Add validation of the number_of_shards parameter in Shrink Action of ILM

client/rest-high-level/src/test/java/org/elasticsearch/client/documentation/ILMDocumentationIT.java

@@ -621,9 +621,6 @@ public void testRetryPolicy() throws Exception {
                     .put("index.lifecycle.name", "my_policy")
                     .build());
             client.indices().create(createIndexRequest, RequestOptions.DEFAULT);
-            assertBusy(() -> assertNotNull(client.indexLifecycle()
-                .explainLifecycle(new ExplainLifecycleRequest("my_index"), RequestOptions.DEFAULT)
-                .getIndexResponses().get("my_index").getFailedStep()), 30, TimeUnit.SECONDS);
         }
 
         // tag::ilm-retry-lifecycle-policy-request
@@ -644,8 +641,8 @@ public void testRetryPolicy() throws Exception {
 
             assertTrue(acknowledged);
         } catch (ElasticsearchException e) {
-            // the retry API might fail as the shrink action steps are retryable (so if the retry API reaches ES when ILM is retrying the
-            // failed `shrink` step, the retry API will fail)
+            // the retry API might fail as the shrink action steps are retryable (ILM will stuck in the `check-target-shards-count` step
+            // with no failure, the retry API will fail)
             // assert that's the exception we encountered (we want to test to fail if there is an actual error with the retry api)
             assertThat(e.getMessage(), containsStringIgnoringCase("reason=cannot retry an action for an index [my_index] that has not " +
                 "encountered an error when running a Lifecycle Policy"));

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/CheckTargetShardsCountStep.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+package org.elasticsearch.xpack.core.ilm;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.elasticsearch.cluster.ClusterState;
+import org.elasticsearch.cluster.metadata.IndexMetadata;
+import org.elasticsearch.index.Index;
+import org.elasticsearch.xpack.core.ilm.step.info.SingleMessageFieldInfo;
+
+import java.util.Locale;
+
+/**
+ * This step checks whether the new shrunken index's shards count is a factor of the source index's shards count.
+ */
+public class CheckTargetShardsCountStep extends ClusterStateWaitStep {
+
+    public static final String NAME = "check-target-shards-count";
+
+    private final Integer numberOfShards;
+
+    private static final Logger logger = LogManager.getLogger(CheckTargetShardsCountStep.class);
+
+    CheckTargetShardsCountStep(StepKey key, StepKey nextStepKey, Integer numberOfShards) {
+        super(key, nextStepKey);
+        this.numberOfShards = numberOfShards;
+    }
+
+    @Override
+    public boolean isRetryable() {
+        return true;
+    }
+
+    public Integer getNumberOfShards() {
+        return numberOfShards;
+    }
+
+    @Override
+    public Result isConditionMet(Index index, ClusterState clusterState) {
+        IndexMetadata indexMetadata = clusterState.metadata().index(index);
+        if (indexMetadata == null) {
+            // Index must have been since deleted, ignore it
+            logger.debug("[{}] lifecycle action for index [{}] executed but index no longer exists",
+                getKey().getAction(), index.getName());
+            return new Result(false, null);
+        }
+        String indexName = indexMetadata.getIndex().getName();
+        if (numberOfShards != null) {
+            int sourceNumberOfShards = indexMetadata.getNumberOfShards();
+            if (sourceNumberOfShards % numberOfShards != 0) {
+                String policyName = indexMetadata.getSettings().get(LifecycleSettings.LIFECYCLE_NAME);
+                String errorMessage = String.format(Locale.ROOT, "lifecycle action of policy [%s] for index [%s] cannot make progress " +
+                        "because the target shards count [%d] must be a factor of the source index's shards count [%d]",
+                    policyName, indexName, numberOfShards, sourceNumberOfShards);
+                logger.debug(errorMessage);
+                return new Result(false, new SingleMessageFieldInfo(errorMessage));
+            }
+        }
+
+        return new Result(true, null);
+    }
+}

