranslog().totalLocal(snapshot.totalOperations());
                    final int recoveredOps = runTranslogRecovery(
                        getEngine(),
                        snapshot,
                        Engine.Operation.Origin.LOCAL_TRANSLOG_RECOVERY,
                        recoveryState.getTranslog()::incrementRecoveredOperations
                    );
                    recoveryState.getTranslog().totalLocal(recoveredOps); // adjust the total local to reflect the actual count
                    return recoveredOps;
                };
                innerOpenEngineAndTranslog(() -> globalCheckpoint);
                getEngine().translogManager()
                    .recoverFromTranslog(translogRecoveryRunner, getEngine().getProcessedLocalCheckpoint(), globalCheckpoint);
                logger.trace("shard locally recovered up to {}", getEngine().getSeqNoStats(globalCheckpoint));
            } finally {
                synchronized (engineMutex) {
                    IOUtils.close(currentEngineReference.getAndSet(null));
                }
            }
        } catch (Exception e) {
            logger.debug(new ParameterizedMessage("failed to recover shard locally up to global checkpoint {}", globalCheckpoint), e);
            return UNASSIGNED_SEQ_NO;
        }
        try {
            // we need to find the safe commit again as we should have created a new one during the local recovery
            final Optional<SequenceNumbers.CommitInfo> newSafeCommit = store.findSafeIndexCommit(globalCheckpoint);
            assert newSafeCommit.isPresent() : "no safe commit found after local recovery";
            return newSafeCommit.get().localCheckpoint + 1;
        } catch (Exception e) {
            logger.debug(
                new ParameterizedMessage(
                    "failed to find the safe commit after recovering shard locally up to global checkpoint {}",
                    globalCheckpoint
                ),
                e
            );
            return UNASSIGNED_SEQ_NO;
        }
    }

    public long recoverLocallyAndFetchStartSeqNo(boolean localTranslog) {
        if (localTranslog) {
            return recoverLocallyUpToGlobalCheckpoint();
        } else {
            return recoverLocallyUptoLastCommit();
        }
    }

    /**
     * The method figures out the sequence number basis the last commit.
     *
     * @return the starting sequence number from which the recovery should start.
     */
    private long recoverLocallyUptoLastCommit() {
        assert isRemoteTranslogEnabled() : "Remote translog store is not enabled";
        long seqNo;
        validateLocalRecoveryState();

        try {
            seqNo = Long.parseLong(store.readLastCommittedSegmentsInfo().getUserData().get(MAX_SEQ_NO));
        } catch (org.apache.lucene.index.IndexNotFoundException e) {
            logger.error("skip local recovery as no index commit found");
            return UNASSIGNED_SEQ_NO;
        } catch (Exception e) {
            logger.error("skip local recovery as failed to find the safe commit", e);
            return UNASSIGNED_SEQ_NO;
        }

        try {
            maybeCheckIndex();
            recoveryState.setStage(RecoveryState.Stage.TRANSLOG);
            recoveryState.getTranslog().totalLocal(0);
        } catch (Exception e) {
            logger.error("check index failed during fetch seqNo", e);
            return UNASSIGNED_SEQ_NO;
        }
        return seqNo;
    }

    private void validateLocalRecoveryState() {
        assert Thread.holdsLock(mutex) == false : "recover locally under mutex";
        if (state != IndexShardState.RECOVERING) {
            throw new IndexShardNotRecoveringException(shardId, state);
        }
        recoveryState.validateCurrentStage(RecoveryState.Stage.INDEX);
        assert routingEntry().recoverySource().getType() == RecoverySource.Type.PEER : "not a peer recovery [" + routingEntry() + "]";
    }

    public void trimOperationOfPreviousPrimaryTerms(long aboveSeqNo) {
        getEngine().translogManager().trimOperationsFromTranslog(getOperationPrimaryTerm(), aboveSeqNo);
    }

    /**
     * Returns the maximum auto_id_timestamp of all append-only requests have been processed by this shard or the auto_id_timestamp received
     * from the primary via {@link #updateMaxUnsafeAutoIdTimestamp(long)} at the beginning of a peer-recovery or a primary-replica resync.
     *
     * @see #updateMaxUnsafeAutoIdTimestamp(long)
     */
    public long getMaxSeenAutoIdTimestamp() {
        return getEngine().getMaxSeenAutoIdTimestamp();
    }

    /**
     * Since operations stored in soft-deletes do not have max_auto_id_timestamp, the primary has to propagate its max_auto_id_timestamp
     * (via {@link #getMaxSeenAutoIdTimestamp()} of all processed append-only requests to replicas at the beginning of a peer-recovery
     * or a primary-replica resync to force a replica to disable optimization for all append-only requests which are replicated via
     * replication while its retry variants are replicated via recovery without auto_id_timestamp.
     * <p>
     * Without this force-update, a replica can generate duplicate documents (for the same id) if it first receives
     * a retry append-only (without timestamp) via recovery, then an original append-only (with timestamp) via replication.
     */
    public void updateMaxUnsafeAutoIdTimestamp(long maxSeenAutoIdTimestampFromPrimary) {
        getEngine().updateMaxUnsafeAutoIdTimestamp(maxSeenAutoIdTimestampFromPrimary);
    }

    public Engine.Result applyTranslogOperation(Translog.Operation operation, Engine.Operation.Origin origin) throws IOException {
        return applyTranslogOperation(getEngine(), operation, origin);
    }

    private Engine.Result applyTranslogOperation(Engine engine, Translog.Operation operation, Engine.Operation.Origin origin)
        throws IOException {
        // If a translog op is replayed on the primary (eg. ccr), we need to use external instead of null for its version type.
        final VersionType versionType = (origin == Engine.Operation.Origin.PRIMARY) ? VersionType.EXTERNAL : null;
        final Engine.Result result;
        switch (operation.opType()) {
            case INDEX:
                final Translog.Index index = (Translog.Index) operation;
                // we set canHaveDuplicates to true all the time such that we de-optimze the translog case and ensure that all
                // autoGeneratedID docs that are coming from the primary are updated correctly.
                result = applyIndexOperation(
                    engine,
                    index.seqNo(),
                    index.primaryTerm(),
                    index.version(),
                    versionType,
                    UNASSIGNED_SEQ_NO,
                    0,
                    index.getAutoGeneratedIdTimestamp(),
                    true,
                    origin,
                    new SourceToParse(
                        shardId.getIndexName(),
                        index.id(),
                        index.source(),
                        MediaTypeRegistry.xContentType(index.source()),
                        index.routing()
                    ),
                    index.id()
                );
                break;
            case DELETE:
                final Translog.Delete delete = (Translog.Delete) operation;
                result = applyDeleteOperation(
                    engine,
                    delete.seqNo(),
                    delete.primaryTerm(),
                    delete.version(),
                    delete.id(),
                    versionType,
                    UNASSIGNED_SEQ_NO,
                    0,
                    origin
                );
                break;
            case NO_OP:
                final Translog.NoOp noOp = (Translog.NoOp) operation;
                result = markSeqNoAsNoop(engine, noOp.seqNo(), noOp.primaryTerm(), noOp.reason(), origin);
                break;
            default:
                throw new IllegalStateException("No operation defined for [" + operation + "]");
        }
        return result;
    }

    /**
     * Replays translog operations from the provided translog {@code snapshot} to the current engine using the given {@code origin}.
     * The callback {@code onOperationRecovered} is notified after each translog operation is replayed successfully.
     */
    int runTranslogRecovery(Engine engine, Translog.Snapshot snapshot, Engine.Operation.Origin origin, Runnable onOperationRecovered)
        throws IOException {
        int opsRecovered = 0;
        Translog.Operation operation;
        while ((operation = snapshot.next()) != null) {
            try {
                logger.trace("[translog] recover op {}", operation);
                Engine.Result result = applyTranslogOperation(engine, operation, origin);
                switch (result.getResultType()) {
                    case FAILURE:
                        throw result.getFailure();
                    case MAPPING_UPDATE_REQUIRED:
                        throw new IllegalArgumentException("unexpected mapping update: " + result.getRequiredMappingUpdate());
                    case SUCCESS:
                        break;
                    default:
                        throw new AssertionError("Unknown result type [" + result.getResultType() + "]");
                }

                opsRecovered++;
                onOperationRecovered.run();
            } catch (Exception e) {
                // TODO: Don't enable this leniency unless users explicitly opt-in
                if (origin == Engine.Operation.Origin.LOCAL_TRANSLOG_RECOVERY && ExceptionsHelper.status(e) == RestStatus.BAD_REQUEST) {
                    // mainly for MapperParsingException and Failure to detect xcontent
                    logger.info("ignoring recovery of a corrupt translog entry", e);
                } else {
                    throw ExceptionsHelper.convertToRuntime(e);
                }
            }
        }
        return opsRecovered;
    }

    private void loadGlobalCheckpointToReplicationTracker() throws IOException {
        // we have to set it before we open an engine and recover from the translog because
        // acquiring a snapshot from the translog causes a sync which causes the global checkpoint to be pulled in,
        // and an engine can be forced to close in ctor which also causes the global checkpoint to be pulled in.
        final String translogUUID = store.readLastCommittedSegmentsInfo().getUserData().get(TRANSLOG_UUID_KEY);
        final long globalCheckpoint = Translog.readGlobalCheckpoint(translogConfig.getTranslogPath(), translogUUID);
        replicationTracker.updateGlobalCheckpointOnReplica(globalCheckpoint, "read from translog checkpoint");
    }

    /**
     * opens the engine on top of the existing lucene engine and translog.
     * Operations from the translog will be replayed to bring lucene up to date.
     **/
    public void openEngineAndRecoverFromTranslog() throws IOException {
        recoveryState.validateCurrentStage(RecoveryState.Stage.INDEX);
        maybeCheckIndex();
        recoveryState.setStage(RecoveryState.Stage.TRANSLOG);
        final RecoveryState.Translog translogRecoveryStats = recoveryState.getTranslog();
        final TranslogRecoveryRunner translogRecoveryRunner = (snapshot) -> {
            translogRecoveryStats.totalOperations(snapshot.totalOperations());
            translogRecoveryStats.totalOperationsOnStart(snapshot.totalOperations());
            return runTranslogRecovery(
                getEngine(),
                snapshot,
                Engine.Operation.Origin.LOCAL_TRANSLOG_RECOVERY,
                translogRecoveryStats::incrementRecoveredOperations
            );
        };

        // Do not load the global checkpoint if this is a remote snapshot index
        if (indexSettings.isRemoteSnapshot() == false && indexSettings.isRemoteTranslogStoreEnabled() == false) {
            loadGlobalCheckpointToReplicationTracker();
        }

        innerOpenEngineAndTranslog(replicationTracker);
        getEngine().translogManager()
            .recoverFromTranslog(translogRecoveryRunner, getEngine().getProcessedLocalCheckpoint(), Long.MAX_VALUE);
    }

    /**
     * Opens the engine on top of the existing lucene engine and translog.
     * The translog is kept but its operations won't be replayed.
     */
    public void openEngineAndSkipTranslogRecovery() throws IOException {
        assert routingEntry().recoverySource().getType() == RecoverySource.Type.PEER : "not a peer recovery [" + routingEntry() + "]";
        recoveryState.validateCurrentStage(RecoveryState.Stage.TRANSLOG);
        loadGlobalCheckpointToReplicationTracker();
        innerOpenEngineAndTranslog(replicationTracker);
        getEngine().translogManager().skipTranslogRecovery();
    }

    public void openEngineAndSkipTranslogRecoveryFromSnapshot() throws IOException {
        assert routingEntry().recoverySource().getType() == RecoverySource.Type.SNAPSHOT : "not a snapshot recovery ["
            + routingEntry()
            + "]";
        recoveryState.validateCurrentStage(RecoveryState.Stage.INDEX);
        maybeCheckIndex();
        recoveryState.setStage(RecoveryState.Stage.TRANSLOG);
        recoveryState.validateCurrentStage(RecoveryState.Stage.TRANSLOG);
        loadGlobalCheckpointToReplicationTracker();
        innerOpenEngineAndTranslog(replicationTracker, false);
        getEngine().translogManager().skipTranslogRecovery();
    }

    private void innerOpenEngineAndTranslog(LongSupplier globalCheckpointSupplier) throws IOException {
        innerOpenEngineAndTranslog(globalCheckpointSupplier, true);
    }

    private void innerOpenEngineAndTranslog(LongSupplier globalCheckpointSupplier, boolean syncFromRemote) throws IOException {
        syncFromRemote = syncFromRemote && indexSettings.isRemoteSnapshot() == false;
        assert Thread.holdsLock(mutex) == false : "opening engine under mutex";
        if (state != IndexShardState.RECOVERING) {
            throw new IndexShardNotRecoveringException(shardId, state);
        }
        final EngineConfig config = newEngineConfig(globalCheckpointSupplier);

