Track Shard-Snapshot Index Generation at Repository Root (#46250)

### Changes to Root-Level index-N (RepositoryData)

This change adds a new field `"shards"` to `RepositoryData` that contains a mapping of `IndexId` to a `String[]`. This string array can be accessed by shard id to get the generation of a shard's shard folder (i.e. the `N` in the name of the currently valid `/indices/${indexId}/${shardId}/index-${N}` for the shard in question).

### Benefits

This allows for creating a new snapshot in the shard without doing any LIST operations on the shard's folder. In the case of AWS S3, this saves about 1/3 of the cost for updating an empty shard (see #45736) and removes one out of two remaining potential issues with eventually consistent blob stores (see #38941 ... now only the root `index-${N}` is determined by listing).

Also and equally if not more important, a number of possible failure modes on eventually consistent blob stores like AWS S3 are eliminated by moving all delete operations to the `master` node and moving from incremental naming of shard level index-N to uuid suffixes for these blobs.

### Only Master Deletes Blobs

This change moves the deleting of the previous shard level `index-${uuid}` blob to the master node instead of the data node allowing for a safe and consistent update of the shard's generation in the `RepositoryData` by first updating `RepositoryData` and then deleting the now unreferenced `index-${newUUID}` blob.
__No deletes are executed on the data nodes at all for any operation with this change.__

Note also: Previous issues with hanging data nodes interfering with master nodes are completely impossible, even on S3 (see next section for details).

### Why Move from index-${N} to index-${uuid} at the Shard Level

This change changes the naming of the shard level `index-${N}` blobs to a uuid suffix `index-${UUID}`. The reason for this is the fact that writing a new shard-level `index-` generation blob is not atomic anymore in its effect. Not only does the blob have to be written to have an effect, it must also be referenced by the root level `index-N` (`RepositoryData`) to become an effective part of the snapshot repository.
This leads to a problem if we were to use incrementing names like we did before. If a blob `index-${N+1}` is written but due to the node/network/cluster/... crashes the root level `RepositoryData` has not been updated then a future operation will determine the shard's generation to be `N` and try to write a new `index-${N+1}` to the already existing path. Updates like that are problematic on S3 for consistency reasons, but also create numerous issues when thinking about stuck data nodes.
Previously stuck data nodes that were tasked to write `index-${N+1}` but got stuck and tried to do so after some other node had already written `index-${N+1}` were prevented form doing so (except for on S3) by us not allowing overwrites for that blob and thus no corruption could occur.
Were we to continue using incrementing names, we could not do this. The stuck node scenario would either allow for overwriting the `N+1` generation or force us to continue using a `LIST` operation to figure out the next `N` (which would make this change pointless).
With uuid naming and moving all deletes to `master` this becomes a non-issue. Data nodes write updated shard generation `index-${uuid}` and `master` makes those `index-${uuid}` part of the `RepositoryData` that it deems correct and cleans up all those `index-` that are unused.

Co-authored-by: Yannick Welsch <[email protected]>
Co-authored-by: Tanguy Leroux <[email protected]>

Author: 3 people

server/src/main/java/org/elasticsearch/action/admin/cluster/repositories/cleanup/TransportCleanupRepositoryAction.java

@@ -42,6 +42,7 @@
 import org.elasticsearch.repositories.Repository;
 import org.elasticsearch.repositories.RepositoryCleanupResult;
 import org.elasticsearch.repositories.blobstore.BlobStoreRepository;
+import org.elasticsearch.snapshots.SnapshotsService;
 import org.elasticsearch.tasks.Task;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.transport.TransportService;
@@ -201,7 +202,9 @@ public void clusterStateProcessed(String source, ClusterState oldState, ClusterS
                     logger.debug("Initialized repository cleanup in cluster state for [{}][{}]", repositoryName, repositoryStateId);
                     threadPool.executor(ThreadPool.Names.SNAPSHOT).execute(ActionRunnable.wrap(listener,
                         l -> blobStoreRepository.cleanup(
-                            repositoryStateId, ActionListener.wrap(result -> after(null, result), e -> after(e, null)))));
+                            repositoryStateId,
+                            newState.nodes().getMinNodeVersion().onOrAfter(SnapshotsService.SHARD_GEN_IN_REPO_DATA_VERSION),
+                            ActionListener.wrap(result -> after(null, result), e -> after(e, null)))));
                 }
 
                 private void after(@Nullable Exception failure, @Nullable RepositoryCleanupResult result) {