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/ShrinkAction.java

@@ -137,6 +137,7 @@ public List<Step> toSteps(Client client, String phase, Step.StepKey nextStepKey)
         StepKey checkNotWriteIndex = new StepKey(phase, NAME, CheckNotDataStreamWriteIndexStep.NAME);
         StepKey waitForNoFollowerStepKey = new StepKey(phase, NAME, WaitForNoFollowersStep.NAME);
         StepKey readOnlyKey = new StepKey(phase, NAME, ReadOnlyAction.NAME);
+        StepKey checkTargetShardsCountKey = new StepKey(phase, NAME, CheckTargetShardsCountStep.NAME);
         StepKey cleanupShrinkIndexKey = new StepKey(phase, NAME, CleanupShrinkIndexStep.NAME);
         StepKey generateShrinkIndexNameKey = new StepKey(phase, NAME, GenerateUniqueIndexNameStep.NAME);
         StepKey setSingleNodeKey = new StepKey(phase, NAME, SetSingleNodeAllocateStep.NAME);
@@ -167,7 +168,9 @@ public List<Step> toSteps(Client client, String phase, Step.StepKey nextStepKey)
         CheckNotDataStreamWriteIndexStep checkNotWriteIndexStep = new CheckNotDataStreamWriteIndexStep(checkNotWriteIndex,
             waitForNoFollowerStepKey);
         WaitForNoFollowersStep waitForNoFollowersStep = new WaitForNoFollowersStep(waitForNoFollowerStepKey, readOnlyKey, client);
-        ReadOnlyStep readOnlyStep = new ReadOnlyStep(readOnlyKey, cleanupShrinkIndexKey, client);
+        ReadOnlyStep readOnlyStep = new ReadOnlyStep(readOnlyKey, checkTargetShardsCountKey, client);
+        CheckTargetShardsCountStep checkTargetShardsCountStep = new CheckTargetShardsCountStep(checkTargetShardsCountKey,
+            cleanupShrinkIndexKey, numberOfShards);
         // we generate a unique shrink index name but we also retry if the allocation of the shrunk index is not possible, so we want to
         // delete the "previously generated" shrink index (this is a no-op if it's the first run of the action and he haven't generated a
         // shrink index name)
@@ -211,9 +214,9 @@ public List<Step> toSteps(Client client, String phase, Step.StepKey nextStepKey)
         DeleteStep deleteSourceIndexStep = new DeleteStep(deleteIndexKey, isShrunkIndexKey, client);
         ShrunkenIndexCheckStep waitOnShrinkTakeover = new ShrunkenIndexCheckStep(isShrunkIndexKey, nextStepKey);
         return Arrays.asList(conditionalSkipShrinkStep, checkNotWriteIndexStep, waitForNoFollowersStep, readOnlyStep,
-            cleanupShrinkIndexStep, generateUniqueIndexNameStep, setSingleNodeStep, checkShrinkReadyStep, shrink, allocated,
-            copyMetadata, isDataStreamBranchingStep, aliasSwapAndDelete, waitOnShrinkTakeover, replaceDataStreamBackingIndex,
-            deleteSourceIndexStep);
+            checkTargetShardsCountStep, cleanupShrinkIndexStep, generateUniqueIndexNameStep, setSingleNodeStep, checkShrinkReadyStep,
+            shrink, allocated, copyMetadata, isDataStreamBranchingStep, aliasSwapAndDelete, waitOnShrinkTakeover,
+            replaceDataStreamBackingIndex, deleteSourceIndexStep);
     }
 