        // we disable deletes since we allow for operations to be executed against the shard while recovering
        // but we need to make sure we don't loose deletes until we are done recovering
        config.setEnableGcDeletes(false);
        updateRetentionLeasesOnReplica(loadRetentionLeases());
        assert recoveryState.getRecoverySource().expectEmptyRetentionLeases() == false || getRetentionLeases().leases().isEmpty()
            : "expected empty set of retention leases with recovery source ["
                + recoveryState.getRecoverySource()
                + "] but got "
                + getRetentionLeases();
        synchronized (engineMutex) {
            assert currentEngineReference.get() == null : "engine is running";
            verifyNotClosed();
            if (indexSettings.isRemoteStoreEnabled()) {
                // Download missing segments from remote segment store.
                if (syncFromRemote) {
                    syncSegmentsFromRemoteSegmentStore(false);
                }
                if (shardRouting.primary()) {
                    if (syncFromRemote) {
                        syncRemoteTranslogAndUpdateGlobalCheckpoint();
                    } else {
                        // we will enter this block when we do not want to recover from remote translog.
                        // currently only during snapshot restore, we are coming into this block.
                        // here, as while initiliazing remote translog we cannot skip downloading translog files,
                        // so before that step, we are deleting the translog files present in remote store.
                        deleteTranslogFilesFromRemoteTranslog();
                    }
                } else if (syncFromRemote) {
                    // For replicas, when we download segments from remote segment store, we need to make sure that local
                    // translog is having the same UUID that is referred by the segments. If they are different, engine open
                    // fails with TranslogCorruptedException. It is safe to create empty translog for remote store enabled
                    // indices as replica would only need to read translog in failover scenario and we always fetch data
                    // from remote translog at the time of failover.
                    final SegmentInfos lastCommittedSegmentInfos = store().readLastCommittedSegmentsInfo();
                    final String translogUUID = lastCommittedSegmentInfos.userData.get(TRANSLOG_UUID_KEY);
                    final long checkpoint = Long.parseLong(lastCommittedSegmentInfos.userData.get(SequenceNumbers.LOCAL_CHECKPOINT_KEY));
                    Translog.createEmptyTranslog(
                        shardPath().resolveTranslog(),
                        shardId(),
                        checkpoint,
                        getPendingPrimaryTerm(),
                        translogUUID,
                        FileChannel::open
                    );
                }
            }
            // we must create a new engine under mutex (see IndexShard#snapshotStoreMetadata).
            final Engine newEngine = engineFactory.newReadWriteEngine(config);
            onNewEngine(newEngine);
            currentEngineReference.set(newEngine);

            if (indexSettings.isSegRepEnabled()) {
                // set initial replication checkpoints into tracker.
                updateReplicationCheckpoint();
            }
            // We set active because we are now writing operations to the engine; this way,
            // we can flush if we go idle after some time and become inactive.
            active.set(true);
        }
        // time elapses after the engine is created above (pulling the config settings) until we set the engine reference, during
        // which settings changes could possibly have happened, so here we forcefully push any config changes to the new engine.
        onSettingsChanged();
        assert assertSequenceNumbersInCommit();
        recoveryState.validateCurrentStage(RecoveryState.Stage.TRANSLOG);
    }

    private boolean assertSequenceNumbersInCommit() throws IOException {
        final Map<String, String> userData = fetchUserData();
        assert userData.containsKey(SequenceNumbers.LOCAL_CHECKPOINT_KEY) : "commit point doesn't contains a local checkpoint";
        assert userData.containsKey(MAX_SEQ_NO) : "commit point doesn't contains a maximum sequence number";
        assert userData.containsKey(Engine.HISTORY_UUID_KEY) : "commit point doesn't contains a history uuid";
        assert userData.get(Engine.HISTORY_UUID_KEY).equals(getHistoryUUID()) : "commit point history uuid ["
            + userData.get(Engine.HISTORY_UUID_KEY)
            + "] is different than engine ["
            + getHistoryUUID()
            + "]";
        assert userData.containsKey(Engine.MAX_UNSAFE_AUTO_ID_TIMESTAMP_COMMIT_ID) : "opening index which was created post 5.5.0 but "
            + Engine.MAX_UNSAFE_AUTO_ID_TIMESTAMP_COMMIT_ID
            + " is not found in commit";
        return true;
    }

    private Map<String, String> fetchUserData() throws IOException {
        if (indexSettings.isRemoteSnapshot() && indexSettings.getExtendedCompatibilitySnapshotVersion() != null) {
            return Lucene.readSegmentInfos(store.directory(), indexSettings.getExtendedCompatibilitySnapshotVersion()).getUserData();
        } else {
            return SegmentInfos.readLatestCommit(store.directory()).getUserData();
        }
    }

    private void onNewEngine(Engine newEngine) {
        assert Thread.holdsLock(engineMutex);
        refreshListeners.setCurrentRefreshLocationSupplier(newEngine.translogManager()::getTranslogLastWriteLocation);
    }

    /**
     * called if recovery has to be restarted after network error / delay **
     */
    public void performRecoveryRestart() throws IOException {
        assert Thread.holdsLock(mutex) == false : "restart recovery under mutex";
        synchronized (engineMutex) {
            assert refreshListeners.pendingCount() == 0 : "we can't restart with pending listeners";
            IOUtils.close(currentEngineReference.getAndSet(null));
            resetRecoveryStage();
        }
    }

    /**
     * If a file-based recovery occurs, a recovery target calls this method to reset the recovery stage.
     */
    public void resetRecoveryStage() {
        assert routingEntry().recoverySource().getType() == RecoverySource.Type.PEER : "not a peer recovery [" + routingEntry() + "]";
        assert currentEngineReference.get() == null;
        if (state != IndexShardState.RECOVERING) {
            throw new IndexShardNotRecoveringException(shardId, state);
        }
        recoveryState().setStage(RecoveryState.Stage.INIT);
    }

    /**
     * returns stats about ongoing recoveries, both source and target
     */
    public RecoveryStats recoveryStats() {
        return recoveryStats;
    }

    /**
     * Returns the current {@link RecoveryState} if this shard is recovering or has been recovering.
     * Returns null if the recovery has not yet started or shard was not recovered (created via an API).
     */
    @Override
    public RecoveryState recoveryState() {
        return this.recoveryState;
    }

    /**
     * perform the last stages of recovery once all translog operations are done.
     * note that you should still call {@link #postRecovery(String)}.
     */
    public void finalizeRecovery() {
        recoveryState().setStage(RecoveryState.Stage.FINALIZE);
        Engine engine = getEngine();
        engine.refresh("recovery_finalization");
        engine.config().setEnableGcDeletes(true);
    }

    /**
     * Returns {@code true} if this shard can ignore a recovery attempt made to it (since the already doing/done it)
     */
    public boolean ignoreRecoveryAttempt() {
        IndexShardState state = state(); // one time volatile read
        return state == IndexShardState.POST_RECOVERY
            || state == IndexShardState.RECOVERING
            || state == IndexShardState.STARTED
            || state == IndexShardState.CLOSED;
    }

    public void readAllowed() throws IllegalIndexShardStateException {
        IndexShardState state = this.state; // one time volatile read
        if (readAllowedStates.contains(state) == false) {
            throw new IllegalIndexShardStateException(
                shardId,
                state,
                "operations only allowed when shard state is one of " + readAllowedStates.toString()
            );
        }
    }

    /** returns true if the {@link IndexShardState} allows reading */
    public boolean isReadAllowed() {
        return readAllowedStates.contains(state);
    }

    private void ensureWriteAllowed(Engine.Operation.Origin origin) throws IllegalIndexShardStateException {
        IndexShardState state = this.state; // one time volatile read

        if (origin.isRecovery()) {
            if (state != IndexShardState.RECOVERING) {
                throw new IllegalIndexShardStateException(
                    shardId,
                    state,
                    "operation only allowed when recovering, origin [" + origin + "]"
                );
            }
        } else {
            if (origin == Engine.Operation.Origin.PRIMARY) {
                assert assertPrimaryMode();
            } else if (origin == Engine.Operation.Origin.REPLICA) {
                assert assertReplicationTarget();
            } else {
                assert origin == Engine.Operation.Origin.LOCAL_RESET;
                assert getActiveOperationsCount() == OPERATIONS_BLOCKED
                    : "locally resetting without blocking operations, active operations are [" + getActiveOperations() + "]";
            }
            if (writeAllowedStates.contains(state) == false) {
                throw new IllegalIndexShardStateException(
                    shardId,
                    state,
                    "operation only allowed when shard state is one of " + writeAllowedStates + ", origin [" + origin + "]"
                );
            }
        }
    }

    private boolean assertPrimaryMode() {
        assert shardRouting.primary() && replicationTracker.isPrimaryMode() : "shard "
            + shardRouting
            + " is not a primary shard in primary mode";
        return true;
    }

    // Returns true if shard routing is primary & replication tracker is in primary mode.
    public boolean isPrimaryMode() {
        return shardRouting.primary() && replicationTracker.isPrimaryMode();
    }

    private boolean assertReplicationTarget() {
        assert replicationTracker.isPrimaryMode() == false : "shard " + shardRouting + " in primary mode cannot be a replication target";
        return true;
    }

    private void verifyNotClosed() throws IllegalIndexShardStateException {
        verifyNotClosed(null);
    }

    private void verifyNotClosed(Exception suppressed) throws IllegalIndexShardStateException {
        IndexShardState state = this.state; // one time volatile read
        if (state == IndexShardState.CLOSED) {
            final IllegalIndexShardStateException exc = new IndexShardClosedException(shardId, "operation only allowed when not closed");
            if (suppressed != null) {
                exc.addSuppressed(suppressed);
            }
            throw exc;
        }
    }

    protected final void verifyActive() throws IllegalIndexShardStateException {
        IndexShardState state = this.state; // one time volatile read
        if (state != IndexShardState.STARTED) {
            throw new IllegalIndexShardStateException(shardId, state, "operation only allowed when shard is active");
        }
    }

    /**
     * Returns number of heap bytes used by the indexing buffer for this shard, or 0 if the shard is closed
     */
    public long getIndexBufferRAMBytesUsed() {
        Engine engine = getEngineOrNull();
        if (engine == null) {
            return 0;
        }
        try {
            return engine.getIndexBufferRAMBytesUsed();
        } catch (AlreadyClosedException ex) {
            return 0;
        }
    }

    public void addShardFailureCallback(Consumer<ShardFailure> onShardFailure) {
        this.shardEventListener.delegates.add(onShardFailure);
    }

    /**
     * Called by {@link IndexingMemoryController} to check whether more than {@code inactiveTimeNS} has passed since the last
     * indexing operation, so we can flush the index.
     */
    public void flushOnIdle(long inactiveTimeNS) {
        Engine engineOrNull = getEngineOrNull();
        if (engineOrNull != null && System.nanoTime() - engineOrNull.getLastWriteNanos() >= inactiveTimeNS) {
            boolean wasActive = active.getAndSet(false);
            if (wasActive) {
                logger.debug("flushing shard on inactive");
                threadPool.executor(ThreadPool.Names.FLUSH).execute(new AbstractRunnable() {
                    @Override
                    public void onFailure(Exception e) {
                        if (state != IndexShardState.CLOSED) {
                            logger.warn("failed to flush shard on inactive", e);
                        }
                    }

                    @Override
                    protected void doRun() {
                        flush(new FlushRequest().waitIfOngoing(false).force(false));
                        periodicFlushMetric.inc();
                    }
                });
            }
        }
    }

    public boolean isActive() {
        return active.get();
    }

    public ShardPath shardPath() {
        return path;
    }

    public void recoverFromLocalShards(
        Consumer<MappingMetadata> mappingUpdateConsumer,
        List<IndexShard> localShards,
        ActionListener<Boolean> listener
    ) throws IOException {
        assert shardRouting.primary() : "recover from local shards only makes sense if the shard is a primary shard";
        assert recoveryState.getRecoverySource().getType() == RecoverySource.Type.LOCAL_SHARDS : "invalid recovery type: "
            + recoveryState.getRecoverySource();
        final List<LocalShardSnapshot> snapshots = new ArrayList<>();
        final ActionListener<Boolean> recoveryListener = ActionListener.runBefore(listener, () -> IOUtils.close(snapshots));
        boolean success = false;
        try {
            for (IndexShard shard : localShards) {
                snapshots.add(new LocalShardSnapshot(shard));
            }
            // we are the first primary, recover from the gateway
            // if its post api allocation, the index should exists
            assert shardRouting.primary() : "recover from local shards only makes sense if the shard is a primary shard";
            StoreRecovery storeRecovery = new StoreRecovery(shardId, logger);
            storeRecovery.recoverFromLocalShards(mappingUpdateConsumer, this, snapshots, recoveryListener);
            success = true;
        } finally {
            if (success == false) {
                IOUtils.close(snapshots);
            }
        }
    }

    public void recoverFromStore(ActionListener<Boolean> listener) {
        // we are the first primary, recover from the gateway
        // if its post api allocation, the index should exists
        assert shardRouting.primary() : "recover from store only makes sense if the shard is a primary shard";
        assert shardRouting.initializing() : "can only start recovery on initializing shard";
        StoreRecovery storeRecovery = new StoreRecovery(shardId, logger);
        storeRecovery.recoverFromStore(this, listener);
    }

    public void restoreFromRemoteStore(ActionListener<Boolean> listener) {
        assert shardRouting.primary() : "recover from store only makes sense if the shard is a primary shard";
        StoreRecovery storeRecovery = new StoreRecovery(shardId, logger);
        storeRecovery.recoverFromRemoteStore(this, listener);
    }

    public void restoreFromSnapshotAndRemoteStore(
        Repository repository,
        RepositoriesService repositoriesService,
        ActionListener<Boolean> listener
    ) {
        try {
            assert shardRouting.primary() : "recover from store only makes sense if the shard is a primary shard";
            assert recoveryState.getRecoverySource().getType() == RecoverySource.Type.SNAPSHOT : "invalid recovery type: "
                + recoveryState.getRecoverySource();
            StoreRecovery storeRecovery = new StoreRecovery(shardId, logger);
            storeRecovery.recoverFromSnapshotAndRemoteStore(this, repository, repositoriesService, listener, threadPool);
        } catch (Exception e) {
            listener.onFailure(e);
        }
    }

    public void restoreFromRepository(Repository repository, ActionListener<Boolean> listener) {
        try {
            assert shardRouting.primary() : "recover from store only makes sense if the shard is a primary shard";
            assert recoveryState.getRecoverySource().getType() == RecoverySource.Type.SNAPSHOT : "invalid recovery type: "
                + recoveryState.getRecoverySource();
            StoreRecovery storeRecovery = new StoreRecovery(shardId, logger);
            storeRecovery.recoverFromRepository(this, repository, listener);
        } catch (Exception e) {
            listener.onFailure(e);
        }
    }

    /**
     * Tests whether or not the engine should be flushed periodically.
     * This test is based on the current size of the translog compared to the configured flush threshold size.
     *
     * @return {@code true} if the engine should be flushed
     */
    boolean shouldPeriodicallyFlush() {
        final Engine engine = getEngineOrNull();
        if (engine != null) {
            try {
                return engine.shouldPeriodicallyFlush();
            } catch (final AlreadyClosedException e) {
                // we are already closed, no need to flush or roll
            }
        }
        return false;
    }

    /**
     * Tests whether or not the translog generation should be rolled to a new generation. This test is based on the size of the current
     * generation compared to the configured generation threshold size.
     *
     * @return {@code true} if the current generation should be rolled to a new generation
     */
    boolean shouldRollTranslogGeneration() {
        final Engine engine = getEngineOrNull();
        if (engine != null) {
            try {
                return engine.translogManager().shouldRollTranslogGeneration();
            } catch (final AlreadyClosedException e) {
                // we are already closed, no need to flush or roll
            }
        }
        return false;
    }

    public void onSettingsChanged() {
        Engine engineOrNull = getEngineOrNull();
        if (engineOrNull != null) {
            final boolean disableTranslogRetention = indexSettings.isSoftDeleteEnabled() && useRetentionLeasesInPeerRecovery;
            engineOrNull.onSettingsChanged(
                disableTranslogRetention ? TimeValue.MINUS_ONE : indexSettings.getTranslogRetentionAge(),
                disableTranslogRetention ? new ByteSizeValue(-1) : indexSettings.getTranslogRetentionSize(),
                indexSettings.getSoftDeleteRetentionOperations()
            );
        }
    }

    private void turnOffTranslogRetention() {
        logger.debug(
            "turn off the translog retention for the replication group {} "
                + "as it starts using retention leases exclusively in peer recoveries",
            shardId
        );
        // Off to the generic threadPool as pruning the delete tombstones can be expensive.
        threadPool.generic().execute(new AbstractRunnable() {
            @Override
            public void onFailure(Exception e) {
                if (state != IndexShardState.CLOSED) {
                    logger.warn("failed to turn off translog retention", e);
                }
            }

