Use static empty store files metadata (#84034)

In a large cluster we expect most nodes not to have a copy of most
shards, but today during replica shard allocation we create a new (and
nontrivial) object for each node that has no copy of a shard. With this
commit we check at deserialization time whether the response is empty
and, if so, avoid the unnecessary instantiation.

Relates #77466

docs/changelog/84034.yaml

@@ -0,0 +1,5 @@
+pr: 84034
+summary: Use static empty store files metadata
+area: Allocation
+type: enhancement
+issues: []

server/src/main/java/org/elasticsearch/index/store/Store.java

@@ -52,6 +52,7 @@
 import org.elasticsearch.common.lucene.store.InputStreamIndexInput;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Setting.Property;
+import org.elasticsearch.common.util.Maps;
 import org.elasticsearch.core.AbstractRefCounted;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.RefCounted;
@@ -759,25 +760,17 @@ public String toString() {
      */
     public static final class MetadataSnapshot implements Iterable<StoreFileMetadata>, Writeable {
         private final Map<String, StoreFileMetadata> metadata;
-
-        public static final MetadataSnapshot EMPTY = new MetadataSnapshot();
-
         private final Map<String, String> commitUserData;
-
         private final long numDocs;
 
+        public static final MetadataSnapshot EMPTY = new MetadataSnapshot(emptyMap(), emptyMap(), 0L);
+
         public MetadataSnapshot(Map<String, StoreFileMetadata> metadata, Map<String, String> commitUserData, long numDocs) {
             this.metadata = metadata;
             this.commitUserData = commitUserData;
             this.numDocs = numDocs;
         }
 
-        MetadataSnapshot() {
-            metadata = emptyMap();
-            commitUserData = emptyMap();
-            numDocs = 0;
-        }
-
         MetadataSnapshot(IndexCommit commit, Directory directory, Logger logger) throws IOException {
             LoadedMetadata loadedMetadata = loadMetadata(commit, directory, logger);
             metadata = loadedMetadata.fileMetadata;
@@ -786,26 +779,21 @@ public MetadataSnapshot(Map<String, StoreFileMetadata> metadata, Map<String, Str
             assert metadata.isEmpty() || numSegmentFiles() == 1 : "numSegmentFiles: " + numSegmentFiles();
         }
 
-        /**
-         * Read from a stream.
-         */
-        public MetadataSnapshot(StreamInput in) throws IOException {
-            final int size = in.readVInt();
-            Map<String, StoreFileMetadata> metadata = new HashMap<>();
-            for (int i = 0; i < size; i++) {
-                StoreFileMetadata meta = new StoreFileMetadata(in);
-                metadata.put(meta.name(), meta);
-            }
-            Map<String, String> commitUserData = new HashMap<>();
-            int num = in.readVInt();
-            for (int i = num; i > 0; i--) {
-                commitUserData.put(in.readString(), in.readString());
+        public static MetadataSnapshot readFrom(StreamInput in) throws IOException {
+            final int metadataSize = in.readVInt();
+            final Map<String, StoreFileMetadata> metadata = metadataSize == 0 ? emptyMap() : Maps.newMapWithExpectedSize(metadataSize);
+            for (int i = 0; i < metadataSize; i++) {
+                final var storeFileMetadata = new StoreFileMetadata(in);
+                metadata.put(storeFileMetadata.name(), storeFileMetadata);
             }
+            final var commitUserData = in.readMap(StreamInput::readString, StreamInput::readString);
+            final var numDocs = in.readLong();
 
-            this.metadata = unmodifiableMap(metadata);
-            this.commitUserData = unmodifiableMap(commitUserData);
-            this.numDocs = in.readLong();
-            assert metadata.isEmpty() || numSegmentFiles() == 1 : "numSegmentFiles: " + numSegmentFiles();
+            if (metadataSize == 0 && commitUserData.size() == 0 && numDocs == 0) {
+                return MetadataSnapshot.EMPTY;
+            } else {
+                return new MetadataSnapshot(metadata, commitUserData, numDocs);
+            }
         }
 
