Enhances get snapshots API to allow retrieving repository index only (#24477)

Currently, the get snapshots API (e.g. /_snapshot/{repositoryName}/_all)
provides information about snapshots in the repository, including the
snapshot state, number of shards snapshotted, failures, etc.  In order
to provide information about each snapshot in the repository, the call
must read the snapshot metadata blob (`snap-{snapshot_uuid}.dat`) for
every snapshot.  In cloud-based repositories, this can be expensive,
both from a cost and performance perspective.  Sometimes, all the user
wants is to retrieve all the names/uuids of each snapshot, and the
indices that went into each snapshot, without any of the other status
information about the snapshot.  This minimal information can be
retrieved from the repository index blob (`index-N`) without needing to
read each snapshot metadata blob.

This commit enhances the get snapshots API with an optional `verbose`
parameter.  If `verbose` is set to false on the request, then the get
snapshots API will only retrieve the minimal information about each
snapshot (the name, uuid, and indices in the snapshot), and only read
this information from the repository index blob, thereby giving users
the option to retrieve the snapshots in a repository in a more
cost-effective and efficient manner.

Closes #24288

Author: Ali Beyad

core/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/get/GetSnapshotsRequest.java

@@ -28,6 +28,7 @@
 import java.io.IOException;
 
 import static org.elasticsearch.action.ValidateActions.addValidationError;
+import static org.elasticsearch.snapshots.SnapshotInfo.VERBOSE_INTRODUCED;
 
 /**
  * Get snapshot request
@@ -43,6 +44,8 @@ public class GetSnapshotsRequest extends MasterNodeRequest<GetSnapshotsRequest>
 
     private boolean ignoreUnavailable;
 
+    private boolean verbose = true;
+
     public GetSnapshotsRequest() {
     }
 
@@ -123,19 +126,44 @@ public GetSnapshotsRequest ignoreUnavailable(boolean ignoreUnavailable) {
         this.ignoreUnavailable = ignoreUnavailable;
         return this;
     }
+
     /**
      * @return Whether snapshots should be ignored when unavailable (corrupt or temporarily not fetchable)
      */
     public boolean ignoreUnavailable() {
         return ignoreUnavailable;
     }
 
+    /**
+     * Set to {@code false} to only show the snapshot names and the indices they contain.
+     * This is useful when the snapshots belong to a cloud-based repository where each
+     * blob read is a concern (cost wise and performance wise), as the snapshot names and
+     * indices they contain can be retrieved from a single index blob in the repository,
+     * whereas the rest of the information requires reading a snapshot metadata file for
+     * each snapshot requested.  Defaults to {@code true}, which returns all information
+     * about each requested snapshot.
+     */
+    public GetSnapshotsRequest verbose(boolean verbose) {
+        this.verbose = verbose;
+        return this;
+    }
+
+    /**
+     * Returns whether the request will return a verbose response.
+     */
+    public boolean verbose() {
+        return verbose;
+    }
+
     @Override
     public void readFrom(StreamInput in) throws IOException {
         super.readFrom(in);
         repository = in.readString();
         snapshots = in.readStringArray();
         ignoreUnavailable = in.readBoolean();
+        if (in.getVersion().onOrAfter(VERBOSE_INTRODUCED)) {
+            verbose = in.readBoolean();
+        }
     }
 
     @Override
@@ -144,5 +172,8 @@ public void writeTo(StreamOutput out) throws IOException {
         out.writeString(repository);
         out.writeStringArray(snapshots);
         out.writeBoolean(ignoreUnavailable);
+        if (out.getVersion().onOrAfter(VERBOSE_INTRODUCED)) {
+            out.writeBoolean(verbose);
+        }
     }
 }

core/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/get/GetSnapshotsRequestBuilder.java

@@ -96,4 +96,18 @@ public GetSnapshotsRequestBuilder setIgnoreUnavailable(boolean ignoreUnavailable
         return this;
     }
 
+    /**
+     * Set to {@code false} to only show the snapshot names and the indices they contain.
+     * This is useful when the snapshots belong to a cloud-based repository where each
+     * blob read is a concern (cost wise and performance wise), as the snapshot names and
+     * indices they contain can be retrieved from a single index blob in the repository,
+     * whereas the rest of the information requires reading a snapshot metadata file for
+     * each snapshot requested.  Defaults to {@code true}, which returns all information
+     * about each requested snapshot.
+     */
+    public GetSnapshotsRequestBuilder setVerbose(boolean verbose) {
+        request.verbose(verbose);
+        return this;
+    }
+
 }

core/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/get/TransportGetSnapshotsAction.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch.action.admin.cluster.snapshots.get;
 
+import org.apache.lucene.util.CollectionUtil;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.support.ActionFilters;
 import org.elasticsearch.action.support.master.TransportMasterNodeAction;
@@ -30,6 +31,7 @@
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.regex.Regex;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.repositories.IndexId;
 import org.elasticsearch.repositories.RepositoryData;
 import org.elasticsearch.snapshots.SnapshotId;
 import org.elasticsearch.snapshots.SnapshotInfo;
@@ -39,11 +41,13 @@
 import org.elasticsearch.transport.TransportService;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.stream.Collectors;
 
 /**
  * Transport Action for get snapshots operation
@@ -76,31 +80,35 @@ protected ClusterBlockException checkBlock(GetSnapshotsRequest request, ClusterS
     }
 
