Cleanup some Serialization Code around Snapshots (#59532) (#59606)

A number of obvious possible simplifications that also improve efficiency
in some cases (better empty collection handling and size hint use).
Also, added a shortcut for writing and reading immutable open maps that
can be used to dry up additional spots.

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/get/GetSnapshotsResponse.java

@@ -32,7 +32,6 @@ import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.snapshots.SnapshotInfo;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
@@ -52,10 +51,7 @@ public class GetSnapshotsResponse extends ActionResponse implements ToXContentOb
             (p, c) -> SnapshotInfo.SNAPSHOT_INFO_PARSER.apply(p, c).build(), new ParseField("snapshots"));
     }
 
-    private List<SnapshotInfo> snapshots = Collections.emptyList();
+    private final List<SnapshotInfo> snapshots;
-
-    GetSnapshotsResponse() {
-    }
 
     public GetSnapshotsResponse(List<SnapshotInfo> snapshots) {
         this.snapshots = Collections.unmodifiableList(snapshots);
@@ -63,12 +59,7 @@ public class GetSnapshotsResponse extends ActionResponse implements ToXContentOb
 
     GetSnapshotsResponse(StreamInput in) throws IOException {
         super(in);
-        int size = in.readVInt();
+        snapshots = Collections.unmodifiableList(in.readList(SnapshotInfo::new));
-        List<SnapshotInfo> builder = new ArrayList<>(size);
-        for (int i = 0; i < size; i++) {
-            builder.add(new SnapshotInfo(in));
-        }
-        snapshots = Collections.unmodifiableList(builder);
     }
 
     /**
@@ -82,10 +73,7 @@ public class GetSnapshotsResponse extends ActionResponse implements ToXContentOb
 
     @Override
     public void writeTo(StreamOutput out) throws IOException {
-        out.writeVInt(snapshots.size());
+        out.writeList(snapshots);
-        for (SnapshotInfo snapshotInfo : snapshots) {
-            snapshotInfo.writeTo(out);
-        }
     }
 
     @Override

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/restore/RestoreSnapshotResponse.java

@@ -43,7 +43,7 @@ import static org.elasticsearch.common.xcontent.ConstructingObjectParser.optiona
 public class RestoreSnapshotResponse extends ActionResponse implements ToXContentObject {
 
     @Nullable
-    private RestoreInfo restoreInfo;
+    private final RestoreInfo restoreInfo;
 
     public RestoreSnapshotResponse(@Nullable RestoreInfo restoreInfo) {
         this.restoreInfo = restoreInfo;

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/status/SnapshotShardsStats.java

@@ -167,7 +167,7 @@ public class SnapshotShardsStats implements ToXContentObject {
         PARSER.declareInt(constructorArg(), new ParseField(Fields.TOTAL));
     }
 
-    public static SnapshotShardsStats fromXContent(XContentParser parser) throws IOException {
+    public static SnapshotShardsStats fromXContent(XContentParser parser) {
         return PARSER.apply(parser, null);
     }
 

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/status/SnapshotStatus.java

@@ -57,11 +57,11 @@ import static org.elasticsearch.common.xcontent.ConstructingObjectParser.optiona
  */
 public class SnapshotStatus implements ToXContentObject, Writeable {
 
-    private Snapshot snapshot;
+    private final Snapshot snapshot;
 
-    private State state;
+    private final State state;
 
-    private List<SnapshotIndexShardStatus> shards;
+    private final List<SnapshotIndexShardStatus> shards;
 
     private Map<String, SnapshotIndexStatus> indicesStatus;
 