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

@@ -91,12 +91,14 @@ public static class Entry implements ToXContent {
         private final ImmutableOpenMap<String, List<ShardId>> waitingIndices;
         private final long startTime;
         private final long repositoryStateId;
+        // see #useShardGenerations
+        private final boolean useShardGenerations;
         @Nullable private final Map<String, Object> userMetadata;
         @Nullable private final String failure;
 
         public Entry(Snapshot snapshot, boolean includeGlobalState, boolean partial, State state, List<IndexId> indices,
                      long startTime, long repositoryStateId, ImmutableOpenMap<ShardId, ShardSnapshotStatus> shards,
-                     String failure, Map<String, Object> userMetadata) {
+                     String failure, Map<String, Object> userMetadata, boolean useShardGenerations) {
             this.state = state;
             this.snapshot = snapshot;
             this.includeGlobalState = includeGlobalState;
@@ -114,6 +116,7 @@ public Entry(Snapshot snapshot, boolean includeGlobalState, boolean partial, Sta
             this.repositoryStateId = repositoryStateId;
             this.failure = failure;
             this.userMetadata = userMetadata;
+            this.useShardGenerations = useShardGenerations;
         }
 
         private static boolean assertShardsConsistent(State state, List<IndexId> indices,
@@ -128,20 +131,22 @@ private static boolean assertShardsConsistent(State state, List<IndexId> indices
                 : "Indices in shards " + indexNamesInShards + " differ from expected indices " + indexNames + " for state [" + state + "]";
             return true;
         }
+
         public Entry(Snapshot snapshot, boolean includeGlobalState, boolean partial, State state, List<IndexId> indices,
                      long startTime, long repositoryStateId, ImmutableOpenMap<ShardId, ShardSnapshotStatus> shards,
-                     Map<String, Object> userMetadata) {
-            this(snapshot, includeGlobalState, partial, state, indices, startTime, repositoryStateId, shards, null, userMetadata);
+                     Map<String, Object> userMetadata, boolean useShardGenerations) {
+            this(snapshot, includeGlobalState, partial, state, indices, startTime, repositoryStateId, shards, null, userMetadata,
+                useShardGenerations);
         }
 
         public Entry(Entry entry, State state, ImmutableOpenMap<ShardId, ShardSnapshotStatus> shards) {
             this(entry.snapshot, entry.includeGlobalState, entry.partial, state, entry.indices, entry.startTime,
-                entry.repositoryStateId, shards, entry.failure, entry.userMetadata);
+                entry.repositoryStateId, shards, entry.failure, entry.userMetadata, entry.useShardGenerations);
         }
 
         public Entry(Entry entry, State state, ImmutableOpenMap<ShardId, ShardSnapshotStatus> shards, String failure) {
             this(entry.snapshot, entry.includeGlobalState, entry.partial, state, entry.indices, entry.startTime,
-                 entry.repositoryStateId, shards, failure, entry.userMetadata);
+                 entry.repositoryStateId, shards, failure, entry.userMetadata, entry.useShardGenerations);
         }
 
         public Entry(Entry entry, ImmutableOpenMap<ShardId, ShardSnapshotStatus> shards) {
@@ -192,6 +197,16 @@ public String failure() {
             return failure;
         }
 