         @Override

server/src/main/java/org/elasticsearch/indices/recovery/RecoveryCleanFilesRequest.java

@@ -43,7 +43,7 @@ public RecoveryCleanFilesRequest(
         super(in);
         recoveryId = in.readLong();
         shardId = new ShardId(in);
-        snapshotFiles = new Store.MetadataSnapshot(in);
+        snapshotFiles = Store.MetadataSnapshot.readFrom(in);
         totalTranslogOps = in.readVInt();
         globalCheckpoint = in.readZLong();
     }

server/src/main/java/org/elasticsearch/indices/recovery/StartRecoveryRequest.java

@@ -40,7 +40,7 @@ public StartRecoveryRequest(StreamInput in) throws IOException {
         targetAllocationId = in.readString();
         sourceNode = new DiscoveryNode(in);
         targetNode = new DiscoveryNode(in);
-        metadataSnapshot = new Store.MetadataSnapshot(in);
+        metadataSnapshot = Store.MetadataSnapshot.readFrom(in);
         primaryRelocation = in.readBoolean();
         startingSeqNo = in.readLong();
         if (in.getVersion().onOrAfter(RecoverySettings.SNAPSHOT_FILE_DOWNLOAD_THROTTLING_SUPPORTED_VERSION)) {

server/src/main/java/org/elasticsearch/indices/store/TransportNodesListShardStoreMetadata.java

@@ -10,6 +10,7 @@
 
 import org.apache.logging.log4j.message.ParameterizedMessage;
 import org.elasticsearch.ElasticsearchException;
+import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionType;
 import org.elasticsearch.action.FailedNodeException;
 import org.elasticsearch.action.support.ActionFilters;
@@ -45,12 +46,13 @@
 import org.elasticsearch.transport.TransportService;
 
 import java.io.IOException;
-import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Objects;
 import java.util.concurrent.TimeUnit;
 
+import static java.util.Collections.emptyList;
+
 public class TransportNodesListShardStoreMetadata extends TransportNodesAction<
     TransportNodesListShardStoreMetadata.Request,
     TransportNodesListShardStoreMetadata.NodesStoreFilesMetadata,
@@ -132,18 +134,17 @@ private StoreFilesMetadata listStoreMetadata(NodeRequest request) throws IOExcep
                 if (indexShard != null) {
                     try {
                         final StoreFilesMetadata storeFilesMetadata = new StoreFilesMetadata(
-                            shardId,
                             indexShard.snapshotStoreMetadata(),
                             indexShard.getPeerRecoveryRetentionLeases()
                         );
                         exists = true;
                         return storeFilesMetadata;
                     } catch (org.apache.lucene.index.IndexNotFoundException e) {
                         logger.trace(new ParameterizedMessage("[{}] node is missing index, responding with empty", shardId), e);
-                        return new StoreFilesMetadata(shardId, Store.MetadataSnapshot.EMPTY, Collections.emptyList());
+                        return StoreFilesMetadata.EMPTY;
                     } catch (IOException e) {
                         logger.warn(new ParameterizedMessage("[{}] can't read metadata from store, responding with empty", shardId), e);
-                        return new StoreFilesMetadata(shardId, Store.MetadataSnapshot.EMPTY, Collections.emptyList());
+                        return StoreFilesMetadata.EMPTY;
                     }
                 }
             }
@@ -166,7 +167,7 @@ private StoreFilesMetadata listStoreMetadata(NodeRequest request) throws IOExcep
             }
             final ShardPath shardPath = ShardPath.loadShardPath(logger, nodeEnv, shardId, customDataPath);
             if (shardPath == null) {
-                return new StoreFilesMetadata(shardId, Store.MetadataSnapshot.EMPTY, Collections.emptyList());
+                return StoreFilesMetadata.EMPTY;
             }
             // note that this may fail if it can't get access to the shard lock. Since we check above there is an active shard, this means:
             // 1) a shard is being constructed, which means the master will not use a copy of this replica
@@ -180,7 +181,7 @@ private StoreFilesMetadata listStoreMetadata(NodeRequest request) throws IOExcep
             );
             // We use peer recovery retention leases from the primary for allocating replicas. We should always have retention leases when
             // we refresh shard info after the primary has started. Hence, we can ignore retention leases if there is no active shard.
-            return new StoreFilesMetadata(shardId, metadataSnapshot, Collections.emptyList());
+            return new StoreFilesMetadata(metadataSnapshot, emptyList());
         } finally {
             TimeValue took = new TimeValue(System.nanoTime() - startTimeNS, TimeUnit.NANOSECONDS);
             if (exists) {
@@ -192,37 +193,43 @@ private StoreFilesMetadata listStoreMetadata(NodeRequest request) throws IOExcep
     }
 