@@ -75,12 +75,7 @@ public class SnapshotStatus implements ToXContentObject, Writeable {
     SnapshotStatus(StreamInput in) throws IOException {
         snapshot = new Snapshot(in);
         state = State.fromValue(in.readByte());
-        int size = in.readVInt();
+        shards = Collections.unmodifiableList(in.readList(SnapshotIndexShardStatus::new));
-        List<SnapshotIndexShardStatus> builder = new ArrayList<>();
-        for (int i = 0; i < size; i++) {
-            builder.add(new SnapshotIndexShardStatus(in));
-        }
-        shards = Collections.unmodifiableList(builder);
         if (in.getVersion().onOrAfter(Version.V_6_2_0)) {
             includeGlobalState = in.readOptionalBoolean();
         }
@@ -185,10 +180,7 @@ public class SnapshotStatus implements ToXContentObject, Writeable {
     public void writeTo(StreamOutput out) throws IOException {
         snapshot.writeTo(out);
         out.writeByte(state.value());
-        out.writeVInt(shards.size());
+        out.writeList(shards);
-        for (SnapshotIndexShardStatus shard : shards) {
-            shard.writeTo(out);
-        }
         if (out.getVersion().onOrAfter(Version.V_6_2_0)) {
             out.writeOptionalBoolean(includeGlobalState);
         }
@@ -299,14 +291,9 @@ public class SnapshotStatus implements ToXContentObject, Writeable {
         if (o == null || getClass() != o.getClass()) return false;
 
         SnapshotStatus that = (SnapshotStatus) o;
-
+        return Objects.equals(snapshot, that.snapshot) && state == that.state && Objects.equals(indicesStatus, that.indicesStatus)
-        if (snapshot != null ? !snapshot.equals(that.snapshot) : that.snapshot != null) return false;
+                && Objects.equals(shardsStats, that.shardsStats) && Objects.equals(stats, that.stats)
-        if (state != that.state) return false;
+                && Objects.equals(includeGlobalState, that.includeGlobalState);
-        if (indicesStatus != null ? !indicesStatus.equals(that.indicesStatus) : that.indicesStatus != null)
-            return false;
-        if (shardsStats != null ? !shardsStats.equals(that.shardsStats) : that.shardsStats != null) return false;
-        if (stats != null ? !stats.equals(that.stats) : that.stats != null) return false;
-        return includeGlobalState != null ? includeGlobalState.equals(that.includeGlobalState) : that.includeGlobalState == null;
     }
 
     @Override

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/status/SnapshotsStatusResponse.java

@@ -32,6 +32,7 @@ import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Objects;
 
 import static org.elasticsearch.common.xcontent.ConstructingObjectParser.constructorArg;
 
@@ -40,7 +41,7 @@ import static org.elasticsearch.common.xcontent.ConstructingObjectParser.constru
  */
 public class SnapshotsStatusResponse extends ActionResponse implements ToXContentObject {
 
-    private List<SnapshotStatus> snapshots = Collections.emptyList();
+    private final List<SnapshotStatus> snapshots;
 
     public SnapshotsStatusResponse(StreamInput in) throws IOException {
         super(in);
@@ -105,9 +106,7 @@ public class SnapshotsStatusResponse extends ActionResponse implements ToXConten
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;
 
-        SnapshotsStatusResponse response = (SnapshotsStatusResponse) o;
+        return Objects.equals(snapshots, ((SnapshotsStatusResponse) o).snapshots);
-
-        return snapshots != null ? snapshots.equals(response.snapshots) : response.snapshots == null;
     }
 
     @Override

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/status/TransportNodesSnapshotsStatus.java

@@ -166,7 +166,7 @@ public class TransportNodesSnapshotsStatus extends TransportNodesAction<Transpor
 
     public static class NodeRequest extends BaseNodeRequest {
 
-        private List<Snapshot> snapshots;
+        private final List<Snapshot> snapshots;
 
         public NodeRequest(StreamInput in) throws IOException {
             super(in);
@@ -186,24 +186,12 @@ public class TransportNodesSnapshotsStatus extends TransportNodesAction<Transpor
 
     public static class NodeSnapshotStatus extends BaseNodeResponse {
 
-        private Map<Snapshot, Map<ShardId, SnapshotIndexShardStatus>> status;
+        private final Map<Snapshot, Map<ShardId, SnapshotIndexShardStatus>> status;
 