+        /**
+         * Whether to write to the repository in a format only understood by versions newer than
+         * {@link SnapshotsService#SHARD_GEN_IN_REPO_DATA_VERSION}.
+         *
+         * @return true if writing to repository in new format
+         */
+        public boolean useShardGenerations() {
+            return useShardGenerations;
+        }
+
         @Override
         public boolean equals(Object o) {
             if (this == o) return true;
@@ -207,6 +222,7 @@ public boolean equals(Object o) {
             if (!snapshot.equals(entry.snapshot)) return false;
             if (state != entry.state) return false;
             if (repositoryStateId != entry.repositoryStateId) return false;
+            if (useShardGenerations != entry.useShardGenerations) return false;
 
             return true;
         }
@@ -221,6 +237,7 @@ public int hashCode() {
             result = 31 * result + indices.hashCode();
             result = 31 * result + Long.hashCode(startTime);
             result = 31 * result + Long.hashCode(repositoryStateId);
+            result = 31 * result + (useShardGenerations ? 1 : 0);
             return result;
         }
 
@@ -503,6 +520,12 @@ public SnapshotsInProgress(StreamInput in) throws IOException {
             if (in.getVersion().onOrAfter(METADATA_FIELD_INTRODUCED)) {
                 userMetadata = in.readMap();
             }
+            final boolean useShardGenerations;
+            if (in.getVersion().onOrAfter(SnapshotsService.SHARD_GEN_IN_REPO_DATA_VERSION)) {
+                useShardGenerations = in.readBoolean();
+            } else {
+                useShardGenerations = false;
+            }
             entries[i] = new Entry(snapshot,
                                    includeGlobalState,
                                    partial,
@@ -512,7 +535,8 @@ public SnapshotsInProgress(StreamInput in) throws IOException {
                                    repositoryStateId,
                                    builder.build(),
                                    failure,
-                                   userMetadata
+                                   userMetadata,
+                                   useShardGenerations
                 );
         }
         this.entries = Arrays.asList(entries);
@@ -541,6 +565,9 @@ public void writeTo(StreamOutput out) throws IOException {
             if (out.getVersion().onOrAfter(METADATA_FIELD_INTRODUCED)) {
                 out.writeMap(entry.userMetadata);
             }
+            if (out.getVersion().onOrAfter(SnapshotsService.SHARD_GEN_IN_REPO_DATA_VERSION)) {
+                out.writeBoolean(entry.useShardGenerations);
+            }
         }
     }
 

server/src/main/java/org/elasticsearch/repositories/FilterRepository.java

@@ -73,16 +73,17 @@ public RepositoryData getRepositoryData() {
     }
 
     @Override
-    public void finalizeSnapshot(SnapshotId snapshotId, List<IndexId> indices, long startTime, String failure, int totalShards,
-                                 List<SnapshotShardFailure> shardFailures, long repositoryStateId, boolean includeGlobalState,
-                                 MetaData metaData, Map<String, Object> userMetadata, ActionListener<SnapshotInfo> listener) {
-        in.finalizeSnapshot(snapshotId, indices, startTime, failure, totalShards, shardFailures, repositoryStateId,
-            includeGlobalState, metaData, userMetadata, listener);
+    public void finalizeSnapshot(SnapshotId snapshotId, ShardGenerations shardGenerations, long startTime, String failure,
+                                 int totalShards, List<SnapshotShardFailure> shardFailures, long repositoryStateId,
+                                 boolean includeGlobalState, MetaData metaData, Map<String, Object> userMetadata,
+                                 boolean writeShardGens, ActionListener<SnapshotInfo> listener) {
+        in.finalizeSnapshot(snapshotId, shardGenerations, startTime, failure, totalShards, shardFailures, repositoryStateId,
+            includeGlobalState, metaData, userMetadata, writeShardGens, listener);
     }
 
     @Override
-    public void deleteSnapshot(SnapshotId snapshotId, long repositoryStateId, ActionListener<Void> listener) {
-        in.deleteSnapshot(snapshotId, repositoryStateId, listener);
+    public void deleteSnapshot(SnapshotId snapshotId, long repositoryStateId, boolean writeShardGens, ActionListener<Void> listener) {
+        in.deleteSnapshot(snapshotId, repositoryStateId, writeShardGens, listener);
     }
 