            @Override
            protected void doRun() {
                onSettingsChanged();
                trimTranslog();
            }
        });
    }

    /**
     * Acquires a lock on the translog files and Lucene soft-deleted documents to prevent them from being trimmed
     */
    public Closeable acquireHistoryRetentionLock() {
        return getEngine().acquireHistoryRetentionLock();
    }

    /**
     * Creates a new history snapshot for reading operations since
     * the provided starting seqno (inclusive) and ending seqno (inclusive)
     * The returned snapshot can be retrieved from either Lucene index or translog files.
     */
    public Translog.Snapshot getHistoryOperations(String reason, long startingSeqNo, long endSeqNo, boolean accurateCount)
        throws IOException {
        return getEngine().newChangesSnapshot(reason, startingSeqNo, endSeqNo, true, accurateCount);
    }

    /**
     * Creates a new history snapshot from the translog instead of the lucene index. Required for cross cluster replication.
     * Use the recommended {@link #getHistoryOperations(String, long, long, boolean)} method for other cases.
     * This method should only be invoked if Segment Replication or Remote Store is not enabled.
     */
    public Translog.Snapshot getHistoryOperationsFromTranslog(long startingSeqNo, long endSeqNo) throws IOException {
        assert (indexSettings.isSegRepEnabled() || indexSettings.isRemoteStoreEnabled()) == false
            : "unsupported operation for segment replication enabled indices or remote store backed indices";
        return getEngine().translogManager().newChangesSnapshot(startingSeqNo, endSeqNo, true);
    }

    /**
     * Checks if we have a completed history of operations since the given starting seqno (inclusive).
     * This method should be called after acquiring the retention lock; See {@link #acquireHistoryRetentionLock()}
     */
    public boolean hasCompleteHistoryOperations(String reason, long startingSeqNo) {
        return getEngine().hasCompleteOperationHistory(reason, startingSeqNo);
    }

    /**
     * Gets the minimum retained sequence number for this engine.
     *
     * @return the minimum retained sequence number
     */
    public long getMinRetainedSeqNo() {
        return getEngine().getMinRetainedSeqNo();
    }

    /**
     * Counts the number of history operations within the provided sequence numbers
     * @param source     source of the requester (e.g., peer-recovery)
     * @param fromSeqNo  from sequence number, included
     * @param toSeqNo    to sequence number, included
     * @return           number of history operations in the sequence number range
     */
    public int countNumberOfHistoryOperations(String source, long fromSeqNo, long toSeqNo) throws IOException {
        return getEngine().countNumberOfHistoryOperations(source, fromSeqNo, toSeqNo);
    }

    /**
     * Creates a new changes snapshot for reading operations whose seq_no are between {@code fromSeqNo}(inclusive)
     * and {@code toSeqNo}(inclusive). The caller has to close the returned snapshot after finishing the reading.
     *
     * @param source            the source of the request
     * @param fromSeqNo         the from seq_no (inclusive) to read
     * @param toSeqNo           the to seq_no (inclusive) to read
     * @param requiredFullRange if {@code true} then {@link Translog.Snapshot#next()} will throw {@link IllegalStateException}
     *                          if any operation between {@code fromSeqNo} and {@code toSeqNo} is missing.
     *                          This parameter should be only enabled when the entire requesting range is below the global checkpoint.
     */
    public Translog.Snapshot newChangesSnapshot(
        String source,
        long fromSeqNo,
        long toSeqNo,
        boolean requiredFullRange,
        boolean accurateCount
    ) throws IOException {
        return getEngine().newChangesSnapshot(source, fromSeqNo, toSeqNo, requiredFullRange, accurateCount);
    }

    public List<Segment> segments(boolean verbose) {
        return getEngine().segments(verbose);
    }

    public String getHistoryUUID() {
        return getEngine().getHistoryUUID();
    }

    public IndexEventListener getIndexEventListener() {
        return indexEventListener;
    }

    public void activateThrottling() {
        try {
            getEngine().activateThrottling();
        } catch (AlreadyClosedException ex) {
            // ignore
        }
    }

    public void deactivateThrottling() {
        try {
            getEngine().deactivateThrottling();
        } catch (AlreadyClosedException ex) {
            // ignore
        }
    }

    private void handleRefreshException(Exception e) {
        if (e instanceof AlreadyClosedException) {
            // ignore
        } else if (e instanceof RefreshFailedEngineException) {
            RefreshFailedEngineException rfee = (RefreshFailedEngineException) e;
            if (rfee.getCause() instanceof InterruptedException) {
                // ignore, we are being shutdown
            } else if (rfee.getCause() instanceof ClosedByInterruptException) {
                // ignore, we are being shutdown
            } else if (rfee.getCause() instanceof ThreadInterruptedException) {
                // ignore, we are being shutdown
            } else {
                if (state != IndexShardState.CLOSED) {
                    logger.warn("Failed to perform engine refresh", e);
                }
            }
        } else {
            if (state != IndexShardState.CLOSED) {
                logger.warn("Failed to perform engine refresh", e);
            }
        }
    }

    /**
     * Called when our shard is using too much heap and should move buffered indexed/deleted documents to disk.
     */
    public void writeIndexingBuffer() {
        try {
            Engine engine = getEngine();
            engine.writeIndexingBuffer();
        } catch (Exception e) {
            handleRefreshException(e);
        }
    }

    /**
     * Notifies the service to update the local checkpoint for the shard with the provided allocation ID. See
     * {@link ReplicationTracker#updateLocalCheckpoint(String, long)} for
     * details.
     *
     * @param allocationId the allocation ID of the shard to update the local checkpoint for
     * @param checkpoint   the local checkpoint for the shard
     */
    public void updateLocalCheckpointForShard(final String allocationId, final long checkpoint) {
        assert assertPrimaryMode();
        verifyNotClosed();
        replicationTracker.updateLocalCheckpoint(allocationId, checkpoint);
    }

    /**
     * Update the local knowledge of the persisted global checkpoint for the specified allocation ID.
     *
     * @param allocationId     the allocation ID to update the global checkpoint for
     * @param globalCheckpoint the global checkpoint
     */
    public void updateGlobalCheckpointForShard(final String allocationId, final long globalCheckpoint) {
        assert assertPrimaryMode();
        verifyNotClosed();
        replicationTracker.updateGlobalCheckpointForShard(allocationId, globalCheckpoint);
    }

    /**
     * Update the local knowledge of the visible global checkpoint for the specified allocation ID.
     *
     * @param allocationId     the allocation ID to update the global checkpoint for
     * @param visibleCheckpoint the visible checkpoint
     */
    public void updateVisibleCheckpointForShard(final String allocationId, final ReplicationCheckpoint visibleCheckpoint) {
        // Update target replication checkpoint only when in active primary mode
        if (this.isPrimaryMode()) {
            verifyNotClosed();
            replicationTracker.updateVisibleCheckpointForShard(allocationId, visibleCheckpoint);
        }
    }

    /**
     * Fetch stats on segment replication.
     * @return {@link Tuple} V1 - TimeValue in ms - mean replication lag for this primary to its entire group,
     * V2 - Set of {@link SegmentReplicationShardStats} per shard in this primary's replication group.
     */
    public Set<SegmentReplicationShardStats> getReplicationStatsForTrackedReplicas() {
        return replicationTracker.getSegmentReplicationStats();
    }

    public ReplicationStats getReplicationStats() {
        if (indexSettings.isSegRepEnabled() && routingEntry().primary()) {
            final Set<SegmentReplicationShardStats> stats = getReplicationStatsForTrackedReplicas();
            long maxBytesBehind = stats.stream().mapToLong(SegmentReplicationShardStats::getBytesBehindCount).max().orElse(0L);
            long totalBytesBehind = stats.stream().mapToLong(SegmentReplicationShardStats::getBytesBehindCount).sum();
            long maxReplicationLag = stats.stream()
                .mapToLong(SegmentReplicationShardStats::getCurrentReplicationLagMillis)
                .max()
                .orElse(0L);
            return new ReplicationStats(maxBytesBehind, totalBytesBehind, maxReplicationLag);
        }
        return new ReplicationStats();
    }

    /**
     * Add a global checkpoint listener. If the global checkpoint is equal to or above the global checkpoint the listener is waiting for,
     * then the listener will be notified immediately via an executor (so possibly not on the current thread). If the specified timeout
     * elapses before the listener is notified, the listener will be notified with an {@link TimeoutException}. A caller may pass null to
     * specify no timeout.
     *
     * @param waitingForGlobalCheckpoint the global checkpoint the listener is waiting for
     * @param listener                   the listener
     * @param timeout                    the timeout
     */
    public void addGlobalCheckpointListener(
        final long waitingForGlobalCheckpoint,
        final GlobalCheckpointListeners.GlobalCheckpointListener listener,
        final TimeValue timeout
    ) {
        this.globalCheckpointListeners.add(waitingForGlobalCheckpoint, listener, timeout);
    }

    private void ensureSoftDeletesEnabled(String feature) {
        if (indexSettings.isSoftDeleteEnabled() == false) {
            String message = feature + " requires soft deletes but " + indexSettings.getIndex() + " does not have soft deletes enabled";
            assert false : message;
            throw new IllegalStateException(message);
        }
    }

    /**
     * Get all retention leases tracked on this shard.
     *
     * @return the retention leases
     */
    public RetentionLeases getRetentionLeases() {
        return getRetentionLeases(false).v2();
    }

    /**
     * If the expire leases parameter is false, gets all retention leases tracked on this shard and otherwise first calculates
     * expiration of existing retention leases, and then gets all non-expired retention leases tracked on this shard. Note that only the
     * primary shard calculates which leases are expired, and if any have expired, syncs the retention leases to any replicas. If the
     * expire leases parameter is true, this replication tracker must be in primary mode.
     *
     * @return a tuple indicating whether or not any retention leases were expired, and the non-expired retention leases
     */
    public Tuple<Boolean, RetentionLeases> getRetentionLeases(final boolean expireLeases) {
        assert expireLeases == false || assertPrimaryMode();
        verifyNotClosed();
        return replicationTracker.getRetentionLeases(expireLeases);
    }

    public RetentionLeaseStats getRetentionLeaseStats() {
        verifyNotClosed();
        return new RetentionLeaseStats(getRetentionLeases());
    }

    /**
     * Adds a new retention lease.
     *
     * @param id                      the identifier of the retention lease
     * @param retainingSequenceNumber the retaining sequence number
     * @param source                  the source of the retention lease
     * @param listener                the callback when the retention lease is successfully added and synced to replicas
     * @return the new retention lease
     * @throws IllegalArgumentException if the specified retention lease already exists
     */
    public RetentionLease addRetentionLease(
        final String id,
        final long retainingSequenceNumber,
        final String source,
        final ActionListener<ReplicationResponse> listener
    ) {
        Objects.requireNonNull(listener);
        assert assertPrimaryMode();
        verifyNotClosed();
        ensureSoftDeletesEnabled("retention leases");
        try (Closeable ignore = acquireHistoryRetentionLock()) {
            final long actualRetainingSequenceNumber = retainingSequenceNumber == RETAIN_ALL
                ? getMinRetainedSeqNo()
                : retainingSequenceNumber;
            return replicationTracker.addRetentionLease(id, actualRetainingSequenceNumber, source, listener);
        } catch (final IOException e) {
            throw new AssertionError(e);
        }
    }

    /**
     * Renews an existing retention lease.
     *
     * @param id                      the identifier of the retention lease
     * @param retainingSequenceNumber the retaining sequence number
     * @param source                  the source of the retention lease
     * @return the renewed retention lease
     * @throws IllegalArgumentException if the specified retention lease does not exist
     */
    public RetentionLease renewRetentionLease(final String id, final long retainingSequenceNumber, final String source) {
        assert assertPrimaryMode();
        verifyNotClosed();
        ensureSoftDeletesEnabled("retention leases");
        try (Closeable ignore = acquireHistoryRetentionLock()) {
            final long actualRetainingSequenceNumber = retainingSequenceNumber == RETAIN_ALL
                ? getMinRetainedSeqNo()
                : retainingSequenceNumber;
            return replicationTracker.renewRetentionLease(id, actualRetainingSequenceNumber, source);
        } catch (final IOException e) {
            throw new AssertionError(e);
        }
    }

    /**
     * Removes an existing retention lease.
     *
     * @param id       the identifier of the retention lease
     * @param listener the callback when the retention lease is successfully removed and synced to replicas
     */
    public void removeRetentionLease(final String id, final ActionListener<ReplicationResponse> listener) {
        Objects.requireNonNull(listener);
        assert assertPrimaryMode();
        verifyNotClosed();
        ensureSoftDeletesEnabled("retention leases");
        replicationTracker.removeRetentionLease(id, listener);
    }

    /**
     * Updates retention leases on a replica.
     *
     * @param retentionLeases the retention leases
     */
    public void updateRetentionLeasesOnReplica(final RetentionLeases retentionLeases) {
        assert assertReplicationTarget();
        verifyNotClosed();
        replicationTracker.updateRetentionLeasesOnReplica(retentionLeases);
    }

    /**
     * Loads the latest retention leases from their dedicated state file.
     *
     * @return the retention leases
     * @throws IOException if an I/O exception occurs reading the retention leases
     */
    public RetentionLeases loadRetentionLeases() throws IOException {
        verifyNotClosed();
        return replicationTracker.loadRetentionLeases(path.getShardStatePath());
    }

    /**
     * Persists the current retention leases to their dedicated state file.
     *
     * @throws WriteStateException if an exception occurs writing the state file
     */
    public void persistRetentionLeases() throws WriteStateException {
        verifyNotClosed();
        replicationTracker.persistRetentionLeases(path.getShardStatePath());
    }

    public boolean assertRetentionLeasesPersisted() throws IOException {
        return replicationTracker.assertRetentionLeasesPersisted(path.getShardStatePath());
    }

    /**
     * Syncs the current retention leases to all replicas.
     */
    public void syncRetentionLeases() {
        assert assertPrimaryMode();
        verifyNotClosed();
        replicationTracker.renewPeerRecoveryRetentionLeases();
        final Tuple<Boolean, RetentionLeases> retentionLeases = getRetentionLeases(true);
        if (retentionLeases.v1()) {
            logger.trace("syncing retention leases [{}] after expiration check", retentionLeases.v2());
            retentionLeaseSyncer.sync(
                shardId,
                shardRouting.allocationId().getId(),
                getPendingPrimaryTerm(),
                retentionLeases.v2(),
                ActionListener.wrap(
                    r -> {},
                    e -> logger.warn(
                        new ParameterizedMessage("failed to sync retention leases [{}] after expiration check", retentionLeases),
                        e
                    )
                )
            );
        } else {
            logger.trace("background syncing retention leases [{}] after expiration check", retentionLeases.v2());
            retentionLeaseSyncer.backgroundSync(
                shardId,
                shardRouting.allocationId().getId(),
                getPendingPrimaryTerm(),
                retentionLeases.v2()
            );
        }
    }