     @Override
-    protected void masterOperation(final GetSnapshotsRequest request, ClusterState state,
+    protected void masterOperation(final GetSnapshotsRequest request, final ClusterState state,
                                    final ActionListener<GetSnapshotsResponse> listener) {
         try {
             final String repository = request.repository();
-            List<SnapshotInfo> snapshotInfoBuilder = new ArrayList<>();
             final Map<String, SnapshotId> allSnapshotIds = new HashMap<>();
-            final List<SnapshotId> currentSnapshotIds = new ArrayList<>();
-            final RepositoryData repositoryData  = snapshotsService.getRepositoryData(repository);
+            final List<SnapshotInfo> currentSnapshots = new ArrayList<>();
             for (SnapshotInfo snapshotInfo : snapshotsService.currentSnapshots(repository)) {
                 SnapshotId snapshotId = snapshotInfo.snapshotId();
                 allSnapshotIds.put(snapshotId.getName(), snapshotId);
-                currentSnapshotIds.add(snapshotId);
+                currentSnapshots.add(snapshotInfo);
             }
+
+            final RepositoryData repositoryData;
             if (isCurrentSnapshotsOnly(request.snapshots()) == false) {
+                repositoryData = snapshotsService.getRepositoryData(repository);
                 for (SnapshotId snapshotId : repositoryData.getAllSnapshotIds()) {
                     allSnapshotIds.put(snapshotId.getName(), snapshotId);
                 }
+            } else {
+                repositoryData = null;
             }
+
             final Set<SnapshotId> toResolve = new HashSet<>();
             if (isAllSnapshots(request.snapshots())) {
                 toResolve.addAll(allSnapshotIds.values());
             } else {
                 for (String snapshotOrPattern : request.snapshots()) {
                     if (GetSnapshotsRequest.CURRENT_SNAPSHOT.equalsIgnoreCase(snapshotOrPattern)) {
-                        toResolve.addAll(currentSnapshotIds);
+                        toResolve.addAll(currentSnapshots.stream().map(SnapshotInfo::snapshotId).collect(Collectors.toList()));
                     } else if (Regex.isSimpleMatchPattern(snapshotOrPattern) == false) {
                         if (allSnapshotIds.containsKey(snapshotOrPattern)) {
                             toResolve.add(allSnapshotIds.get(snapshotOrPattern));
@@ -121,9 +129,23 @@ protected void masterOperation(final GetSnapshotsRequest request, ClusterState s
                 }
             }
 
-            snapshotInfoBuilder.addAll(snapshotsService.snapshots(
-                repository, new ArrayList<>(toResolve), repositoryData.getIncompatibleSnapshotIds(), request.ignoreUnavailable()));
-            listener.onResponse(new GetSnapshotsResponse(snapshotInfoBuilder));
+            final List<SnapshotInfo> snapshotInfos;
+            if (request.verbose()) {
+                final Set<SnapshotId> incompatibleSnapshots = repositoryData != null ?
+                    new HashSet<>(repositoryData.getIncompatibleSnapshotIds()) : Collections.emptySet();
+                snapshotInfos = snapshotsService.snapshots(repository, new ArrayList<>(toResolve),
+                    incompatibleSnapshots, request.ignoreUnavailable());
+            } else {
+                if (repositoryData != null) {
+                    // want non-current snapshots as well, which are found in the repository data
+                    snapshotInfos = buildSimpleSnapshotInfos(toResolve, repositoryData, currentSnapshots);
+                } else {
+                    // only want current snapshots
+                    snapshotInfos = currentSnapshots.stream().map(SnapshotInfo::basic).collect(Collectors.toList());
+                    CollectionUtil.timSort(snapshotInfos);
+                }
+            }
+            listener.onResponse(new GetSnapshotsResponse(snapshotInfos));
         } catch (Exception e) {
             listener.onFailure(e);
         }
@@ -136,4 +158,32 @@ private boolean isAllSnapshots(String[] snapshots) {
     private boolean isCurrentSnapshotsOnly(String[] snapshots) {
         return (snapshots.length == 1 && GetSnapshotsRequest.CURRENT_SNAPSHOT.equalsIgnoreCase(snapshots[0]));
     }
+
+    private List<SnapshotInfo> buildSimpleSnapshotInfos(final Set<SnapshotId> toResolve,
+                                                        final RepositoryData repositoryData,
+                                                        final List<SnapshotInfo> currentSnapshots) {
+        List<SnapshotInfo> snapshotInfos = new ArrayList<>();
+        for (SnapshotInfo snapshotInfo : currentSnapshots) {
+            if (toResolve.remove(snapshotInfo.snapshotId())) {
+                snapshotInfos.add(snapshotInfo.basic());
+            }
+        }
+        Map<SnapshotId, List<String>> snapshotsToIndices = new HashMap<>();
+        for (IndexId indexId : repositoryData.getIndices().values()) {
+            for (SnapshotId snapshotId : repositoryData.getSnapshots(indexId)) {
+                if (toResolve.contains(snapshotId)) {
+                    snapshotsToIndices.computeIfAbsent(snapshotId, (k) -> new ArrayList<>())
+                                      .add(indexId.getName());
+                }
+            }
+        }
+        for (Map.Entry<SnapshotId, List<String>> entry : snapshotsToIndices.entrySet()) {
+            final List<String> indices = entry.getValue();
+            CollectionUtil.timSort(indices);
+            final SnapshotId snapshotId = entry.getKey();
+            snapshotInfos.add(new SnapshotInfo(snapshotId, indices, repositoryData.getSnapshotState(snapshotId)));
+        }
+        CollectionUtil.timSort(snapshotInfos);
+        return Collections.unmodifiableList(snapshotInfos);
+    }
 }