     public static class StoreFilesMetadata implements Iterable<StoreFileMetadata>, Writeable {
-        private final ShardId shardId;
         private final Store.MetadataSnapshot metadataSnapshot;
         private final List<RetentionLease> peerRecoveryRetentionLeases;
 
-        public StoreFilesMetadata(
-            ShardId shardId,
-            Store.MetadataSnapshot metadataSnapshot,
-            List<RetentionLease> peerRecoveryRetentionLeases
-        ) {
-            this.shardId = shardId;
+        private static final ShardId FAKE_SHARD_ID = new ShardId("_na_", "_na_", 0);
+        public static final StoreFilesMetadata EMPTY = new StoreFilesMetadata(Store.MetadataSnapshot.EMPTY, emptyList());
+
+        public StoreFilesMetadata(Store.MetadataSnapshot metadataSnapshot, List<RetentionLease> peerRecoveryRetentionLeases) {
             this.metadataSnapshot = metadataSnapshot;
             this.peerRecoveryRetentionLeases = peerRecoveryRetentionLeases;
         }
 
-        public StoreFilesMetadata(StreamInput in) throws IOException {
-            this.shardId = new ShardId(in);
-            this.metadataSnapshot = new Store.MetadataSnapshot(in);
-            this.peerRecoveryRetentionLeases = in.readList(RetentionLease::new);
+        public static StoreFilesMetadata readFrom(StreamInput in) throws IOException {
+            if (in.getVersion().before(Version.V_8_2_0)) {
+                new ShardId(in);
+            }
+            final var metadataSnapshot = Store.MetadataSnapshot.readFrom(in);
+            final var peerRecoveryRetentionLeases = in.readList(RetentionLease::new);
+            if (metadataSnapshot == Store.MetadataSnapshot.EMPTY && peerRecoveryRetentionLeases.isEmpty()) {
+                return EMPTY;
+            } else {
+                return new StoreFilesMetadata(metadataSnapshot, peerRecoveryRetentionLeases);
+            }
         }
 
         @Override
         public void writeTo(StreamOutput out) throws IOException {
-            shardId.writeTo(out);
+            if (out.getVersion().before(Version.V_8_2_0)) {
+                // no compatible version cares about the shard ID, we can just make one up
+                FAKE_SHARD_ID.writeTo(out);
+
+                // NB only checked this for versions back to 7.17.0, we are assuming that we don't use this with earlier versions:
+                assert out.getVersion().onOrAfter(Version.V_7_17_0) : out.getVersion();
+            }
             metadataSnapshot.writeTo(out);
             out.writeList(peerRecoveryRetentionLeases);
         }
 