    /**
     * Called when the recovery process for a shard has opened the engine on the target shard. Ensures that the right data structures
     * have been set up locally to track local checkpoint information for the shard and that the shard is added to the replication group.
     *
     * @param allocationId  the allocation ID of the shard for which recovery was initiated
     */
    public void initiateTracking(final String allocationId) {
        assert assertPrimaryMode();
        replicationTracker.initiateTracking(allocationId);
    }

    /**
     * Marks the shard with the provided allocation ID as in-sync with the primary shard. See
     * {@link ReplicationTracker#markAllocationIdAsInSync(String, long)}
     * for additional details.
     *
     * @param allocationId    the allocation ID of the shard to mark as in-sync
     * @param localCheckpoint the current local checkpoint on the shard
     */
    public void markAllocationIdAsInSync(final String allocationId, final long localCheckpoint) throws InterruptedException {
        assert assertPrimaryMode();
        replicationTracker.markAllocationIdAsInSync(allocationId, localCheckpoint);
    }

    /**
     * Returns the persisted local checkpoint for the shard.
     *
     * @return the local checkpoint
     */
    public long getLocalCheckpoint() {
        return getEngine().getPersistedLocalCheckpoint();
    }

    /**
     * Fetch the latest checkpoint that has been processed but not necessarily persisted.
     * Also see {@link #getLocalCheckpoint()}.
     */
    public long getProcessedLocalCheckpoint() {
        return getEngine().getProcessedLocalCheckpoint();
    }

    /**
     * Returns the global checkpoint for the shard.
     *
     * @return the global checkpoint
     */
    public long getLastKnownGlobalCheckpoint() {
        return replicationTracker.getGlobalCheckpoint();
    }

    /**
     * Returns the latest global checkpoint value that has been persisted in the underlying storage (i.e. translog's checkpoint)
     */
    public long getLastSyncedGlobalCheckpoint() {
        return getEngine().getLastSyncedGlobalCheckpoint();
    }

    /**
     * Get the local knowledge of the global checkpoints for all in-sync allocation IDs.
     *
     * @return a map from allocation ID to the local knowledge of the global checkpoint for that allocation ID
     */
    public Map<String, Long> getInSyncGlobalCheckpoints() {
        assert assertPrimaryMode();
        verifyNotClosed();
        return replicationTracker.getInSyncGlobalCheckpoints();
    }

    /**
     * Syncs the global checkpoint to the replicas if the global checkpoint on at least one replica is behind the global checkpoint on the
     * primary.
     */
    public void maybeSyncGlobalCheckpoint(final String reason) {
        verifyNotClosed();
        assert shardRouting.primary() : "only call maybeSyncGlobalCheckpoint on primary shard";
        if (replicationTracker.isPrimaryMode() == false) {
            return;
        }
        assert assertPrimaryMode();
        // only sync if there are no operations in flight, or when using async durability
        final SeqNoStats stats = getEngine().getSeqNoStats(replicationTracker.getGlobalCheckpoint());
        final boolean asyncDurability = indexSettings().getTranslogDurability() == Durability.ASYNC;
        if (stats.getMaxSeqNo() == stats.getGlobalCheckpoint() || asyncDurability) {
            final Map<String, Long> globalCheckpoints = getInSyncGlobalCheckpoints();
            final long globalCheckpoint = replicationTracker.getGlobalCheckpoint();
            // async durability means that the local checkpoint might lag (as it is only advanced on fsync)
            // periodically ask for the newest local checkpoint by syncing the global checkpoint, so that ultimately the global
            // checkpoint can be synced. Also take into account that a shard might be pending sync, which means that it isn't
            // in the in-sync set just yet but might be blocked on waiting for its persisted local checkpoint to catch up to
            // the global checkpoint.
            final boolean syncNeeded = (asyncDurability
                && (stats.getGlobalCheckpoint() < stats.getMaxSeqNo() || replicationTracker.pendingInSync()))
                // check if the persisted global checkpoint
                || StreamSupport.stream(globalCheckpoints.values().spliterator(), false).anyMatch(v -> v < globalCheckpoint);
            // only sync if index is not closed and there is a shard lagging the primary
            if (syncNeeded && indexSettings.getIndexMetadata().getState() == IndexMetadata.State.OPEN) {
                logger.trace("syncing global checkpoint for [{}]", reason);
                globalCheckpointSyncer.run();
            }
        }
    }

    /**
     * Returns the current replication group for the shard.
     *
     * @return the replication group
     */
    public ReplicationGroup getReplicationGroup() {
        assert assertPrimaryMode();
        verifyNotClosed();
        ReplicationGroup replicationGroup = replicationTracker.getReplicationGroup();
        // PendingReplicationActions is dependent on ReplicationGroup. Every time we expose ReplicationGroup,
        // ensure PendingReplicationActions is updated with the newest version to prevent races.
        pendingReplicationActions.accept(replicationGroup);
        return replicationGroup;
    }

    /**
     * Returns the pending replication actions for the shard.
     *
     * @return the pending replication actions
     */
    public PendingReplicationActions getPendingReplicationActions() {
        assert assertPrimaryMode();
        verifyNotClosed();
        return pendingReplicationActions;
    }

    /**
     * Updates the global checkpoint on a replica shard after it has been updated by the primary.
     *
     * @param globalCheckpoint the global checkpoint
     * @param reason           the reason the global checkpoint was updated
     */
    public void updateGlobalCheckpointOnReplica(final long globalCheckpoint, final String reason) {
        assert assertReplicationTarget();
        final long localCheckpoint = getLocalCheckpoint();
        if (globalCheckpoint > localCheckpoint) {
            /*
             * This can happen during recovery when the shard has started its engine but recovery is not finalized and is receiving global
             * checkpoint updates. However, since this shard is not yet contributing to calculating the global checkpoint, it can be the
             * case that the global checkpoint update from the primary is ahead of the local checkpoint on this shard. In this case, we
             * ignore the global checkpoint update. This can happen if we are in the translog stage of recovery. Prior to this, the engine
             * is not opened and this shard will not receive global checkpoint updates, and after this the shard will be contributing to
             * calculations of the global checkpoint. However, we can not assert that we are in the translog stage of recovery here as
             * while the global checkpoint update may have emanated from the primary when we were in that state, we could subsequently move
             * to recovery finalization, or even finished recovery before the update arrives here.
             * When remote translog is enabled for an index, replication operation is limited to primary term validation and does not
             * update local checkpoint at replica, so the local checkpoint at replica can be less than globalCheckpoint.
             */
            assert (state() != IndexShardState.POST_RECOVERY && state() != IndexShardState.STARTED)
                || indexSettings.isRemoteTranslogStoreEnabled() : "supposedly in-sync shard copy received a global checkpoint ["
                    + globalCheckpoint
                    + "] "
                    + "that is higher than its local checkpoint ["
                    + localCheckpoint
                    + "]";
            return;
        }
        replicationTracker.updateGlobalCheckpointOnReplica(globalCheckpoint, reason);
    }

    /**
     * Updates the known allocation IDs and the local checkpoints for the corresponding allocations from a primary relocation source.
     *
     * @param primaryContext the sequence number context
     */
    public void activateWithPrimaryContext(final ReplicationTracker.PrimaryContext primaryContext) {
        assert shardRouting.primary() && shardRouting.isRelocationTarget()
            : "only primary relocation target can update allocation IDs from primary context: " + shardRouting;
        assert primaryContext.getCheckpointStates().containsKey(routingEntry().allocationId().getId()) : "primary context ["
            + primaryContext
            + "] does not contain relocation target ["
            + routingEntry()
            + "]";
        assert getLocalCheckpoint() == primaryContext.getCheckpointStates().get(routingEntry().allocationId().getId()).getLocalCheckpoint()
            || indexSettings().getTranslogDurability() == Durability.ASYNC : "local checkpoint ["
                + getLocalCheckpoint()
                + "] does not match checkpoint from primary context ["
                + primaryContext
                + "]";
        synchronized (mutex) {
            replicationTracker.activateWithPrimaryContext(primaryContext); // make changes to primaryMode flag only under mutex
        }
        postActivatePrimaryMode();
    }

    private void postActivatePrimaryMode() {
        if (indexSettings.isRemoteStoreEnabled()) {
            // We make sure to upload translog (even if it does not contain any operations) to remote translog.
            // This helps to get a consistent state in remote store where both remote segment store and remote
            // translog contains data.
            try {
                getEngine().translogManager().syncTranslog();
            } catch (IOException e) {
                logger.error("Failed to sync translog to remote from new primary", e);
            }
        }
        ensurePeerRecoveryRetentionLeasesExist();
    }

    private void ensurePeerRecoveryRetentionLeasesExist() {
        threadPool.generic()
            .execute(
                () -> replicationTracker.createMissingPeerRecoveryRetentionLeases(
                    ActionListener.wrap(
                        r -> logger.trace("created missing peer recovery retention leases"),
                        e -> logger.debug("failed creating missing peer recovery retention leases", e)
                    )
                )
            );
    }

    /**
     * Check if there are any recoveries pending in-sync.
     *
     * @return {@code true} if there is at least one shard pending in-sync, otherwise false
     */
    public boolean pendingInSync() {
        assert assertPrimaryMode();
        return replicationTracker.pendingInSync();
    }

    /**
     * Should be called for each no-op update operation to increment relevant statistics.
     */
    public void noopUpdate() {
        internalIndexingStats.noopUpdate();
    }

    public void maybeCheckIndex() {
        recoveryState.setStage(RecoveryState.Stage.VERIFY_INDEX);
        if (Booleans.isTrue(checkIndexOnStartup) || "checksum".equals(checkIndexOnStartup)) {
            try {
                checkIndex();
            } catch (IOException ex) {
                throw new RecoveryFailedException(recoveryState, "check index failed", ex);
            }
        }
    }

    void checkIndex() throws IOException {
        if (store.tryIncRef()) {
            try {
                doCheckIndex();
            } catch (IOException e) {
                store.markStoreCorrupted(e);
                throw e;
            } finally {
                store.decRef();
            }
        }
    }

    private void doCheckIndex() throws IOException {
        long timeNS = System.nanoTime();
        if (!Lucene.indexExists(store.directory())) {
            return;
        }

        try (BytesStreamOutput os = new BytesStreamOutput(); PrintStream out = new PrintStream(os, false, StandardCharsets.UTF_8.name())) {
            if ("checksum".equals(checkIndexOnStartup)) {
                // physical verification only: verify all checksums for the latest commit
                IOException corrupt = null;
                MetadataSnapshot metadata = snapshotStoreMetadata();
                for (Map.Entry<String, StoreFileMetadata> entry : metadata.asMap().entrySet()) {
                    try {
                        Store.checkIntegrity(entry.getValue(), store.directory());
                        out.println("checksum passed: " + entry.getKey());
                    } catch (IOException exc) {
                        out.println("checksum failed: " + entry.getKey());
                        exc.printStackTrace(out);
                        corrupt = exc;
                    }
                }
                out.flush();
                if (corrupt != null) {
                    logger.warn("check index [failure]\n{}", os.bytes().utf8ToString());
                    throw corrupt;
                }
            } else {
                // full checkindex
                final CheckIndex.Status status = store.checkIndex(out);
                out.flush();
                if (!status.clean) {
                    if (state == IndexShardState.CLOSED) {
                        // ignore if closed....
                        return;
                    }
                    logger.warn("check index [failure]\n{}", os.bytes().utf8ToString());
                    throw new IOException("index check failure");
                }
            }

            if (logger.isDebugEnabled()) {
                logger.debug("check index [success]\n{}", os.bytes().utf8ToString());
            }
        }

        recoveryState.getVerifyIndex().checkIndexTime(Math.max(0, TimeValue.nsecToMSec(System.nanoTime() - timeNS)));
    }

    Engine getEngine() {
        Engine engine = getEngineOrNull();
        if (engine == null) {
            throw new AlreadyClosedException("engine is closed");
        }
        return engine;
    }

    /**
     * NOTE: returns null if engine is not yet started (e.g. recovery phase 1, copying over index files, is still running), or if engine is
     * closed.
     */
    protected Engine getEngineOrNull() {
        return this.currentEngineReference.get();
    }

    public void startRecovery(
        RecoveryState recoveryState,
        PeerRecoveryTargetService recoveryTargetService,
        RecoveryListener recoveryListener,
        RepositoriesService repositoriesService,
        Consumer<MappingMetadata> mappingUpdateConsumer,
        IndicesService indicesService
    ) {
        // TODO: Create a proper object to encapsulate the recovery context
        // all of the current methods here follow a pattern of:
        // resolve context which isn't really dependent on the local shards and then async
        // call some external method with this pointer.
        // with a proper recovery context object we can simply change this to:
        // startRecovery(RecoveryState recoveryState, ShardRecoverySource source ) {
        // markAsRecovery("from " + source.getShortDescription(), recoveryState);
        // threadPool.generic().execute() {
        // onFailure () { listener.failure() };
        // doRun() {
        // if (source.recover(this)) {
        // recoveryListener.onRecoveryDone(recoveryState);
        // }
        // }
        // }}
        // }
        logger.debug("startRecovery type={}", recoveryState.getRecoverySource().getType());
        assert recoveryState.getRecoverySource().equals(shardRouting.recoverySource());
        switch (recoveryState.getRecoverySource().getType()) {
            case EMPTY_STORE:
            case EXISTING_STORE:
                executeRecovery("from store", recoveryState, recoveryListener, this::recoverFromStore);
                break;
            case REMOTE_STORE:
                executeRecovery("from remote store", recoveryState, recoveryListener, l -> restoreFromRemoteStore(l));
                break;
            case PEER:
                try {
                    markAsRecovering("from " + recoveryState.getSourceNode(), recoveryState);
                    recoveryTargetService.startRecovery(this, recoveryState.getSourceNode(), recoveryListener);
                } catch (Exception e) {
                    failShard("corrupted preexisting index", e);
                    recoveryListener.onFailure(recoveryState, new RecoveryFailedException(recoveryState, null, e), true);
                }
                break;
            case SNAPSHOT:
                final SnapshotRecoverySource recoverySource = (SnapshotRecoverySource) recoveryState.getRecoverySource();
                if (recoverySource.isSearchableSnapshot()) {
                    executeRecovery("from snapshot (remote)", recoveryState, recoveryListener, this::recoverFromStore);
                } else if (recoverySource.remoteStoreIndexShallowCopy()) {
                    final String repo = recoverySource.snapshot().getRepository();
                    executeRecovery(
                        "from snapshot and remote store",
                        recoveryState,
                        recoveryListener,
                        l -> restoreFromSnapshotAndRemoteStore(repositoriesService.repository(repo), repositoriesService, l)
                    );
                    // indicesService.indexService(shardRouting.shardId().getIndex()).addMetadataListener();
                } else {
                    final String repo = recoverySource.snapshot().getRepository();
                    executeRecovery(
                        "from snapshot",
                        recoveryState,
                        recoveryListener,
                        l -> restoreFromRepository(repositoriesService.repository(repo), l)
                    );
                }
                break;
            case LOCAL_SHARDS:
                final IndexMetadata indexMetadata = indexSettings().getIndexMetadata();
                final Index resizeSourceIndex = indexMetadata.getResizeSourceIndex();
                final List<IndexShard> startedShards = new ArrayList<>();
                final IndexService sourceIndexService = indicesService.indexService(resizeSourceIndex);
                final Set<ShardId> requiredShards;
                final int numShards;
                if (sourceIndexService != null) {
                    requiredShards = IndexMetadata.selectRecoverFromShards(
                        shardId().id(),
                        sourceIndexService.getMetadata(),
                        indexMetadata.getNumberOfShards()
                    );
                    for (IndexShard shard : sourceIndexService) {
                        if (shard.state() == IndexShardState.STARTED && requiredShards.contains(shard.shardId())) {
                            startedShards.add(shard);
                        }
                    }
                    numShards = requiredShards.size();
                } else {
                    numShards = -1;
                    requiredShards = Collections.emptySet();
                }