         public NodeSnapshotStatus(StreamInput in) throws IOException {
             super(in);
-            int numberOfSnapshots = in.readVInt();
+            status = unmodifiableMap(
-            Map<Snapshot, Map<ShardId, SnapshotIndexShardStatus>> snapshotMapBuilder = new HashMap<>(numberOfSnapshots);
+                    in.readMap(Snapshot::new, input -> unmodifiableMap(input.readMap(ShardId::new, SnapshotIndexShardStatus::new))));
-            for (int i = 0; i < numberOfSnapshots; i++) {
-                Snapshot snapshot = new Snapshot(in);
-                int numberOfShards = in.readVInt();
-                Map<ShardId, SnapshotIndexShardStatus> shardMapBuilder = new HashMap<>(numberOfShards);
-                for (int j = 0; j < numberOfShards; j++) {
-                    ShardId shardId =  new ShardId(in);
-                    SnapshotIndexShardStatus status = new SnapshotIndexShardStatus(in);
-                    shardMapBuilder.put(shardId, status);
-                }
-                snapshotMapBuilder.put(snapshot, unmodifiableMap(shardMapBuilder));
-            }
-            status = unmodifiableMap(snapshotMapBuilder);
         }
 
         public NodeSnapshotStatus(DiscoveryNode node, Map<Snapshot, Map<ShardId, SnapshotIndexShardStatus>> status) {
@@ -219,15 +207,8 @@ public class TransportNodesSnapshotsStatus extends TransportNodesAction<Transpor
         public void writeTo(StreamOutput out) throws IOException {
             super.writeTo(out);
             if (status != null) {
-                out.writeVInt(status.size());
+                out.writeMap(status, (o, s) -> s.writeTo(o),
-                for (Map.Entry<Snapshot, Map<ShardId, SnapshotIndexShardStatus>> entry : status.entrySet()) {
+                        (output, v) -> output.writeMap(v, (o, shardId) -> shardId.writeTo(o), (o, sis) -> sis.writeTo(o)));
-                    entry.getKey().writeTo(out);
-                    out.writeVInt(entry.getValue().size());
-                    for (Map.Entry<ShardId, SnapshotIndexShardStatus> shardEntry : entry.getValue().entrySet()) {
-                        shardEntry.getKey().writeTo(out);
-                        shardEntry.getValue().writeTo(out);
-                    }
-                }
             } else {
                 out.writeVInt(0);
             }

server/src/main/java/org/elasticsearch/cluster/RestoreInProgress.java

@@ -27,13 +27,13 @@ import org.elasticsearch.cluster.ClusterState.Custom;
 import org.elasticsearch.common.collect.ImmutableOpenMap;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.common.xcontent.ToXContent;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.snapshots.Snapshot;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
@@ -69,12 +69,7 @@ public class RestoreInProgress extends AbstractNamedDiffable<Custom> implements
     public boolean equals(Object o) {
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;
-
+        return entries.equals(((RestoreInProgress) o).entries);
-        RestoreInProgress that = (RestoreInProgress) o;
-
-        if (!entries.equals(that.entries)) return false;
-
-        return true;
     }
 
     @Override
@@ -225,7 +220,7 @@ public class RestoreInProgress extends AbstractNamedDiffable<Custom> implements
     /**
      * Represents status of a restored shard
      */
-    public static class ShardRestoreStatus {
+    public static class ShardRestoreStatus implements Writeable {
         private State state;
         private String nodeId;
         private String reason;
@@ -320,6 +315,7 @@ public class RestoreInProgress extends AbstractNamedDiffable<Custom> implements
          *
          * @param out stream input
          */
+        @Override
         public void writeTo(StreamOutput out) throws IOException {
             out.writeOptionalString(nodeId);
             out.writeByte(state.value);
@@ -368,7 +364,7 @@ public class RestoreInProgress extends AbstractNamedDiffable<Custom> implements
          */
         FAILURE((byte) 3);
 
-        private byte value;
+        private final byte value;
 