     @Override
@@ -117,8 +118,9 @@ public boolean isReadOnly() {
 
     @Override
     public void snapshotShard(Store store, MapperService mapperService, SnapshotId snapshotId, IndexId indexId,
-                              IndexCommit snapshotIndexCommit, IndexShardSnapshotStatus snapshotStatus, ActionListener<String> listener) {
-        in.snapshotShard(store, mapperService, snapshotId, indexId, snapshotIndexCommit, snapshotStatus, listener);
+                              IndexCommit snapshotIndexCommit, IndexShardSnapshotStatus snapshotStatus, boolean writeShardGens,
+                              ActionListener<String> listener) {
+        in.snapshotShard(store, mapperService, snapshotId, indexId, snapshotIndexCommit, snapshotStatus, writeShardGens, listener);
     }
     @Override
     public void restoreShard(Store store, SnapshotId snapshotId, IndexId indexId, ShardId snapshotShardId, RecoveryState recoveryState) {

server/src/main/java/org/elasticsearch/repositories/Repository.java

@@ -112,28 +112,33 @@ default Repository create(RepositoryMetaData metaData, Function<String, Reposito
      * <p>
      * This method is called on master after all shards are snapshotted.
      *
-     * @param snapshotId    snapshot id
-     * @param indices       list of indices in the snapshot
-     * @param startTime     start time of the snapshot
-     * @param failure       global failure reason or null
-     * @param totalShards   total number of shards
-     * @param shardFailures list of shard failures
-     * @param repositoryStateId the unique id identifying the state of the repository when the snapshot began
+     * @param snapshotId         snapshot id
+     * @param shardGenerations   updated shard generations
+     * @param startTime          start time of the snapshot
+     * @param failure            global failure reason or null
+     * @param totalShards        total number of shards
+     * @param shardFailures      list of shard failures
+     * @param repositoryStateId  the unique id identifying the state of the repository when the snapshot began
      * @param includeGlobalState include cluster global state
+     * @param clusterMetaData    cluster metadata
+     * @param userMetadata       user metadata
+     * @param writeShardGens     if shard generations should be written to the repository
      * @param listener listener to be called on completion of the snapshot
      */
-    void finalizeSnapshot(SnapshotId snapshotId, List<IndexId> indices, long startTime, String failure, int totalShards,
-                          List<SnapshotShardFailure> shardFailures, long repositoryStateId, boolean includeGlobalState,
-                          MetaData clusterMetaData, Map<String, Object> userMetadata, ActionListener<SnapshotInfo> listener);
+    void finalizeSnapshot(SnapshotId snapshotId, ShardGenerations shardGenerations, long startTime, String failure,
+                          int totalShards, List<SnapshotShardFailure> shardFailures, long repositoryStateId,
+                          boolean includeGlobalState, MetaData clusterMetaData, Map<String, Object> userMetadata,
+                          boolean writeShardGens, ActionListener<SnapshotInfo> listener);
 
     /**
      * Deletes snapshot
      *
-     * @param snapshotId snapshot id
+     * @param snapshotId        snapshot id
      * @param repositoryStateId the unique id identifying the state of the repository when the snapshot deletion began
-     * @param listener completion listener
+     * @param writeShardGens    if shard generations should be written to the repository
+     * @param listener          completion listener
      */
-    void deleteSnapshot(SnapshotId snapshotId, long repositoryStateId, ActionListener<Void> listener);
+    void deleteSnapshot(SnapshotId snapshotId, long repositoryStateId, boolean writeShardGens, ActionListener<Void> listener);
 
     /**
      * Returns snapshot throttle time in nanoseconds
@@ -195,7 +200,7 @@ void finalizeSnapshot(SnapshotId snapshotId, List<IndexId> indices, long startTi
      * @param listener            listener invoked on completion
      */
     void snapshotShard(Store store, MapperService mapperService, SnapshotId snapshotId, IndexId indexId, IndexCommit snapshotIndexCommit,
-                       IndexShardSnapshotStatus snapshotStatus, ActionListener<String> listener);
+                       IndexShardSnapshotStatus snapshotStatus, boolean writeShardGens, ActionListener<String> listener);
 
     /**
      * Restores snapshot of the shard.