                if (numShards == startedShards.size()) {
                    assert requiredShards.isEmpty() == false;
                    executeRecovery(
                        "from local shards",
                        recoveryState,
                        recoveryListener,
                        l -> recoverFromLocalShards(
                            mappingUpdateConsumer,
                            startedShards.stream().filter((s) -> requiredShards.contains(s.shardId())).collect(Collectors.toList()),
                            l
                        )
                    );
                } else {
                    final RuntimeException e;
                    if (numShards == -1) {
                        e = new IndexNotFoundException(resizeSourceIndex);
                    } else {
                        e = new IllegalStateException(
                            "not all required shards of index "
                                + resizeSourceIndex
                                + " are started yet, expected "
                                + numShards
                                + " found "
                                + startedShards.size()
                                + " can't recover shard "
                                + shardId()
                        );
                    }
                    throw e;
                }
                break;
            default:
                throw new IllegalArgumentException("Unknown recovery source " + recoveryState.getRecoverySource());
        }
    }

    private void executeRecovery(
        String reason,
        RecoveryState recoveryState,
        RecoveryListener recoveryListener,
        CheckedConsumer<ActionListener<Boolean>, Exception> action
    ) {
        markAsRecovering(reason, recoveryState); // mark the shard as recovering on the cluster state thread
        threadPool.generic().execute(ActionRunnable.wrap(ActionListener.wrap(r -> {
            if (r) {
                recoveryListener.onDone(recoveryState);
            }
        }, e -> recoveryListener.onFailure(recoveryState, new RecoveryFailedException(recoveryState, null, e), true)), action));
    }

    /**
     * Returns whether the shard is a relocated primary, i.e. not in charge anymore of replicating changes (see {@link ReplicationTracker}).
     */
    public boolean isRelocatedPrimary() {
        assert shardRouting.primary() : "only call isRelocatedPrimary on primary shard";
        return replicationTracker.isRelocated();
    }

    public RetentionLease addPeerRecoveryRetentionLease(
        String nodeId,
        long globalCheckpoint,
        ActionListener<ReplicationResponse> listener
    ) {
        assert assertPrimaryMode();
        // only needed for BWC reasons involving rolling upgrades from versions that do not support PRRLs:
        assert indexSettings.isSoftDeleteEnabled() == false;
        return replicationTracker.addPeerRecoveryRetentionLease(nodeId, globalCheckpoint, listener);
    }

    public RetentionLease cloneLocalPeerRecoveryRetentionLease(String nodeId, ActionListener<ReplicationResponse> listener) {
        assert assertPrimaryMode();
        return replicationTracker.cloneLocalPeerRecoveryRetentionLease(nodeId, listener);
    }

    public void removePeerRecoveryRetentionLease(String nodeId, ActionListener<ReplicationResponse> listener) {
        assert assertPrimaryMode();
        replicationTracker.removePeerRecoveryRetentionLease(nodeId, listener);
    }

    /**
     * Returns a list of retention leases for peer recovery installed in this shard copy.
     */
    public List<RetentionLease> getPeerRecoveryRetentionLeases() {
        return replicationTracker.getPeerRecoveryRetentionLeases();
    }

    public boolean useRetentionLeasesInPeerRecovery() {
        return useRetentionLeasesInPeerRecovery;
    }

    private SafeCommitInfo getSafeCommitInfo() {
        final Engine engine = getEngineOrNull();
        return engine == null ? SafeCommitInfo.EMPTY : engine.getSafeCommitInfo();
    }

    class ShardEventListener implements Engine.EventListener {
        private final CopyOnWriteArrayList<Consumer<ShardFailure>> delegates = new CopyOnWriteArrayList<>();

        // called by the current engine
        @Override
        public void onFailedEngine(String reason, @Nullable Exception failure) {
            final ShardFailure shardFailure = new ShardFailure(shardRouting, reason, failure);
            for (Consumer<ShardFailure> listener : delegates) {
                try {
                    listener.accept(shardFailure);
                } catch (Exception inner) {
                    inner.addSuppressed(failure);
                    logger.warn("exception while notifying engine failure", inner);
                }
            }
        }
    }

    private static void persistMetadata(
        final ShardPath shardPath,
        final IndexSettings indexSettings,
        final ShardRouting newRouting,
        final @Nullable ShardRouting currentRouting,
        final Logger logger
    ) throws IOException {
        assert newRouting != null : "newRouting must not be null";

        // only persist metadata if routing information that is persisted in shard state metadata actually changed
        final ShardId shardId = newRouting.shardId();
        if (currentRouting == null
            || currentRouting.primary() != newRouting.primary()
            || currentRouting.allocationId().equals(newRouting.allocationId()) == false) {
            assert currentRouting == null || currentRouting.isSameAllocation(newRouting);
            final String writeReason;
            if (currentRouting == null) {
                writeReason = "initial state with allocation id [" + newRouting.allocationId() + "]";
            } else {
                writeReason = "routing changed from " + currentRouting + " to " + newRouting;
            }
            logger.trace("{} writing shard state, reason [{}]", shardId, writeReason);

            final ShardStateMetadata.IndexDataLocation indexDataLocation = IndexSettings.SEARCHABLE_SNAPSHOT_REPOSITORY.exists(
                indexSettings.getSettings()
            ) ? ShardStateMetadata.IndexDataLocation.REMOTE : ShardStateMetadata.IndexDataLocation.LOCAL;
            final ShardStateMetadata newShardStateMetadata = new ShardStateMetadata(
                newRouting.primary(),
                indexSettings.getUUID(),
                newRouting.allocationId(),
                indexDataLocation
            );
            ShardStateMetadata.FORMAT.writeAndCleanup(newShardStateMetadata, shardPath.getShardStatePath());
        } else {
            logger.trace("{} skip writing shard state, has been written before", shardId);
        }
    }

    private DocumentMapperForType docMapper() {
        return mapperService.documentMapperWithAutoCreate();
    }

    private EngineConfig newEngineConfig(LongSupplier globalCheckpointSupplier) throws IOException {
        final Sort indexSort = indexSortSupplier.get();
        final Engine.Warmer warmer = reader -> {
            assert Thread.holdsLock(mutex) == false : "warming engine under mutex";
            assert reader != null;
            if (this.warmer != null) {
                this.warmer.warm(reader);
            }
        };

        internalRefreshListener.clear();
        internalRefreshListener.add(new RefreshMetricUpdater(refreshMetric));
        if (indexSettings.isSegRepEnabled()) {
            internalRefreshListener.add(new ReplicationCheckpointUpdater());
        }
        if (this.checkpointPublisher != null && shardRouting.primary() && indexSettings.isSegRepLocalEnabled()) {
            internalRefreshListener.add(new CheckpointRefreshListener(this, this.checkpointPublisher));
        }

        if (isRemoteStoreEnabled()) {
            internalRefreshListener.add(
                new RemoteStoreRefreshListener(
                    this,
                    this.checkpointPublisher,
                    remoteStoreStatsTrackerFactory.getRemoteSegmentTransferTracker(shardId())
                )
            );
        }

        /*
          With segment replication enabled for primary relocation, recover replica shard initially as read only and
          change to a writeable engine during relocation handoff after a round of segment replication.
         */
        boolean isReadOnlyReplica = indexSettings.isSegRepEnabled()
            && (shardRouting.primary() == false
                || (shardRouting.isRelocationTarget() && recoveryState.getStage() != RecoveryState.Stage.FINALIZE));

        return this.engineConfigFactory.newEngineConfig(
            shardId,
            threadPool,
            indexSettings,
            warmer,
            store,
            indexSettings.getMergePolicy(isTimeSeriesIndex),
            mapperService != null ? mapperService.indexAnalyzer() : null,
            similarityService.similarity(mapperService),
            engineConfigFactory.newCodecServiceOrDefault(indexSettings, mapperService, logger, codecService),
            shardEventListener,
            indexCache != null ? indexCache.query() : null,
            cachingPolicy,
            translogConfig,
            IndexingMemoryController.SHARD_INACTIVE_TIME_SETTING.get(indexSettings.getSettings()),
            Arrays.asList(refreshListeners, refreshPendingLocationListener),
            internalRefreshListener,
            indexSort,
            circuitBreakerService,
            globalCheckpointSupplier,
            replicationTracker::getRetentionLeases,
            this::getOperationPrimaryTerm,
            tombstoneDocSupplier(),
            isReadOnlyReplica,
            this::isStartedPrimary,
            translogFactorySupplier.apply(indexSettings, shardRouting),
            isTimeSeriesDescSortOptimizationEnabled() ? DataStream.TIMESERIES_LEAF_SORTER : null // DESC @timestamp default order for
            // timeseries
        );
    }

    private boolean isRemoteStoreEnabled() {
        return (remoteStore != null && shardRouting.primary());
    }

    public boolean isRemoteTranslogEnabled() {
        return indexSettings() != null && indexSettings().isRemoteTranslogStoreEnabled();
    }

    /**
     * This checks if we are in state to upload to remote store. Until the cluster-manager informs the shard through
     * cluster state, the shard will not be in STARTED state. This method is used to prevent pre-emptive segment or
     * translog uploads.
     */
    public boolean isStartedPrimary() {
        return getReplicationTracker().isPrimaryMode() && state() == IndexShardState.STARTED;
    }

    /**
     * @return true if segment reverse search optimization is enabled for time series based workload.
     */
    public boolean isTimeSeriesDescSortOptimizationEnabled() {
        // Do not change segment order in case of index sort.
        return isTimeSeriesIndex && getIndexSort() == null;
    }

    /**
     * @return True if settings indicate this shard is backed by a remote snapshot, false otherwise.
     */
    public boolean isRemoteSnapshot() {
        return indexSettings != null && indexSettings.isRemoteSnapshot();
    }

    /**
     * Acquire a primary operation permit whenever the shard is ready for indexing. If a permit is directly available, the provided
     * ActionListener will be called on the calling thread. During relocation hand-off, permit acquisition can be delayed. The provided
     * ActionListener will then be called using the provided executor.
     *
     * @param debugInfo an extra information that can be useful when tracing an unreleased permit. When assertions are enabled
     *                  the tracing will capture the supplied object's {@link Object#toString()} value. Otherwise the object
     *                  isn't used
     */
    public void acquirePrimaryOperationPermit(ActionListener<Releasable> onPermitAcquired, String executorOnDelay, Object debugInfo) {
        acquirePrimaryOperationPermit(onPermitAcquired, executorOnDelay, debugInfo, false);
    }

    public void acquirePrimaryOperationPermit(
        ActionListener<Releasable> onPermitAcquired,
        String executorOnDelay,
        Object debugInfo,
        boolean forceExecution
    ) {
        verifyNotClosed();
        assert shardRouting.primary() : "acquirePrimaryOperationPermit should only be called on primary shard: " + shardRouting;

        indexShardOperationPermits.acquire(
            wrapPrimaryOperationPermitListener(onPermitAcquired),
            executorOnDelay,
            forceExecution,
            debugInfo
        );
    }

    /**
     * Acquire all primary operation permits. Once all permits are acquired, the provided ActionListener is called.
     * It is the responsibility of the caller to close the {@link Releasable}.
     */
    public void acquireAllPrimaryOperationsPermits(final ActionListener<Releasable> onPermitAcquired, final TimeValue timeout) {
        verifyNotClosed();
        assert shardRouting.primary() : "acquireAllPrimaryOperationsPermits should only be called on primary shard: " + shardRouting;

        asyncBlockOperations(wrapPrimaryOperationPermitListener(onPermitAcquired), timeout.duration(), timeout.timeUnit());
    }

    /**
     * Wraps the action to run on a primary after acquiring permit. This wrapping is used to check if the shard is in primary mode before
     * executing the action.
     *
     * @param listener the listener to wrap
     * @return the wrapped listener
     */
    private ActionListener<Releasable> wrapPrimaryOperationPermitListener(final ActionListener<Releasable> listener) {
        return ActionListener.delegateFailure(listener, (l, r) -> {
            if (replicationTracker.isPrimaryMode()) {
                l.onResponse(r);
            } else {
                r.close();
                l.onFailure(new ShardNotInPrimaryModeException(shardId, state));
            }
        });
    }

    private void asyncBlockOperations(ActionListener<Releasable> onPermitAcquired, long timeout, TimeUnit timeUnit) {
        final Releasable forceRefreshes = refreshListeners.forceRefreshes();
        final ActionListener<Releasable> wrappedListener = ActionListener.wrap(r -> {
            forceRefreshes.close();
            onPermitAcquired.onResponse(r);
        }, e -> {
            forceRefreshes.close();
            onPermitAcquired.onFailure(e);
        });
        try {
            indexShardOperationPermits.asyncBlockOperations(wrappedListener, timeout, timeUnit);
        } catch (Exception e) {
            forceRefreshes.close();
            throw e;
        }
    }

