Cache resolved index for mgets

Mgets need to resolve the provided index to an actual index. This is
done for every item in an mget request, even if all mgets target the
same index. With this commit we eliminate this overhead when only one
index (or a small number of indices with items sorted by index) is
affected.

server/src/main/java/org/elasticsearch/action/get/TransportMultiGetAction.java

@@ -21,6 +21,7 @@
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.util.concurrent.AtomicArray;
 import org.elasticsearch.common.util.concurrent.EsExecutors;
+import org.elasticsearch.core.Tuple;
 import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.indices.IndicesService;
 import org.elasticsearch.tasks.Task;
@@ -60,16 +61,24 @@ protected void doExecute(Task task, final MultiGetRequest request, final ActionL
 
         final AtomicArray<MultiGetItemResponse> responses = new AtomicArray<>(request.items.size());
         final Map<ShardId, MultiGetShardRequest> shardRequests = new HashMap<>();
+        // single item cache that maps the provided index name to the resolved one
+        Tuple<String, String> lastResolvedIndex = Tuple.tuple(null, null);
 
         for (int i = 0; i < request.items.size(); i++) {
             MultiGetRequest.Item item = request.items.get(i);
 
             ShardId shardId;
             try {
-                String concreteSingleIndex = indexNameExpressionResolver.concreteSingleIndex(clusterState, item).getName();
+                String concreteSingleIndex;
+                if (item.index().equals(lastResolvedIndex.v1())) {
+                    concreteSingleIndex = lastResolvedIndex.v2();
+                } else {
+                    concreteSingleIndex = indexNameExpressionResolver.concreteSingleIndex(clusterState, item).getName();
+                    lastResolvedIndex = Tuple.tuple(item.index(), concreteSingleIndex);
+                }
                 item.routing(clusterState.metadata().resolveIndexRouting(item.routing(), item.index()));
                 shardId = clusterService.operationRouting()
-                    .getShards(clusterState, concreteSingleIndex, item.id(), item.routing(), null)
+                    .getIndexShardRoutingTable(clusterState, concreteSingleIndex, item.id(), item.routing())
                     .shardId();
             } catch (RoutingMissingException e) {
                 responses.set(i, newItemFailure(e.getIndex().getName(), e.getId(), e));

server/src/main/java/org/elasticsearch/cluster/routing/OperationRouting.java

@@ -56,6 +56,16 @@ void setUseAdaptiveReplicaSelection(boolean useAdaptiveReplicaSelection) {
         this.useAdaptiveReplicaSelection = useAdaptiveReplicaSelection;
     }
 
+    public IndexShardRoutingTable getIndexShardRoutingTable(
+        ClusterState clusterState,
+        String index,
+        String id,
+        @Nullable String routing
+    ) {
+        IndexRouting indexRouting = IndexRouting.fromIndexMetadata(indexMetadata(clusterState, index));
+        return clusterState.getRoutingTable().shardRoutingTable(index, indexRouting.getShard(id, routing));
+    }
+
     /**
      * Shards to use for a {@code GET} operation.
      * @return A shard iterator that can be used for GETs, or null if e.g. due to preferences no match is found.
@@ -67,8 +77,7 @@ public ShardIterator getShards(
         @Nullable String routing,
         @Nullable String preference
     ) {
-        IndexRouting indexRouting = IndexRouting.fromIndexMetadata(indexMetadata(clusterState, index));
-        IndexShardRoutingTable shards = clusterState.getRoutingTable().shardRoutingTable(index, indexRouting.getShard(id, routing));
+        IndexShardRoutingTable shards = getIndexShardRoutingTable(clusterState, index, id, routing);
         return preferenceActiveShardIterator(shards, clusterState.nodes().getLocalNodeId(), clusterState.nodes(), preference, null, null);
     }