     @Override

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ilm/CheckTargetShardsCountStepTests.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+package org.elasticsearch.xpack.core.ilm;
+
+import org.elasticsearch.Version;
+import org.elasticsearch.cluster.ClusterState;
+import org.elasticsearch.cluster.metadata.IndexMetadata;
+import org.elasticsearch.cluster.metadata.Metadata;
+import org.elasticsearch.xpack.core.ilm.Step.StepKey;
+import org.elasticsearch.xpack.core.ilm.step.info.SingleMessageFieldInfo;
+
+import static org.hamcrest.Matchers.is;
+
+public class CheckTargetShardsCountStepTests extends AbstractStepTestCase<CheckTargetShardsCountStep> {
+
+    @Override
+    protected CheckTargetShardsCountStep createRandomInstance() {
+        return new CheckTargetShardsCountStep(randomStepKey(), randomStepKey(), null);
+    }
+
+    @Override
+    protected CheckTargetShardsCountStep mutateInstance(CheckTargetShardsCountStep instance) {
+        StepKey key = instance.getKey();
+        StepKey nextKey = instance.getNextStepKey();
+
+        switch (between(0, 1)) {
+        case 0:
+            key = new StepKey(key.getPhase(), key.getAction(), key.getName() + randomAlphaOfLength(5));
+            break;
+        case 1:
+            nextKey = new StepKey(key.getPhase(), key.getAction(), key.getName() + randomAlphaOfLength(5));
+            break;
+        default:
+            throw new AssertionError("Illegal randomisation branch");
+        }
+
+        return new CheckTargetShardsCountStep(key, nextKey, null);
+    }
+
+    @Override
+    protected CheckTargetShardsCountStep copyInstance(CheckTargetShardsCountStep instance) {
+        return new CheckTargetShardsCountStep(instance.getKey(), instance.getNextStepKey(), instance.getNumberOfShards());
+    }
+
+    public void testStepCompleteIfTargetShardsCountIsValid() {
+        String policyName = "test-ilm-policy";
+        IndexMetadata indexMetadata =
+            IndexMetadata.builder(randomAlphaOfLength(10)).settings(settings(Version.CURRENT)
+                .put(LifecycleSettings.LIFECYCLE_NAME, policyName))
+                .numberOfShards(10).numberOfReplicas(randomIntBetween(0, 5)).build();
+
+        ClusterState clusterState = ClusterState.builder(emptyClusterState()).metadata(
+            Metadata.builder().put(indexMetadata, true).build()).build();
+
+        CheckTargetShardsCountStep checkTargetShardsCountStep = new CheckTargetShardsCountStep(randomStepKey(), randomStepKey(), 2);
+
+        ClusterStateWaitStep.Result result = checkTargetShardsCountStep.isConditionMet(indexMetadata.getIndex(), clusterState);
+        assertThat(result.isComplete(), is(true));
+    }
+
+    public void testStepIncompleteIfTargetShardsCountNotValid() {
+        String indexName = randomAlphaOfLength(10);
+        String policyName = "test-ilm-policy";
+        IndexMetadata indexMetadata =
+            IndexMetadata.builder(indexName).settings(settings(Version.CURRENT).put(LifecycleSettings.LIFECYCLE_NAME, policyName))
+                .numberOfShards(10).numberOfReplicas(randomIntBetween(0, 5)).build();
+
+        ClusterState clusterState = ClusterState.builder(emptyClusterState()).metadata(
+            Metadata.builder().put(indexMetadata, true).build()).build();
+
+        CheckTargetShardsCountStep checkTargetShardsCountStep = new CheckTargetShardsCountStep(randomStepKey(), randomStepKey(), 3);
+
+        ClusterStateWaitStep.Result result = checkTargetShardsCountStep.isConditionMet(indexMetadata.getIndex(), clusterState);
+        assertThat(result.isComplete(), is(false));
+        SingleMessageFieldInfo info = (SingleMessageFieldInfo) result.getInfomationContext();
+        assertThat(info.getMessage(), is("lifecycle action of policy [" + policyName + "] for index [" + indexName +
+            "] cannot make progress because the target shards count [3] must be a factor of the source index's shards count [10]"));
+    }
+}