    /**
     * Runs the specified runnable under a permit and otherwise calling back the specified failure callback. This method is really a
     * convenience for {@link #acquirePrimaryOperationPermit(ActionListener, String, Object)} where the listener equates to
     * try-with-resources closing the releasable after executing the runnable on successfully acquiring the permit, an otherwise calling
     * back the failure callback.
     *
     * @param runnable the runnable to execute under permit
     * @param onFailure the callback on failure
     * @param executorOnDelay the executor to execute the runnable on if permit acquisition is blocked
     * @param debugInfo debug info
     */
    public void runUnderPrimaryPermit(
        final Runnable runnable,
        final Consumer<Exception> onFailure,
        final String executorOnDelay,
        final Object debugInfo
    ) {
        verifyNotClosed();
        assert shardRouting.primary() : "runUnderPrimaryPermit should only be called on primary shard but was " + shardRouting;
        final ActionListener<Releasable> onPermitAcquired = ActionListener.wrap(releasable -> {
            try (Releasable ignore = releasable) {
                runnable.run();
            }
        }, onFailure);
        acquirePrimaryOperationPermit(onPermitAcquired, executorOnDelay, debugInfo);
    }

    private <E extends Exception> void bumpPrimaryTerm(
        final long newPrimaryTerm,
        final CheckedRunnable<E> onBlocked,
        @Nullable ActionListener<Releasable> combineWithAction
    ) {
        assert Thread.holdsLock(mutex);
        assert newPrimaryTerm > pendingPrimaryTerm || (newPrimaryTerm >= pendingPrimaryTerm && combineWithAction != null);
        assert getOperationPrimaryTerm() <= pendingPrimaryTerm;
        final CountDownLatch termUpdated = new CountDownLatch(1);
        asyncBlockOperations(new ActionListener<Releasable>() {
            @Override
            public void onFailure(final Exception e) {
                try {
                    innerFail(e);
                } finally {
                    if (combineWithAction != null) {
                        combineWithAction.onFailure(e);
                    }
                }
            }

            private void innerFail(final Exception e) {
                try {
                    failShard("exception during primary term transition", e);
                } catch (AlreadyClosedException ace) {
                    // ignore, shard is already closed
                }
            }

            @Override
            public void onResponse(final Releasable releasable) {
                final RunOnce releaseOnce = new RunOnce(releasable::close);
                try {
                    assert getOperationPrimaryTerm() <= pendingPrimaryTerm;
                    termUpdated.await();
                    // indexShardOperationPermits doesn't guarantee that async submissions are executed
                    // in the order submitted. We need to guard against another term bump
                    if (getOperationPrimaryTerm() < newPrimaryTerm) {
                        replicationTracker.setOperationPrimaryTerm(newPrimaryTerm);
                        onBlocked.run();
                    }
                } catch (final Exception e) {
                    if (combineWithAction == null) {
                        // otherwise leave it to combineWithAction to release the permit
                        releaseOnce.run();
                    }
                    innerFail(e);
                } finally {
                    if (combineWithAction != null) {
                        combineWithAction.onResponse(releasable);
                    } else {
                        releaseOnce.run();
                    }
                }
            }
        }, 30, TimeUnit.MINUTES);
        pendingPrimaryTerm = newPrimaryTerm;
        termUpdated.countDown();
    }

    /**
     * Acquire a replica operation permit whenever the shard is ready for indexing (see
     * {@link #acquirePrimaryOperationPermit(ActionListener, String, Object)}). If the given primary term is lower than then one in
     * {@link #shardRouting}, the {@link ActionListener#onFailure(Exception)} method of the provided listener is invoked with an
     * {@link IllegalStateException}. If permit acquisition is delayed, the listener will be invoked on the executor with the specified
     * name.
     *
     * @param opPrimaryTerm              the operation primary term
     * @param globalCheckpoint           the global checkpoint associated with the request
     * @param maxSeqNoOfUpdatesOrDeletes the max seq_no of updates (index operations overwrite Lucene) or deletes captured on the primary
     *                                   after this replication request was executed on it (see {@link #getMaxSeqNoOfUpdatesOrDeletes()}
     * @param onPermitAcquired           the listener for permit acquisition
     * @param executorOnDelay            the name of the executor to invoke the listener on if permit acquisition is delayed
     * @param debugInfo                  an extra information that can be useful when tracing an unreleased permit. When assertions are
     *                                   enabled the tracing will capture the supplied object's {@link Object#toString()} value.
     *                                   Otherwise the object isn't used
     */
    public void acquireReplicaOperationPermit(
        final long opPrimaryTerm,
        final long globalCheckpoint,
        final long maxSeqNoOfUpdatesOrDeletes,
        final ActionListener<Releasable> onPermitAcquired,
        final String executorOnDelay,
        final Object debugInfo
    ) {
        innerAcquireReplicaOperationPermit(
            opPrimaryTerm,
            globalCheckpoint,
            maxSeqNoOfUpdatesOrDeletes,
            onPermitAcquired,
            false,
            (listener) -> indexShardOperationPermits.acquire(listener, executorOnDelay, true, debugInfo)
        );
    }

    /**
     * Acquire all replica operation permits whenever the shard is ready for indexing (see
     * {@link #acquireAllPrimaryOperationsPermits(ActionListener, TimeValue)}. If the given primary term is lower than then one in
     * {@link #shardRouting}, the {@link ActionListener#onFailure(Exception)} method of the provided listener is invoked with an
     * {@link IllegalStateException}.
     *
     * @param opPrimaryTerm              the operation primary term
     * @param globalCheckpoint           the global checkpoint associated with the request
     * @param maxSeqNoOfUpdatesOrDeletes the max seq_no of updates (index operations overwrite Lucene) or deletes captured on the primary
     *                                   after this replication request was executed on it (see {@link #getMaxSeqNoOfUpdatesOrDeletes()}
     * @param onPermitAcquired           the listener for permit acquisition
     * @param timeout                    the maximum time to wait for the in-flight operations block
     */
    public void acquireAllReplicaOperationsPermits(
        final long opPrimaryTerm,
        final long globalCheckpoint,
        final long maxSeqNoOfUpdatesOrDeletes,
        final ActionListener<Releasable> onPermitAcquired,
        final TimeValue timeout
    ) {
        innerAcquireReplicaOperationPermit(
            opPrimaryTerm,
            globalCheckpoint,
            maxSeqNoOfUpdatesOrDeletes,
            onPermitAcquired,
            true,
            listener -> asyncBlockOperations(listener, timeout.duration(), timeout.timeUnit())
        );
    }

    private void innerAcquireReplicaOperationPermit(
        final long opPrimaryTerm,
        final long globalCheckpoint,
        final long maxSeqNoOfUpdatesOrDeletes,
        final ActionListener<Releasable> onPermitAcquired,
        final boolean allowCombineOperationWithPrimaryTermUpdate,
        final Consumer<ActionListener<Releasable>> operationExecutor
    ) {
        verifyNotClosed();

        // This listener is used for the execution of the operation. If the operation requires all the permits for its
        // execution and the primary term must be updated first, we can combine the operation execution with the
        // primary term update. Since indexShardOperationPermits doesn't guarantee that async submissions are executed
        // in the order submitted, combining both operations ensure that the term is updated before the operation is
        // executed. It also has the side effect of acquiring all the permits one time instead of two.
        final ActionListener<Releasable> operationListener = ActionListener.delegateFailure(
            onPermitAcquired,
            (delegatedListener, releasable) -> {
                if (opPrimaryTerm < getOperationPrimaryTerm()) {
                    releasable.close();
                    final String message = String.format(
                        Locale.ROOT,
                        "%s operation primary term [%d] is too old (current [%d])",
                        shardId,
                        opPrimaryTerm,
                        getOperationPrimaryTerm()
                    );
                    delegatedListener.onFailure(new IllegalStateException(message));
                } else {
                    assert assertReplicationTarget();
                    try {
                        updateGlobalCheckpointOnReplica(globalCheckpoint, "operation");
                        advanceMaxSeqNoOfUpdatesOrDeletes(maxSeqNoOfUpdatesOrDeletes);
                    } catch (Exception e) {
                        releasable.close();
                        delegatedListener.onFailure(e);
                        return;
                    }
                    delegatedListener.onResponse(releasable);
                }
            }
        );

        if (requirePrimaryTermUpdate(opPrimaryTerm, allowCombineOperationWithPrimaryTermUpdate)) {
            synchronized (mutex) {
                if (requirePrimaryTermUpdate(opPrimaryTerm, allowCombineOperationWithPrimaryTermUpdate)) {
                    final IndexShardState shardState = state();
                    // only roll translog and update primary term if shard has made it past recovery
                    // Having a new primary term here means that the old primary failed and that there is a new primary, which again
                    // means that the cluster-manager will fail this shard as all initializing shards are failed when a primary is selected
                    // We abort early here to prevent an ongoing recovery from the failed primary to mess with the global / local checkpoint
                    if (shardState != IndexShardState.POST_RECOVERY && shardState != IndexShardState.STARTED) {
                        throw new IndexShardNotStartedException(shardId, shardState);
                    }

                    bumpPrimaryTerm(opPrimaryTerm, () -> {
                        updateGlobalCheckpointOnReplica(globalCheckpoint, "primary term transition");
                        final long currentGlobalCheckpoint = getLastKnownGlobalCheckpoint();
                        final long maxSeqNo = seqNoStats().getMaxSeqNo();
                        logger.info(
                            "detected new primary with primary term [{}], global checkpoint [{}], max_seq_no [{}]",
                            opPrimaryTerm,
                            currentGlobalCheckpoint,
                            maxSeqNo
                        );
                        // With Segment Replication enabled, we never want to reset a replica's engine unless
                        // it is promoted to primary.
                        if (currentGlobalCheckpoint < maxSeqNo && indexSettings.isSegRepEnabled() == false) {
                            resetEngineToGlobalCheckpoint();
                        } else {
                            getEngine().translogManager().rollTranslogGeneration();
                        }
                    }, allowCombineOperationWithPrimaryTermUpdate ? operationListener : null);

                    if (allowCombineOperationWithPrimaryTermUpdate) {
                        logger.debug("operation execution has been combined with primary term update");
                        return;
                    }
                }
            }
        }
        assert opPrimaryTerm <= pendingPrimaryTerm : "operation primary term ["
            + opPrimaryTerm
            + "] should be at most ["
            + pendingPrimaryTerm
            + "]";
        operationExecutor.accept(operationListener);
    }

    private boolean requirePrimaryTermUpdate(final long opPrimaryTerm, final boolean allPermits) {
        return (opPrimaryTerm > pendingPrimaryTerm) || (allPermits && opPrimaryTerm > getOperationPrimaryTerm());
    }

    public static final int OPERATIONS_BLOCKED = -1;

    /**
     * Obtain the active operation count, or {@link IndexShard#OPERATIONS_BLOCKED} if all permits are held (even if there are
     * outstanding operations in flight).
     *
     * @return the active operation count, or {@link IndexShard#OPERATIONS_BLOCKED} when all permits are held.
     */
    public int getActiveOperationsCount() {
        return indexShardOperationPermits.getActiveOperationsCount();
    }

    /**
     * @return a list of describing each permit that wasn't released yet. The description consist of the debugInfo supplied
     *         when the permit was acquired plus a stack traces that was captured when the permit was request.
     */
    public List<String> getActiveOperations() {
        return indexShardOperationPermits.getActiveOperations();
    }

    private final AsyncIOProcessor<Translog.Location> translogSyncProcessor;

    private static AsyncIOProcessor<Translog.Location> createTranslogSyncProcessor(
        Logger logger,
        ThreadPool threadPool,
        Supplier<Engine> engineSupplier,
        boolean bufferAsyncIoProcessor,
        Supplier<TimeValue> bufferIntervalSupplier
    ) {
        assert bufferAsyncIoProcessor == false || Objects.nonNull(bufferIntervalSupplier)
            : "If bufferAsyncIoProcessor is true, then the bufferIntervalSupplier needs to be non null";
        ThreadContext threadContext = threadPool.getThreadContext();
        CheckedConsumer<List<Tuple<Translog.Location, Consumer<Exception>>>, IOException> writeConsumer = candidates -> {
            try {
                engineSupplier.get().translogManager().ensureTranslogSynced(candidates.stream().map(Tuple::v1));
            } catch (AlreadyClosedException ex) {
                // that's fine since we already synced everything on engine close - this also is conform with the methods
                // documentation
            } catch (IOException ex) { // if this fails we are in deep shit - fail the request
                logger.debug("failed to sync translog", ex);
                throw ex;
            }
        };
        if (bufferAsyncIoProcessor) {
            return new BufferedAsyncIOProcessor<>(logger, 102400, threadContext, threadPool, bufferIntervalSupplier) {
                @Override
                protected void write(List<Tuple<Translog.Location, Consumer<Exception>>> candidates) throws IOException {
                    writeConsumer.accept(candidates);
                }

                @Override
                protected String getBufferProcessThreadPoolName() {
                    return ThreadPool.Names.TRANSLOG_SYNC;
                }
            };
        }

        return new AsyncIOProcessor<>(logger, 1024, threadContext) {
            @Override
            protected void write(List<Tuple<Translog.Location, Consumer<Exception>>> candidates) throws IOException {
                writeConsumer.accept(candidates);
            }
        };
    }

    /**
     * Syncs the given location with the underlying storage unless already synced. This method might return immediately without
     * actually fsyncing the location until the sync listener is called. Yet, unless there is already another thread fsyncing
     * the transaction log the caller thread will be hijacked to run the fsync for all pending fsync operations.
     * This method allows indexing threads to continue indexing without blocking on fsync calls. We ensure that there is only
     * one thread blocking on the sync an all others can continue indexing.
     * NOTE: if the syncListener throws an exception when it's processed the exception will only be logged. Users should make sure that the
     * listener handles all exception cases internally.
     */
    public final void sync(Translog.Location location, Consumer<Exception> syncListener) {
        verifyNotClosed();
        translogSyncProcessor.put(location, syncListener);
    }

    public void sync() throws IOException {
        verifyNotClosed();
        getEngine().translogManager().syncTranslog();
    }

    /**
     * Checks if the underlying storage sync is required.
     */
    public boolean isSyncNeeded() {
        return getEngine().translogManager().isTranslogSyncNeeded();
    }

    /**
     * Returns the current translog durability mode
     */
    public Durability getTranslogDurability() {
        return indexSettings.getTranslogDurability();
    }

    // we can not protect with a lock since we "release" on a different thread
    private final AtomicBoolean flushOrRollRunning = new AtomicBoolean();

    /**
     * Schedules a flush or translog generation roll if needed but will not schedule more than one concurrently. The operation will be
     * executed asynchronously on the flush thread pool.
     */
    public void afterWriteOperation() {
        if (shouldPeriodicallyFlush() || shouldRollTranslogGeneration()) {
            if (flushOrRollRunning.compareAndSet(false, true)) {
                /*
                 * We have to check again since otherwise there is a race when a thread passes the first check next to another thread which
                 * performs the operation quickly enough to  finish before the current thread could flip the flag. In that situation, we
                 * have an extra operation.
                 *
                 * Additionally, a flush implicitly executes a translog generation roll so if we execute a flush then we do not need to
                 * check if we should roll the translog generation.
                 */
                if (shouldPeriodicallyFlush()) {
                    logger.debug("submitting async flush request");
                    final AbstractRunnable flush = new AbstractRunnable() {
                        @Override
                        public void onFailure(final Exception e) {
                            if (state != IndexShardState.CLOSED) {
                                logger.warn("failed to flush index", e);
                            }
                        }