         /**
          * Constructs new state
@@ -448,19 +444,9 @@ public class RestoreInProgress extends AbstractNamedDiffable<Custom> implements
             }
             Snapshot snapshot = new Snapshot(in);
             State state = State.fromValue(in.readByte());
-            int indices = in.readVInt();
+            List<String> indexBuilder = in.readStringList();
-            List<String> indexBuilder = new ArrayList<>();
+            entriesBuilder.put(uuid, new Entry(uuid, snapshot, state, Collections.unmodifiableList(indexBuilder),
-            for (int j = 0; j < indices; j++) {
+                    in.readImmutableMap(ShardId::new, ShardRestoreStatus::readShardRestoreStatus)));
-                indexBuilder.add(in.readString());
-            }
-            ImmutableOpenMap.Builder<ShardId, ShardRestoreStatus> builder = ImmutableOpenMap.builder();
-            int shards = in.readVInt();
-            for (int j = 0; j < shards; j++) {
-                ShardId shardId = new ShardId(in);
-                ShardRestoreStatus shardState = ShardRestoreStatus.readShardRestoreStatus(in);
-                builder.put(shardId, shardState);
-            }
-            entriesBuilder.put(uuid, new Entry(uuid, snapshot, state, Collections.unmodifiableList(indexBuilder), builder.build()));
         }
         this.entries = entriesBuilder.build();
     }
@@ -475,15 +461,8 @@ public class RestoreInProgress extends AbstractNamedDiffable<Custom> implements
             }
             entry.snapshot().writeTo(out);
             out.writeByte(entry.state().value());
-            out.writeVInt(entry.indices().size());
+            out.writeStringCollection(entry.indices);
-            for (String index : entry.indices()) {
+            out.writeMap(entry.shards);
-                out.writeString(index);
-            }
-            out.writeVInt(entry.shards().size());
-            for (ObjectObjectCursor<ShardId, ShardRestoreStatus> shardEntry : entry.shards()) {
-                shardEntry.key.writeTo(out);
-                shardEntry.value.writeTo(out);
-            }
         }
     }
 

server/src/main/java/org/elasticsearch/cluster/SnapshotsInProgress.java

@@ -29,6 +29,7 @@ import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.collect.ImmutableOpenMap;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.common.xcontent.ToXContent;
 import org.elasticsearch.common.xcontent.XContentBuilder;
@@ -39,7 +40,6 @@ import org.elasticsearch.snapshots.Snapshot;
 import org.elasticsearch.snapshots.SnapshotsService;
 
 import java.io.IOException;
-import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
@@ -88,7 +88,7 @@ public class SnapshotsInProgress extends AbstractNamedDiffable<Custom> implement
         return builder.append("]").toString();
     }
 
-    public static class Entry implements ToXContent, RepositoryOperation {
+    public static class Entry implements Writeable, ToXContent, RepositoryOperation {
         private final State state;
         private final Snapshot snapshot;
         private final boolean includeGlobalState;
@@ -122,6 +122,38 @@ public class SnapshotsInProgress extends AbstractNamedDiffable<Custom> implement
             this.version = version;
         }
 
+        private Entry(StreamInput in) throws IOException {
+            snapshot = new Snapshot(in);
+            includeGlobalState = in.readBoolean();
+            partial = in.readBoolean();
+            state = State.fromValue(in.readByte());
+            indices = in.readList(IndexId::new);
+            startTime = in.readLong();
+            shards = in.readImmutableMap(ShardId::new, ShardSnapshotStatus::new);
+            repositoryStateId = in.readLong();
+            failure = in.readOptionalString();
+            if (in.getVersion().onOrAfter(METADATA_FIELD_INTRODUCED)) {
+                userMetadata = in.readMap();
+            } else {
+                userMetadata = null;
+            }
+            if (in.getVersion().onOrAfter(VERSION_IN_SNAPSHOT_VERSION)) {
+                version = Version.readVersion(in);
+            } else if (in.getVersion().onOrAfter(SnapshotsService.SHARD_GEN_IN_REPO_DATA_VERSION)) {
+                // If an older master informs us that shard generations are supported we use the minimum shard generation compatible
+                // version. If shard generations are not supported yet we use a placeholder for a version that does not use shard
+                // generations.
+                version = in.readBoolean() ? SnapshotsService.SHARD_GEN_IN_REPO_DATA_VERSION : SnapshotsService.OLD_SNAPSHOT_FORMAT;
+            } else {
+                version = SnapshotsService.OLD_SNAPSHOT_FORMAT;
+            }
+            if (in.getVersion().onOrAfter(DATA_STREAMS_IN_SNAPSHOT)) {
+                dataStreams = in.readStringList();
+            } else {
+                dataStreams = Collections.emptyList();
+            }
+        }
+
         private static boolean assertShardsConsistent(State state, List<IndexId> indices,
                                                       ImmutableOpenMap<ShardId, ShardSnapshotStatus> shards) {
             if ((state == State.INIT || state == State.ABORTED) && shards.isEmpty()) {
@@ -316,6 +348,30 @@ public class SnapshotsInProgress extends AbstractNamedDiffable<Custom> implement
             return builder;
         }
 