-        public ShardId shardId() {
-            return this.shardId;
-        }
-
         public boolean isEmpty() {
             return metadataSnapshot.size() == 0;
         }
@@ -267,8 +274,6 @@ public String syncId() {
         @Override
         public String toString() {
             return "StoreFilesMetadata{"
-                + ", shardId="
-                + shardId
                 + ", metadataSnapshot{size="
                 + metadataSnapshot.size()
                 + ", syncId="
@@ -385,7 +390,7 @@ public static class NodeStoreFilesMetadata extends BaseNodeResponse {
 
         public NodeStoreFilesMetadata(StreamInput in, DiscoveryNode node) throws IOException {
             super(in, node);
-            storeFilesMetadata = new StoreFilesMetadata(in);
+            storeFilesMetadata = StoreFilesMetadata.readFrom(in);
         }
 
         public NodeStoreFilesMetadata(DiscoveryNode node, StoreFilesMetadata storeFilesMetadata) {

server/src/test/java/org/elasticsearch/gateway/ReplicaShardAllocatorTests.java

@@ -684,7 +684,6 @@ TestAllocator addData(
             data.put(
                 node,
                 new TransportNodesListShardStoreMetadata.StoreFilesMetadata(
-                    shardId,
                     new Store.MetadataSnapshot(unmodifiableMap(filesAsMap), unmodifiableMap(commitData), randomInt()),
                     peerRecoveryRetentionLeases
                 )

server/src/test/java/org/elasticsearch/index/store/StoreTests.java

@@ -80,7 +80,10 @@
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.LongUnaryOperator;
 
+import static java.util.Collections.emptyList;
+import static java.util.Collections.emptyMap;
 import static java.util.Collections.unmodifiableMap;
+import static org.elasticsearch.test.VersionUtils.randomCompatibleVersion;
 import static org.elasticsearch.test.VersionUtils.randomVersion;
 import static org.hamcrest.Matchers.anyOf;
 import static org.hamcrest.Matchers.containsString;
@@ -93,6 +96,7 @@
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.notNullValue;
+import static org.hamcrest.Matchers.sameInstance;
 
 public class StoreTests extends ESTestCase {
 
@@ -918,7 +922,7 @@ public void testMetadataSnapshotStreaming() throws Exception {
         ByteArrayInputStream inBuffer = new ByteArrayInputStream(outBuffer.toByteArray());
         InputStreamStreamInput in = new InputStreamStreamInput(inBuffer);
         in.setVersion(targetNodeVersion);
-        Store.MetadataSnapshot inMetadataSnapshot = new Store.MetadataSnapshot(in);
+        Store.MetadataSnapshot inMetadataSnapshot = Store.MetadataSnapshot.readFrom(in);
         Map<String, StoreFileMetadata> origEntries = new HashMap<>();
         origEntries.putAll(outMetadataSnapshot.asMap());
         for (Map.Entry<String, StoreFileMetadata> entry : inMetadataSnapshot.asMap().entrySet()) {
@@ -928,6 +932,21 @@ public void testMetadataSnapshotStreaming() throws Exception {
         assertThat(inMetadataSnapshot.getCommitUserData(), equalTo(outMetadataSnapshot.getCommitUserData()));
     }
 