                        @Override
                        protected void doRun() throws IOException {
                            flush(new FlushRequest());
                            periodicFlushMetric.inc();
                        }

                        @Override
                        public void onAfter() {
                            flushOrRollRunning.compareAndSet(true, false);
                            afterWriteOperation();
                        }
                    };
                    threadPool.executor(ThreadPool.Names.FLUSH).execute(flush);
                } else if (shouldRollTranslogGeneration()) {
                    logger.debug("submitting async roll translog generation request");
                    final AbstractRunnable roll = new AbstractRunnable() {
                        @Override
                        public void onFailure(final Exception e) {
                            if (state != IndexShardState.CLOSED) {
                                logger.warn("failed to roll translog generation", e);
                            }
                        }

                        @Override
                        protected void doRun() throws Exception {
                            rollTranslogGeneration();
                        }

                        @Override
                        public void onAfter() {
                            flushOrRollRunning.compareAndSet(true, false);
                            afterWriteOperation();
                        }
                    };
                    threadPool.executor(ThreadPool.Names.FLUSH).execute(roll);
                } else {
                    flushOrRollRunning.compareAndSet(true, false);
                }
            }
        }
    }

    /**
     * Build {@linkplain RefreshListeners} for this shard.
     */
    private RefreshListeners buildRefreshListeners() {
        return new RefreshListeners(
            indexSettings::getMaxRefreshListeners,
            () -> refresh("too_many_listeners"),
            logger,
            threadPool.getThreadContext(),
            externalRefreshMetric
        );
    }

    /**
     * Simple struct encapsulating a shard failure
     *
     * @see IndexShard#addShardFailureCallback(Consumer)
     *
     * @opensearch.api
     */
    @PublicApi(since = "1.0.0")
    public static final class ShardFailure {
        public final ShardRouting routing;
        public final String reason;
        @Nullable
        public final Exception cause;

        public ShardFailure(ShardRouting routing, String reason, @Nullable Exception cause) {
            this.routing = routing;
            this.reason = reason;
            this.cause = cause;
        }
    }

    EngineFactory getEngineFactory() {
        return engineFactory;
    }

    EngineConfigFactory getEngineConfigFactory() {
        return engineConfigFactory;
    }

    // for tests
    ReplicationTracker getReplicationTracker() {
        return replicationTracker;
    }

    /**
     * Executes a scheduled refresh if necessary.
     *
     * @return <code>true</code> iff the engine got refreshed otherwise <code>false</code>
     */
    public boolean scheduledRefresh() {
        verifyNotClosed();
        boolean listenerNeedsRefresh = refreshListeners.refreshNeeded();
        if (isReadAllowed() && (listenerNeedsRefresh || getEngine().refreshNeeded())) {
            if (listenerNeedsRefresh == false // if we have a listener that is waiting for a refresh we need to force it
                && isSearchIdleSupported()
                && isSearchIdle()
                && indexSettings.isExplicitRefresh() == false
                && active.get()) { // it must be active otherwise we might not free up segment memory once the shard became inactive
                // lets skip this refresh since we are search idle and
                // don't necessarily need to refresh. the next searcher access will register a refreshListener and that will
                // cause the next schedule to refresh.
                final Engine engine = getEngine();
                engine.maybePruneDeletes(); // try to prune the deletes in the engine if we accumulated some
                setRefreshPending(engine);
                return false;
            } else {
                if (logger.isTraceEnabled()) {
                    logger.trace("refresh with source [schedule]");
                }
                return getEngine().maybeRefresh("schedule");
            }
        }
        final Engine engine = getEngine();
        engine.maybePruneDeletes(); // try to prune the deletes in the engine if we accumulated some
        return false;
    }

    /**
     * Returns true if this shards is search idle
     */
    public final boolean isSearchIdle() {
        return (threadPool.relativeTimeInMillis() - lastSearcherAccess.get()) >= indexSettings.getSearchIdleAfter().getMillis();
    }

    /**
     * Returns true if this shard supports search idle.
     * <p>
     * Indices using Segment Replication will ignore search idle unless there are no replicas.
     * Primary shards push out new segments only
     * after a refresh, so we don't want to wait for a search to trigger that cycle. Replicas will only refresh after receiving
     * a new set of segments.
     */
    public final boolean isSearchIdleSupported() {
        // If the index is remote store backed, then search idle is not supported. This is to ensure that async refresh
        // task continues to upload to remote store periodically.
        if (isRemoteTranslogEnabled()) {
            return false;
        }
        return indexSettings.isSegRepEnabled() == false || indexSettings.getNumberOfReplicas() == 0;
    }

    /**
     * Returns the last timestamp the searcher was accessed. This is a relative timestamp in milliseconds.
     */
    final long getLastSearcherAccess() {
        return lastSearcherAccess.get();
    }

    /**
     * Returns true if this shard has some scheduled refresh that is pending because of search-idle.
     */
    public final boolean hasRefreshPending() {
        return pendingRefreshLocation.get() != null;
    }

    private void setRefreshPending(Engine engine) {
        final Translog.Location lastWriteLocation = engine.translogManager().getTranslogLastWriteLocation();
        pendingRefreshLocation.updateAndGet(curr -> {
            if (curr == null || curr.compareTo(lastWriteLocation) <= 0) {
                return lastWriteLocation;
            } else {
                return curr;
            }
        });
    }

    private class RefreshPendingLocationListener implements ReferenceManager.RefreshListener {
        Translog.Location lastWriteLocation;

        @Override
        public void beforeRefresh() {
            try {
                lastWriteLocation = getEngine().translogManager().getTranslogLastWriteLocation();
            } catch (AlreadyClosedException exc) {
                // shard is closed - no location is fine
                lastWriteLocation = null;
            }
        }

        @Override
        public void afterRefresh(boolean didRefresh) {
            if (didRefresh && lastWriteLocation != null) {
                pendingRefreshLocation.updateAndGet(pendingLocation -> {
                    if (pendingLocation == null || pendingLocation.compareTo(lastWriteLocation) <= 0) {
                        return null;
                    } else {
                        return pendingLocation;
                    }
                });
            }
        }
    }

    /**
     * Registers the given listener and invokes it once the shard is active again and all
     * pending refresh translog location has been refreshed. If there is no pending refresh location registered the listener will be
     * invoked immediately.
     * @param listener the listener to invoke once the pending refresh location is visible. The listener will be called with
     *                 <code>true</code> if the listener was registered to wait for a refresh.
     */
    public final void awaitShardSearchActive(Consumer<Boolean> listener) {
        markSearcherAccessed(); // move the shard into non-search idle
        final Translog.Location location = pendingRefreshLocation.get();
        if (location != null) {
            addRefreshListener(location, (b) -> {
                pendingRefreshLocation.compareAndSet(location, null);
                listener.accept(true);
            });
        } else {
            listener.accept(false);
        }
    }

    /**
     * Add a listener for refreshes.
     *
     * @param location the location to listen for
     * @param listener for the refresh. Called with true if registering the listener ran it out of slots and forced a refresh. Called with
     *        false otherwise.
     */
    public void addRefreshListener(Translog.Location location, Consumer<Boolean> listener) {
        final boolean readAllowed;
        if (isReadAllowed()) {
            readAllowed = true;
        } else {
            // check again under postRecoveryMutex. this is important to create a happens before relationship
            // between the switch to POST_RECOVERY + associated refresh. Otherwise we may respond
            // to a listener before a refresh actually happened that contained that operation.
            synchronized (postRecoveryMutex) {
                readAllowed = isReadAllowed();
            }
        }
        // NRT Replicas will not accept refresh listeners.
        if (readAllowed && isSegmentReplicationAllowed() == false) {
            refreshListeners.addOrNotify(location, listener);
        } else {
            // we're not yet ready fo ready for reads, just ignore refresh cycles
            listener.accept(false);
        }
    }

    /**
     * Metrics updater for a refresh
     *
     * @opensearch.internal
     */
    private static class RefreshMetricUpdater implements ReferenceManager.RefreshListener {

        private final MeanMetric refreshMetric;
        private long currentRefreshStartTime;
        private Thread callingThread = null;

        private RefreshMetricUpdater(MeanMetric refreshMetric) {
            this.refreshMetric = refreshMetric;
        }

        @Override
        public void beforeRefresh() throws IOException {
            if (Assertions.ENABLED) {
                assert callingThread == null : "beforeRefresh was called by "
                    + callingThread.getName()
                    + " without a corresponding call to afterRefresh";
                callingThread = Thread.currentThread();
            }
            currentRefreshStartTime = System.nanoTime();
        }

        @Override
        public void afterRefresh(boolean didRefresh) throws IOException {
            if (Assertions.ENABLED) {
                assert callingThread != null : "afterRefresh called but not beforeRefresh";
                assert callingThread == Thread.currentThread() : "beforeRefreshed called by a different thread. current ["
                    + Thread.currentThread().getName()
                    + "], thread that called beforeRefresh ["
                    + callingThread.getName()
                    + "]";
                callingThread = null;
            }
            refreshMetric.inc(System.nanoTime() - currentRefreshStartTime);
        }
    }

    /**
     * Refresh listener to update the Shard's ReplicationCheckpoint post refresh.
     */
    private class ReplicationCheckpointUpdater implements ReferenceManager.RefreshListener {
        @Override
        public void beforeRefresh() throws IOException {}

        @Override
        public void afterRefresh(boolean didRefresh) throws IOException {
            if (didRefresh) {
                // We're only starting to track the replication checkpoint. The timers for replication are started when
                // the checkpoint is published. This is done so that the timers do not include the time spent by primary
                // in uploading the segments to remote store.
                updateReplicationCheckpoint();
            }
        }
    }

    private void updateReplicationCheckpoint() {
        final Tuple<GatedCloseable<SegmentInfos>, ReplicationCheckpoint> tuple = getLatestSegmentInfosAndCheckpoint();
        try (final GatedCloseable<SegmentInfos> ignored = tuple.v1()) {
            replicationTracker.setLatestReplicationCheckpoint(tuple.v2());
        } catch (IOException e) {
            throw new OpenSearchException("Error Closing SegmentInfos Snapshot", e);
        }
    }

    private EngineConfig.TombstoneDocSupplier tombstoneDocSupplier() {
        final RootObjectMapper.Builder noopRootMapper = new RootObjectMapper.Builder("__noop");
        final DocumentMapper noopDocumentMapper = mapperService != null
            ? new DocumentMapper.Builder(noopRootMapper, mapperService).build(mapperService)
            : null;
        return new EngineConfig.TombstoneDocSupplier() {
            @Override
            public ParsedDocument newDeleteTombstoneDoc(String id) {
                return docMapper().getDocumentMapper().createDeleteTombstoneDoc(shardId.getIndexName(), id);
            }

            @Override
            public ParsedDocument newNoopTombstoneDoc(String reason) {
                return noopDocumentMapper.createNoopTombstoneDoc(shardId.getIndexName(), reason);
            }
        };
    }

    /**
     * Rollback the current engine to the safe commit, then replay local translog up to the global checkpoint.
     */
    void resetEngineToGlobalCheckpoint() throws IOException {
        assert Thread.holdsLock(mutex) == false : "resetting engine under mutex";
        assert getActiveOperationsCount() == OPERATIONS_BLOCKED : "resetting engine without blocking operations; active operations are ["
            + getActiveOperations()
            + ']';
        sync(); // persist the global checkpoint to disk
        final SeqNoStats seqNoStats = seqNoStats();
        final TranslogStats translogStats = translogStats();
        // flush to make sure the latest commit, which will be opened by the read-only engine, includes all operations.
        flush(new FlushRequest().waitIfOngoing(true));

        SetOnce<Engine> newEngineReference = new SetOnce<>();
        final long globalCheckpoint = getLastKnownGlobalCheckpoint();
        assert globalCheckpoint == getLastSyncedGlobalCheckpoint();
        synchronized (engineMutex) {
            verifyNotClosed();
            // we must create both new read-only engine and new read-write engine under engineMutex to ensure snapshotStoreMetadata,
            // acquireXXXCommit and close works.
            final Engine readOnlyEngine = new ReadOnlyEngine(
                newEngineConfig(replicationTracker),
                seqNoStats,
                translogStats,
                false,
                Function.identity(),
                true
            ) {
                @Override
                public GatedCloseable<IndexCommit> acquireLastIndexCommit(boolean flushFirst) {
                    synchronized (engineMutex) {
                        if (newEngineReference.get() == null) {
                            throw new AlreadyClosedException("engine was closed");
                        }
                        // ignore flushFirst since we flushed above and we do not want to interfere with ongoing translog replay
                        return newEngineReference.get().acquireLastIndexCommit(false);
                    }
                }

                @Override
                public GatedCloseable<IndexCommit> acquireSafeIndexCommit() {
                    synchronized (engineMutex) {
                        if (newEngineReference.get() == null) {
                            throw new AlreadyClosedException("engine was closed");
                        }
                        return newEngineReference.get().acquireSafeIndexCommit();
                    }
                }

                @Override
                public GatedCloseable<SegmentInfos> getSegmentInfosSnapshot() {
                    synchronized (engineMutex) {
                        if (newEngineReference.get() == null) {
                            throw new AlreadyClosedException("engine was closed");
                        }
                        return newEngineReference.get().getSegmentInfosSnapshot();
                    }
                }

                @Override
                public void close() throws IOException {
                    assert Thread.holdsLock(engineMutex);

                    Engine newEngine = newEngineReference.get();
                    if (newEngine == currentEngineReference.get()) {
                        // we successfully installed the new engine so do not close it.
                        newEngine = null;
                    }
                    IOUtils.close(super::close, newEngine);
                }
            };
            IOUtils.close(currentEngineReference.getAndSet(readOnlyEngine));
            if (indexSettings.isRemoteStoreEnabled()) {
                syncSegmentsFromRemoteSegmentStore(false);
            }
            if (indexSettings.isRemoteTranslogStoreEnabled() && shardRouting.primary()) {
                syncRemoteTranslogAndUpdateGlobalCheckpoint();
            }
            newEngineReference.set(engineFactory.newReadWriteEngine(newEngineConfig(replicationTracker)));
            onNewEngine(newEngineReference.get());
        }
        final TranslogRecoveryRunner translogRunner = (snapshot) -> runTranslogRecovery(
            newEngineReference.get(),
            snapshot,
            Engine.Operation.Origin.LOCAL_RESET,
            () -> {
                // TODO: add a dedicate recovery stats for the reset translog
            }
        );