+        @Override
+        public void writeTo(StreamOutput out) throws IOException {
+            snapshot.writeTo(out);
+            out.writeBoolean(includeGlobalState);
+            out.writeBoolean(partial);
+            out.writeByte(state.value());
+            out.writeList(indices);
+            out.writeLong(startTime);
+            out.writeMap(shards);
+            out.writeLong(repositoryStateId);
+            out.writeOptionalString(failure);
+            if (out.getVersion().onOrAfter(METADATA_FIELD_INTRODUCED)) {
+                out.writeMap(userMetadata);
+            }
+            if (out.getVersion().onOrAfter(VERSION_IN_SNAPSHOT_VERSION)) {
+                Version.writeVersion(version, out);
+            } else if (out.getVersion().onOrAfter(SnapshotsService.SHARD_GEN_IN_REPO_DATA_VERSION)) {
+                out.writeBoolean(SnapshotsService.useShardGenerations(version));
+            }
+            if (out.getVersion().onOrAfter(DATA_STREAMS_IN_SNAPSHOT)) {
+                out.writeStringCollection(dataStreams);
+            }
+        }
+
         @Override
         public boolean isFragment() {
             return false;
@@ -337,7 +393,7 @@ public class SnapshotsInProgress extends AbstractNamedDiffable<Custom> implement
         return true;
     }
 
-    public static class ShardSnapshotStatus {
+    public static class ShardSnapshotStatus implements Writeable {
 
         /**
          * Shard snapshot status for shards that are waiting for another operation to finish before they can be assigned to a node.
@@ -417,6 +473,7 @@ public class SnapshotsInProgress extends AbstractNamedDiffable<Custom> implement
             return state == ShardState.INIT || state == ShardState.ABORTED || state == ShardState.WAITING;
         }
 
+        @Override
         public void writeTo(StreamOutput out) throws IOException {
             out.writeOptionalString(nodeId);
             out.writeByte(state.value);
@@ -548,114 +605,12 @@ public class SnapshotsInProgress extends AbstractNamedDiffable<Custom> implement
     }
 
     public SnapshotsInProgress(StreamInput in) throws IOException {
-        Entry[] entries = new Entry[in.readVInt()];
+        this.entries = in.readList(SnapshotsInProgress.Entry::new);
-        for (int i = 0; i < entries.length; i++) {
-            Snapshot snapshot = new Snapshot(in);
-            boolean includeGlobalState = in.readBoolean();
-            boolean partial = in.readBoolean();
-            State state = State.fromValue(in.readByte());
-            List<IndexId> indexBuilder = in.readList(IndexId::new);
-            long startTime = in.readLong();
-            int shards = in.readVInt();
-            ImmutableOpenMap.Builder<ShardId, ShardSnapshotStatus> builder = ImmutableOpenMap.builder(shards);
-            for (int j = 0; j < shards; j++) {
-                ShardId shardId = new ShardId(in);
-                if (in.getVersion().onOrAfter(Version.V_6_0_0_beta1)) {
-                    builder.put(shardId, new ShardSnapshotStatus(in));
-                } else {
-                    String nodeId = in.readOptionalString();
-                    ShardState shardState = ShardState.fromValue(in.readByte());
-                    // Workaround for https://github.com/elastic/elasticsearch/issues/25878
-                    // Some old snapshot might still have null in shard failure reasons
-                    String reason = shardState.failed() ? "" : null;
-                    builder.put(shardId, new ShardSnapshotStatus(nodeId, shardState, reason));
-                }
-            }
-            long repositoryStateId = in.readLong();
-            final String failure;
-            if (in.getVersion().onOrAfter(Version.V_6_7_0)) {
-                failure = in.readOptionalString();
-            } else {
-                failure = null;
-            }
-            Map<String, Object> userMetadata = null;
-            if (in.getVersion().onOrAfter(METADATA_FIELD_INTRODUCED)) {
-                userMetadata = in.readMap();
-            }
-            final Version version;
-            if (in.getVersion().onOrAfter(VERSION_IN_SNAPSHOT_VERSION)) {
-                version = Version.readVersion(in);
-            } else if (in.getVersion().onOrAfter(SnapshotsService.SHARD_GEN_IN_REPO_DATA_VERSION)) {
-                // If an older master informs us that shard generations are supported we use the minimum shard generation compatible
-                // version. If shard generations are not supported yet we use a placeholder for a version that does not use shard
-                // generations.
-                version = in.readBoolean() ? SnapshotsService.SHARD_GEN_IN_REPO_DATA_VERSION : SnapshotsService.OLD_SNAPSHOT_FORMAT;
-            } else {
-                version = SnapshotsService.OLD_SNAPSHOT_FORMAT;
-            }
-
-            List<String> dataStreams;
-            if (in.getVersion().onOrAfter(DATA_STREAMS_IN_SNAPSHOT)) {
-                dataStreams = in.readStringList();
-            } else {
-                dataStreams = Collections.emptyList();
-            }
-            entries[i] = new Entry(snapshot,
-                                   includeGlobalState,
-                                   partial,
-                                   state,
-                                   Collections.unmodifiableList(indexBuilder),
-                                   dataStreams,
-                                   startTime,
-                                   repositoryStateId,
-                                   builder.build(),
-                                   failure,
-                                   userMetadata,
-                                   version
-                );
-        }
-        this.entries = Arrays.asList(entries);
     }
 