+    public void testEmptyMetadataSnapshotStreaming() throws Exception {
+        var outMetadataSnapshot = randomBoolean() ? Store.MetadataSnapshot.EMPTY : new Store.MetadataSnapshot(emptyMap(), emptyMap(), 0L);
+        var targetNodeVersion = randomCompatibleVersion(random(), org.elasticsearch.Version.CURRENT);
+
+        var outBuffer = new ByteArrayOutputStream();
+        var out = new OutputStreamStreamOutput(outBuffer);
+        out.setVersion(targetNodeVersion);
+        outMetadataSnapshot.writeTo(out);
+
+        var inBuffer = new ByteArrayInputStream(outBuffer.toByteArray());
+        var in = new InputStreamStreamInput(inBuffer);
+        in.setVersion(targetNodeVersion);
+        assertThat(Store.MetadataSnapshot.readFrom(in), sameInstance(Store.MetadataSnapshot.EMPTY));
+    }
+
     protected Store.MetadataSnapshot createMetadataSnapshot() {
         StoreFileMetadata storeFileMetadata1 = new StoreFileMetadata("segments", 1, "666", MIN_SUPPORTED_LUCENE_VERSION.toString());
         StoreFileMetadata storeFileMetadata2 = new StoreFileMetadata("no_segments", 1, "666", MIN_SUPPORTED_LUCENE_VERSION.toString());
@@ -978,21 +997,16 @@ public void testStreamStoreFilesMetadata() throws Exception {
             );
         }
         TransportNodesListShardStoreMetadata.StoreFilesMetadata outStoreFileMetadata =
-            new TransportNodesListShardStoreMetadata.StoreFilesMetadata(
-                new ShardId("test", "_na_", 0),
-                metadataSnapshot,
-                peerRecoveryRetentionLeases
-            );
+            new TransportNodesListShardStoreMetadata.StoreFilesMetadata(metadataSnapshot, peerRecoveryRetentionLeases);
         ByteArrayOutputStream outBuffer = new ByteArrayOutputStream();
         OutputStreamStreamOutput out = new OutputStreamStreamOutput(outBuffer);
-        org.elasticsearch.Version targetNodeVersion = randomVersion(random());
+        org.elasticsearch.Version targetNodeVersion = randomCompatibleVersion(random(), org.elasticsearch.Version.CURRENT);
         out.setVersion(targetNodeVersion);
         outStoreFileMetadata.writeTo(out);
         ByteArrayInputStream inBuffer = new ByteArrayInputStream(outBuffer.toByteArray());
         InputStreamStreamInput in = new InputStreamStreamInput(inBuffer);
         in.setVersion(targetNodeVersion);
-        TransportNodesListShardStoreMetadata.StoreFilesMetadata inStoreFileMetadata =
-            new TransportNodesListShardStoreMetadata.StoreFilesMetadata(in);
+        var inStoreFileMetadata = TransportNodesListShardStoreMetadata.StoreFilesMetadata.readFrom(in);
         Iterator<StoreFileMetadata> outFiles = outStoreFileMetadata.iterator();
         for (StoreFileMetadata inFile : inStoreFileMetadata) {
             assertThat(inFile.name(), equalTo(outFiles.next().name()));
@@ -1001,6 +1015,25 @@ public void testStreamStoreFilesMetadata() throws Exception {
         assertThat(outStoreFileMetadata.peerRecoveryRetentionLeases(), equalTo(peerRecoveryRetentionLeases));
     }
 
+    public void testStreamEmptyStoreFilesMetadata() throws Exception {
+        var outStoreFileMetadata = randomBoolean()
+            ? TransportNodesListShardStoreMetadata.StoreFilesMetadata.EMPTY
+            : new TransportNodesListShardStoreMetadata.StoreFilesMetadata(Store.MetadataSnapshot.EMPTY, emptyList());
+        var outBuffer = new ByteArrayOutputStream();
+        var out = new OutputStreamStreamOutput(outBuffer);
+        var targetNodeVersion = randomCompatibleVersion(random(), org.elasticsearch.Version.CURRENT);
+        out.setVersion(targetNodeVersion);
+        outStoreFileMetadata.writeTo(out);
+
+        var inBuffer = new ByteArrayInputStream(outBuffer.toByteArray());
+        var in = new InputStreamStreamInput(inBuffer);
+        in.setVersion(targetNodeVersion);
+        assertThat(
+            TransportNodesListShardStoreMetadata.StoreFilesMetadata.readFrom(in),
+            sameInstance(TransportNodesListShardStoreMetadata.StoreFilesMetadata.EMPTY)
+        );
+    }
+
     public void testMarkCorruptedOnTruncatedSegmentsFile() throws IOException {
         IndexWriterConfig iwc = newIndexWriterConfig();
         final ShardId shardId = new ShardId("index", "_na_", 1);

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/repositories/PutCcrRestoreSessionAction.java

@@ -114,7 +114,7 @@ public static class PutCcrRestoreSessionResponse extends ActionResponse {
         PutCcrRestoreSessionResponse(StreamInput in) throws IOException {
             super(in);
             node = new DiscoveryNode(in);
-            storeFileMetadata = new Store.MetadataSnapshot(in);
+            storeFileMetadata = Store.MetadataSnapshot.readFrom(in);
             mappingVersion = in.readVLong();
         }