        // When the new engine is created, translogs are synced from remote store onto local. Since remote store is the source
        // of truth for translog, we play all translogs that exists locally. Otherwise, the recoverUpto happens upto global checkpoint.
        // We also replay all local translog ops with Segment replication, because on engine swap our local translog may
        // hold more ops than the global checkpoint.
        long recoverUpto = this.isRemoteTranslogEnabled() || indexSettings().isSegRepEnabled() ? Long.MAX_VALUE : globalCheckpoint;
        newEngineReference.get()
            .translogManager()
            .recoverFromTranslog(translogRunner, newEngineReference.get().getProcessedLocalCheckpoint(), recoverUpto);
        newEngineReference.get().refresh("reset_engine");
        synchronized (engineMutex) {
            verifyNotClosed();
            IOUtils.close(currentEngineReference.getAndSet(newEngineReference.get()));
            // We set active because we are now writing operations to the engine; this way,
            // if we go idle after some time and become inactive, we still give sync'd flush a chance to run.
            active.set(true);
        }
        // time elapses after the engine is created above (pulling the config settings) until we set the engine reference, during
        // which settings changes could possibly have happened, so here we forcefully push any config changes to the new engine.
        onSettingsChanged();
    }

    private void syncRemoteTranslogAndUpdateGlobalCheckpoint() throws IOException {
        syncTranslogFilesFromRemoteTranslog();
        loadGlobalCheckpointToReplicationTracker();
    }

    public void deleteTranslogFilesFromRemoteTranslog() throws IOException {
        TranslogFactory translogFactory = translogFactorySupplier.apply(indexSettings, shardRouting);
        assert translogFactory instanceof RemoteBlobStoreInternalTranslogFactory;
        Repository repository = ((RemoteBlobStoreInternalTranslogFactory) translogFactory).getRepository();
        RemoteFsTranslog.cleanup(repository, shardId, getThreadPool());
    }

    public void syncTranslogFilesFromRemoteTranslog() throws IOException {
        TranslogFactory translogFactory = translogFactorySupplier.apply(indexSettings, shardRouting);
        assert translogFactory instanceof RemoteBlobStoreInternalTranslogFactory;
        Repository repository = ((RemoteBlobStoreInternalTranslogFactory) translogFactory).getRepository();
        RemoteFsTranslog.download(repository, shardId, getThreadPool(), shardPath().resolveTranslog(), logger);
    }

    /**
     * Downloads segments from remote segment store
     * @param overrideLocal flag to override local segment files with those in remote store.
     * @throws IOException if exception occurs while reading segments from remote store.
     */
    public void syncSegmentsFromRemoteSegmentStore(boolean overrideLocal) throws IOException {
        syncSegmentsFromRemoteSegmentStore(overrideLocal, () -> {});
    }

    /**
     * Downloads segments from remote segment store along with updating the access time of the recovery target.
     * @param overrideLocal flag to override local segment files with those in remote store.
     * @param onFileSync runnable that updates the access time when run.
     * @throws IOException if exception occurs while reading segments from remote store.
     */
    public void syncSegmentsFromRemoteSegmentStore(boolean overrideLocal, final Runnable onFileSync) throws IOException {
        boolean syncSegmentSuccess = false;
        long startTimeMs = System.currentTimeMillis();
        assert indexSettings.isRemoteStoreEnabled();
        logger.trace("Downloading segments from remote segment store");
        RemoteSegmentStoreDirectory remoteDirectory = getRemoteDirectory();
        // We need to call RemoteSegmentStoreDirectory.init() in order to get latest metadata of the files that
        // are uploaded to the remote segment store.
        RemoteSegmentMetadata remoteSegmentMetadata = remoteDirectory.init();

        Map<String, RemoteSegmentStoreDirectory.UploadedSegmentMetadata> uploadedSegments = remoteDirectory
            .getSegmentsUploadedToRemoteStore()
            .entrySet()
            .stream()
            .filter(entry -> entry.getKey().startsWith(IndexFileNames.SEGMENTS) == false)
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
        store.incRef();
        remoteStore.incRef();
        try {
            final Directory storeDirectory;
            if (recoveryState.getStage() == RecoveryState.Stage.INDEX) {
                storeDirectory = new StoreRecovery.StatsDirectoryWrapper(store.directory(), recoveryState.getIndex());
                for (String file : uploadedSegments.keySet()) {
                    long checksum = Long.parseLong(uploadedSegments.get(file).getChecksum());
                    if (overrideLocal || localDirectoryContains(storeDirectory, file, checksum) == false) {
                        recoveryState.getIndex().addFileDetail(file, uploadedSegments.get(file).getLength(), false);
                    } else {
                        recoveryState.getIndex().addFileDetail(file, uploadedSegments.get(file).getLength(), true);
                    }
                }
            } else {
                storeDirectory = store.directory();
            }
            copySegmentFiles(storeDirectory, remoteDirectory, null, uploadedSegments, overrideLocal, onFileSync);

            if (remoteSegmentMetadata != null) {
                final SegmentInfos infosSnapshot = store.buildSegmentInfos(
                    remoteSegmentMetadata.getSegmentInfosBytes(),
                    remoteSegmentMetadata.getGeneration()
                );
                long processedLocalCheckpoint = Long.parseLong(infosSnapshot.getUserData().get(LOCAL_CHECKPOINT_KEY));
                // delete any other commits, we want to start the engine only from a new commit made with the downloaded infos bytes.
                // Extra segments will be wiped on engine open.
                for (String file : List.of(store.directory().listAll())) {
                    if (file.startsWith(IndexFileNames.SEGMENTS)) {
                        store.deleteQuiet(file);
                    }
                }
                assert Arrays.stream(store.directory().listAll()).filter(f -> f.startsWith(IndexFileNames.SEGMENTS)).findAny().isEmpty()
                    : "There should not be any segments file in the dir";
                store.commitSegmentInfos(infosSnapshot, processedLocalCheckpoint, processedLocalCheckpoint);
            }
            syncSegmentSuccess = true;
        } catch (IOException e) {
            throw new IndexShardRecoveryException(shardId, "Exception while copying segment files from remote segment store", e);
        } finally {
            logger.trace(
                "syncSegmentsFromRemoteSegmentStore success={} elapsedTime={}",
                syncSegmentSuccess,
                (System.currentTimeMillis() - startTimeMs)
            );
            store.decRef();
            remoteStore.decRef();
        }
    }

    /**
     * Downloads segments from given remote segment store for a specific commit.
     * @param overrideLocal flag to override local segment files with those in remote store
     * @param sourceRemoteDirectory RemoteSegmentDirectory Instance from which we need to sync segments
     * @param primaryTerm Primary Term for shard at the time of commit operation for which we are syncing segments
     * @param commitGeneration commit generation at the time of commit operation for which we are syncing segments
     * @throws IOException if exception occurs while reading segments from remote store
     */
    public void syncSegmentsFromGivenRemoteSegmentStore(
        boolean overrideLocal,
        RemoteSegmentStoreDirectory sourceRemoteDirectory,
        long primaryTerm,
        long commitGeneration
    ) throws IOException {
        logger.trace("Downloading segments from given remote segment store");
        RemoteSegmentStoreDirectory remoteDirectory = null;
        if (remoteStore != null) {
            remoteDirectory = getRemoteDirectory();
            remoteDirectory.init();
            remoteStore.incRef();
        }
        Map<String, RemoteSegmentStoreDirectory.UploadedSegmentMetadata> uploadedSegments = sourceRemoteDirectory
            .getSegmentsUploadedToRemoteStore();
        final Directory storeDirectory = store.directory();
        store.incRef();

        try {
            String segmentsNFile = copySegmentFiles(
                storeDirectory,
                sourceRemoteDirectory,
                remoteDirectory,
                uploadedSegments,
                overrideLocal,
                () -> {}
            );
            if (segmentsNFile != null) {
                try (
                    ChecksumIndexInput indexInput = new BufferedChecksumIndexInput(
                        storeDirectory.openInput(segmentsNFile, IOContext.DEFAULT)
                    )
                ) {
                    SegmentInfos infosSnapshot = SegmentInfos.readCommit(store.directory(), indexInput, commitGeneration);
                    long processedLocalCheckpoint = Long.parseLong(infosSnapshot.getUserData().get(LOCAL_CHECKPOINT_KEY));
                    if (remoteStore != null) {
                        store.commitSegmentInfos(infosSnapshot, processedLocalCheckpoint, processedLocalCheckpoint);
                    } else {
                        store.directory().sync(infosSnapshot.files(true));
                        store.directory().syncMetaData();
                    }
                }
            }
        } catch (IOException e) {
            throw new IndexShardRecoveryException(shardId, "Exception while copying segment files from remote segment store", e);
        } finally {
            store.decRef();
            if (remoteStore != null) {
                remoteStore.decRef();
            }
        }
    }

    private String copySegmentFiles(
        Directory storeDirectory,
        RemoteSegmentStoreDirectory sourceRemoteDirectory,
        RemoteSegmentStoreDirectory targetRemoteDirectory,
        Map<String, RemoteSegmentStoreDirectory.UploadedSegmentMetadata> uploadedSegments,
        boolean overrideLocal,
        final Runnable onFileSync
    ) throws IOException {
        Set<String> toDownloadSegments = new HashSet<>();
        Set<String> skippedSegments = new HashSet<>();
        String segmentNFile = null;

        try {
            if (overrideLocal) {
                for (String file : storeDirectory.listAll()) {
                    storeDirectory.deleteFile(file);
                }
            }

            for (String file : uploadedSegments.keySet()) {
                long checksum = Long.parseLong(uploadedSegments.get(file).getChecksum());
                if (overrideLocal || localDirectoryContains(storeDirectory, file, checksum) == false) {
                    toDownloadSegments.add(file);
                } else {
                    skippedSegments.add(file);
                }

                if (file.startsWith(IndexFileNames.SEGMENTS)) {
                    assert segmentNFile == null : "There should be only one SegmentInfosSnapshot file";
                    segmentNFile = file;
                }
            }

            if (toDownloadSegments.isEmpty() == false) {
                try {
                    fileDownloader.download(sourceRemoteDirectory, storeDirectory, targetRemoteDirectory, toDownloadSegments, onFileSync);
                } catch (Exception e) {
                    throw new IOException("Error occurred when downloading segments from remote store", e);
                }
            }
        } finally {
            logger.trace("Downloaded segments here: {}", toDownloadSegments);
            logger.trace("Skipped download for segments here: {}", skippedSegments);
        }

        return segmentNFile;
    }

    // Visible for testing
    boolean localDirectoryContains(Directory localDirectory, String file, long checksum) throws IOException {
        try (IndexInput indexInput = localDirectory.openInput(file, IOContext.DEFAULT)) {
            if (checksum == CodecUtil.retrieveChecksum(indexInput)) {
                return true;
            } else {
                logger.warn("Checksum mismatch between local and remote segment file: {}, will override local file", file);
                // If there is a checksum mismatch and we are not serving reads it is safe to go ahead and delete the file now.
                // Outside of engine resets this method will be invoked during recovery so this is safe.
                if (isReadAllowed() == false) {
                    localDirectory.deleteFile(file);
                } else {
                    // segment conflict with remote store while the shard is serving reads.
                    failShard("Local copy of segment " + file + " has a different checksum than the version in remote store", null);
                }
            }
        } catch (NoSuchFileException | FileNotFoundException e) {
            logger.debug("File {} does not exist in local FS, downloading from remote store", file);
        } catch (IOException e) {
            logger.warn("Exception while reading checksum of file: {}, this can happen if file is corrupted", file);
            // For any other exception on reading checksum, we delete the file to re-download again
            localDirectory.deleteFile(file);
        }
        return false;
    }

    /**
     * Returns the maximum sequence number of either update or delete operations have been processed in this shard
     * or the sequence number from {@link #advanceMaxSeqNoOfUpdatesOrDeletes(long)}. An index request is considered
     * as an update operation if it overwrites the existing documents in Lucene index with the same document id.
     * <p>
     * The primary captures this value after executes a replication request, then transfers it to a replica before
     * executing that replication request on a replica.
     */
    public long getMaxSeqNoOfUpdatesOrDeletes() {
        return getEngine().getMaxSeqNoOfUpdatesOrDeletes();
    }

    /**
     * A replica calls this method to advance the max_seq_no_of_updates marker of its engine to at least the max_seq_no_of_updates
     * value (piggybacked in a replication request) that it receives from its primary before executing that replication request.
     * The receiving value is at least as high as the max_seq_no_of_updates on the primary was when any of the operations of that
     * replication request were processed on it.
     * <p>
     * A replica shard also calls this method to bootstrap the max_seq_no_of_updates marker with the value that it received from
     * the primary in peer-recovery, before it replays remote translog operations from the primary. The receiving value is at least
     * as high as the max_seq_no_of_updates on the primary was when any of these operations were processed on it.
     * <p>
     * These transfers guarantee that every index/delete operation when executing on a replica engine will observe this marker a value
     * which is at least the value of the max_seq_no_of_updates marker on the primary after that operation was executed on the primary.
     *
     * @see #acquireReplicaOperationPermit(long, long, long, ActionListener, String, Object)
     * @see RecoveryTarget#indexTranslogOperations(List, int, long, long, RetentionLeases, long, ActionListener)
     */
    public void advanceMaxSeqNoOfUpdatesOrDeletes(long seqNo) {
        getEngine().advanceMaxSeqNoOfUpdatesOrDeletes(seqNo);
    }

    /**
     * Performs the pre-closing checks on the {@link IndexShard}.
     *
     * @throws IllegalStateException if the sanity checks failed
     */
    public void verifyShardBeforeIndexClosing() throws IllegalStateException {
        getEngine().verifyEngineBeforeIndexClosing();
    }

    RetentionLeaseSyncer getRetentionLeaseSyncer() {
        return retentionLeaseSyncer;
    }

    /**
     * Fetch the latest SegmentInfos held by the shard's underlying Engine, wrapped
     * by a a {@link GatedCloseable} to ensure files are not deleted/merged away.
     *
     * @throws EngineException - When segment infos cannot be safely retrieved
     */
    public GatedCloseable<SegmentInfos> getSegmentInfosSnapshot() {
        return getEngine().getSegmentInfosSnapshot();
    }

    private TimeValue getRemoteTranslogUploadBufferInterval(Supplier<TimeValue> clusterRemoteTranslogBufferIntervalSupplier) {
        assert Objects.nonNull(clusterRemoteTranslogBufferIntervalSupplier) : "remote translog buffer interval supplier is null";
        if (indexSettings().isRemoteTranslogBufferIntervalExplicit()) {
            return indexSettings().getRemoteTranslogUploadBufferInterval();
        }
        return clusterRemoteTranslogBufferIntervalSupplier.get();
    }

    // Exclusively for testing, please do not use it elsewhere.
    public AsyncIOProcessor<Translog.Location> getTranslogSyncProcessor() {
        return translogSyncProcessor;
    }
}