     @Override
     public void writeTo(StreamOutput out) throws IOException {
-        out.writeVInt(entries.size());
+        out.writeList(entries);
-        for (Entry entry : entries) {
-            entry.snapshot().writeTo(out);
-            out.writeBoolean(entry.includeGlobalState());
-            out.writeBoolean(entry.partial());
-            out.writeByte(entry.state().value());
-            out.writeVInt(entry.indices().size());
-            for (IndexId index : entry.indices()) {
-                index.writeTo(out);
-            }
-            out.writeLong(entry.startTime());
-            out.writeVInt(entry.shards().size());
-            for (ObjectObjectCursor<ShardId, ShardSnapshotStatus> shardEntry : entry.shards()) {
-                shardEntry.key.writeTo(out);
-                if (out.getVersion().onOrAfter(Version.V_6_0_0_beta1)) {
-                    shardEntry.value.writeTo(out);
-                } else {
-                    out.writeOptionalString(shardEntry.value.nodeId());
-                    out.writeByte(shardEntry.value.state().value);
-                }
-            }
-            out.writeLong(entry.repositoryStateId);
-            if (out.getVersion().onOrAfter(Version.V_6_7_0)) {
-                out.writeOptionalString(entry.failure);
-            }
-            if (out.getVersion().onOrAfter(METADATA_FIELD_INTRODUCED)) {
-                out.writeMap(entry.userMetadata);
-            }
-            if (out.getVersion().onOrAfter(VERSION_IN_SNAPSHOT_VERSION)) {
-                Version.writeVersion(entry.version, out);
-            } else if (out.getVersion().onOrAfter(SnapshotsService.SHARD_GEN_IN_REPO_DATA_VERSION)) {
-                out.writeBoolean(SnapshotsService.useShardGenerations(entry.version));
-            }
-            if (out.getVersion().onOrAfter(DATA_STREAMS_IN_SNAPSHOT)) {
-                out.writeStringCollection(entry.dataStreams);
-            }
-        }
     }
 
     private static final String REPOSITORY = "repository";

server/src/main/java/org/elasticsearch/cluster/metadata/RepositoriesMetadata.java

@@ -36,7 +36,6 @@ import org.elasticsearch.repositories.RepositoryData;
 
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.List;
@@ -165,11 +164,7 @@ public class RepositoriesMetadata extends AbstractNamedDiffable<Custom> implemen
     }
 
     public RepositoriesMetadata(StreamInput in) throws IOException {
-        RepositoryMetadata[] repository = new RepositoryMetadata[in.readVInt()];
+        this.repositories = in.readList(RepositoryMetadata::new);
-        for (int i = 0; i < repository.length; i++) {
-            repository[i] = new RepositoryMetadata(in);
-        }
-        this.repositories = Collections.unmodifiableList(Arrays.asList(repository));
     }
 
     public static NamedDiff<Custom> readDiffFrom(StreamInput in) throws  IOException {
@@ -181,10 +176,7 @@ public class RepositoriesMetadata extends AbstractNamedDiffable<Custom> implemen
      */
     @Override
     public void writeTo(StreamOutput out) throws IOException {
-        out.writeVInt(repositories.size());
+        out.writeList(repositories);
-        for (RepositoryMetadata repository : repositories) {
-            repository.writeTo(out);
-        }
     }
 
     public static RepositoriesMetadata fromXContent(XContentParser parser) throws IOException {

server/src/main/java/org/elasticsearch/cluster/metadata/RepositoryMetadata.java

@@ -21,6 +21,7 @@ package org.elasticsearch.cluster.metadata;
 import org.elasticsearch.Version;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.repositories.RepositoryData;
 
@@ -30,7 +31,7 @@ import java.util.Objects;
 /**
  * Metadata about registered repository
  */
-public class RepositoryMetadata {
+public class RepositoryMetadata implements Writeable {
 
     public static final Version REPO_GEN_IN_CS_VERSION = Version.V_7_6_0;
 
@@ -142,6 +143,7 @@ public class RepositoryMetadata {
      *
      * @param out stream output
      */
+    @Override
     public void writeTo(StreamOutput out) throws IOException {
         out.writeString(name);
         out.writeString(type);

server/src/main/java/org/elasticsearch/common/io/stream/StreamInput.java

@@ -35,6 +35,7 @@ import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
+import org.elasticsearch.common.collect.ImmutableOpenMap;
 import org.elasticsearch.common.geo.GeoPoint;
 import org.elasticsearch.common.settings.SecureString;
 import org.elasticsearch.common.text.Text;
@@ -678,6 +679,25 @@ public abstract class StreamInput extends InputStream {
         return (Map<String, Object>) readGenericValue();
     }
 
+    /**
+     * Read {@link ImmutableOpenMap} using given key and value readers.
+     *
+     * @param keyReader   key reader
+     * @param valueReader value reader
+     */
+    public <K, V> ImmutableOpenMap<K, V> readImmutableMap(Writeable.Reader<K> keyReader, Writeable.Reader<V> valueReader)
+            throws IOException {
+        final int size = readVInt();
+        if (size == 0) {
+            return ImmutableOpenMap.of();
+        }
+        final ImmutableOpenMap.Builder<K,V> builder = ImmutableOpenMap.builder(size);
+        for (int i = 0; i < size; i++) {
+            builder.put(keyReader.read(this), valueReader.read(this));
+        }
+        return builder.build();
+    }
+
     /**
      * Reads a value of unspecified type. If a collection is read then the collection will be mutable if it contains any entry but might
      * be immutable if it is empty.

server/src/main/java/org/elasticsearch/common/io/stream/StreamOutput.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch.common.io.stream;
 
+import com.carrotsearch.hppc.cursors.ObjectObjectCursor;
 import org.apache.lucene.index.CorruptIndexException;
 import org.apache.lucene.index.IndexFormatTooNewException;
 import org.apache.lucene.index.IndexFormatTooOldException;
@@ -35,6 +36,7 @@ import org.elasticsearch.common.CharArrays;
 import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
+import org.elasticsearch.common.collect.ImmutableOpenMap;
 import org.elasticsearch.common.geo.GeoPoint;
 import org.elasticsearch.common.io.stream.Writeable.Writer;
 import org.elasticsearch.common.settings.SecureString;
@@ -628,6 +630,28 @@ public abstract class StreamOutput extends OutputStream {
         }
     }
 
+    /**
+     * Write a {@link ImmutableOpenMap} of {@code K}-type keys to {@code V}-type.
+     *
+     * @param keyWriter The key writer
+     * @param valueWriter The value writer
+     */
+    public final <K, V> void writeMap(final ImmutableOpenMap<K, V> map, final Writer<K> keyWriter, final Writer<V> valueWriter)
+            throws IOException {
+        writeVInt(map.size());
+        for (final ObjectObjectCursor<K, V> entry : map) {
+            keyWriter.write(this, entry.key);
+            valueWriter.write(this, entry.value);
+        }
+    }
+
+    /**
+     * Write a {@link ImmutableOpenMap} of {@code K}-type keys to {@code V}-type.
+     */
+    public final <K extends Writeable, V extends Writeable> void writeMap(final ImmutableOpenMap<K, V> map) throws IOException {
+        writeMap(map, (o, k) -> k.writeTo(o), (o, v) -> v.writeTo(o));
+    }
+
     /**
      * Writes an {@link Instant} to the stream with nanosecond resolution
      */

server/src/main/java/org/elasticsearch/snapshots/RestoreInfo.java

@@ -30,7 +30,6 @@ import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.rest.RestStatus;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
@@ -61,12 +60,7 @@ public class RestoreInfo implements ToXContentObject, Writeable {
 
     public RestoreInfo(StreamInput in) throws IOException {
         name = in.readString();
-        int size = in.readVInt();
+        indices = Collections.unmodifiableList(in.readStringList());
-        List<String> indicesListBuilder = new ArrayList<>();
-        for (int i = 0; i < size; i++) {
-            indicesListBuilder.add(in.readString());
-        }
-        indices = Collections.unmodifiableList(indicesListBuilder);
         totalShards = in.readVInt();
         successfulShards = in.readVInt();
     }
@@ -173,10 +167,7 @@ public class RestoreInfo implements ToXContentObject, Writeable {
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         out.writeString(name);
-        out.writeVInt(indices.size());
+        out.writeStringCollection(indices);
-        for (String index : indices) {
-            out.writeString(index);
-        }
         out.writeVInt(totalShards);
         out.writeVInt(successfulShards);
     }

server/src/test/java/org/elasticsearch/common/io/stream/BytesStreamsTests.java

@@ -24,6 +24,7 @@ import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.Constants;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
+import org.elasticsearch.common.collect.ImmutableOpenMap;
 import org.elasticsearch.common.geo.GeoPoint;
 import org.elasticsearch.common.lucene.BytesRefs;
 import org.elasticsearch.common.unit.TimeValue;
@@ -498,6 +499,38 @@ public class BytesStreamsTests extends ESTestCase {
         assertThat(expected, equalTo(loaded));
     }
 
+    public void testWriteImmutableMap() throws IOException {
+        final int size = randomIntBetween(0, 100);
+        final ImmutableOpenMap.Builder<String, String> expectedBuilder = ImmutableOpenMap.builder(randomIntBetween(0, 100));
+        for (int i = 0; i < size; ++i) {
+            expectedBuilder.put(randomAlphaOfLength(2), randomAlphaOfLength(5));
+        }
+
+        final ImmutableOpenMap<String, String> expected = expectedBuilder.build();
+        final BytesStreamOutput out = new BytesStreamOutput();
+        out.writeMap(expected, StreamOutput::writeString, StreamOutput::writeString);
+        final StreamInput in = StreamInput.wrap(BytesReference.toBytes(out.bytes()));
+        final ImmutableOpenMap<String, String> loaded = in.readImmutableMap(StreamInput::readString, StreamInput::readString);
+
+        assertThat(expected, equalTo(loaded));
+    }
+
+    public void testWriteImmutableMapOfWritable() throws IOException {
+        final int size = randomIntBetween(0, 100);
+        final ImmutableOpenMap.Builder<TestWriteable, TestWriteable> expectedBuilder = ImmutableOpenMap.builder(randomIntBetween(0, 100));
+        for (int i = 0; i < size; ++i) {
+            expectedBuilder.put(new TestWriteable(randomBoolean()), new TestWriteable(randomBoolean()));
+        }
+
+        final ImmutableOpenMap<TestWriteable, TestWriteable> expected = expectedBuilder.build();
+        final BytesStreamOutput out = new BytesStreamOutput();
+        out.writeMap(expected);
+        final StreamInput in = StreamInput.wrap(BytesReference.toBytes(out.bytes()));
+        final ImmutableOpenMap<TestWriteable, TestWriteable> loaded = in.readImmutableMap(TestWriteable::new, TestWriteable::new);
+
+        assertThat(expected, equalTo(loaded));
+    }
+
     public void testWriteMapOfLists() throws IOException {
         final int size = randomIntBetween(0, 5);
         final Map<String, List<String>> expected = new HashMap<>(size);
@@ -628,6 +661,16 @@ public class BytesStreamsTests extends ESTestCase {
         public void writeTo(StreamOutput out) throws IOException {
             out.writeBoolean(value);
         }
+
+        @Override
+        public boolean equals(Object o) {
+            return o instanceof TestWriteable && value == ((TestWriteable) o).value;
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(value);
+        }
     }
 
     public void testWriteMapWithConsistentOrder() throws IOException {