Change default search preference to _primary for searchable snapshot indices

CHANGELOG.md

@@ -124,6 +124,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Moved concurrent-search from sandbox plugin to server module behind feature flag ([#7203](https://github.com/opensearch-project/OpenSearch/pull/7203))
 - Allow access to indices cache clear APIs for read only indexes ([#7303](https://github.com/opensearch-project/OpenSearch/pull/7303))
 - Changed concurrent-search threadpool type to be resizable and support task resource tracking ([#7502](https://github.com/opensearch-project/OpenSearch/pull/7502))
+- Default search preference to _primary for searchable snapshot indices ([#7628](https://github.com/opensearch-project/OpenSearch/pull/7628))
 
 ### Deprecated
 

server/src/internalClusterTest/java/org/opensearch/snapshots/SearchableSnapshotIT.java

@@ -10,6 +10,8 @@
 import org.opensearch.action.admin.cluster.node.stats.NodeStats;
 import org.opensearch.action.admin.cluster.node.stats.NodesStatsRequest;
 import org.opensearch.action.admin.cluster.node.stats.NodesStatsResponse;
+import org.opensearch.action.admin.cluster.shards.ClusterSearchShardsGroup;
+import org.opensearch.action.admin.cluster.shards.ClusterSearchShardsRequest;
 import org.opensearch.action.admin.cluster.snapshots.create.CreateSnapshotResponse;
 import org.opensearch.action.admin.cluster.snapshots.delete.DeleteSnapshotRequest;
 import org.opensearch.action.admin.cluster.snapshots.get.GetSnapshotsResponse;
@@ -41,6 +43,7 @@
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
 import java.util.stream.Collectors;
@@ -664,6 +667,58 @@ public void testCacheIndexFilesClearedOnDelete() throws Exception {
         logger.info("--> validated that the cache file path doesn't exist");
     }
 
+    /**
+     * Test scenario that validates that the default search preference for searchable snapshot
+     * is primary shards
+     */
+    public void testDefaultShardPreference() throws Exception {
+        final int numReplicas = 1;
+        final String indexName = "test-idx";
+        final String restoredIndexName = indexName + "-copy";
+        final String repoName = "test-repo";
+        final String snapshotName = "test-snap";
+        final Client client = client();
+
+        // Create an index, snapshot and restore as a searchable snapshot index
+        internalCluster().ensureAtLeastNumSearchAndDataNodes(numReplicas + 1);
+        createIndexWithDocsAndEnsureGreen(numReplicas, 100, indexName);
+        createRepositoryWithSettings(null, repoName);
+        takeSnapshot(client, snapshotName, repoName, indexName);
+        restoreSnapshotAndEnsureGreen(client, snapshotName, repoName);
+        assertDocCount(restoredIndexName, 100L);
+        assertRemoteSnapshotIndexSettings(client, restoredIndexName);
+
+        // ClusterSearchShards API returns a list of shards that will be used
+        // when querying a particular index
+        ClusterSearchShardsGroup[] shardGroups = client.admin()
+            .cluster()
+            .searchShards(new ClusterSearchShardsRequest(restoredIndexName))
+            .actionGet()
+            .getGroups();
+
+        // Ensure when no preferences are set (default preference), the only compatible shards are primary
+        for (ClusterSearchShardsGroup shardsGroup : shardGroups) {
+            assertEquals(1, shardsGroup.getShards().length);
+            assertTrue(shardsGroup.getShards()[0].primary());
+        }
+
+        // Ensure when preferences are set, all the compatible shards are returned
+        shardGroups = client.admin()
+            .cluster()
+            .searchShards(new ClusterSearchShardsRequest(restoredIndexName).preference("foo"))
+            .actionGet()
+            .getGroups();
+
+        // Ensures that the compatible shards are not just primaries
+        for (ClusterSearchShardsGroup shardsGroup : shardGroups) {
+            assertTrue(shardsGroup.getShards().length > 1);
+            boolean containsReplica = Arrays.stream(shardsGroup.getShards())
+                .map(shardRouting -> !shardRouting.primary())
+                .reduce(false, (s1, s2) -> s1 || s2);
+            assertTrue(containsReplica);
+        }
+    }
+
     /**
      * Asserts the cache folder count to match the number of shards and the number of indices within the cache folder
      * as provided.

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

@@ -42,6 +42,7 @@
 import org.opensearch.common.settings.Setting;
 import org.opensearch.common.settings.Settings;
 import org.opensearch.core.common.Strings;
+import org.opensearch.index.IndexModule;
 import org.opensearch.index.IndexNotFoundException;
 import org.opensearch.index.shard.ShardId;
 import org.opensearch.node.ResponseCollectorService;
@@ -238,6 +239,13 @@ public GroupShardsIterator<ShardIterator> searchShards(
         final Set<IndexShardRoutingTable> shards = computeTargetedShards(clusterState, concreteIndices, routing);
         final Set<ShardIterator> set = new HashSet<>(shards.size());
         for (IndexShardRoutingTable shard : shards) {
+            IndexMetadata indexMetadataForShard = indexMetadata(clusterState, shard.shardId.getIndex().getName());
+            if (IndexModule.Type.REMOTE_SNAPSHOT.match(
+                indexMetadataForShard.getSettings().get(IndexModule.INDEX_STORE_TYPE_SETTING.getKey())
+            ) && (preference == null || preference.isEmpty())) {
+                preference = Preference.PRIMARY.type();
+            }
+
             ShardIterator iterator = preferenceActiveShardIterator(
                 shard,
                 clusterState.nodes().getLocalNodeId(),

server/src/test/java/org/opensearch/cluster/routing/OperationRoutingTests.java

@@ -46,11 +46,13 @@
 import org.opensearch.common.unit.TimeValue;
 import org.opensearch.common.util.io.IOUtils;
 import org.opensearch.index.Index;
+import org.opensearch.index.IndexModule;
 import org.opensearch.index.shard.ShardId;
 import org.opensearch.node.ResponseCollectorService;
 import org.opensearch.test.ClusterServiceUtils;
 import org.opensearch.test.OpenSearchTestCase;
 import org.opensearch.threadpool.TestThreadPool;
+import org.opensearch.threadpool.ThreadPool;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -992,6 +994,66 @@ public void testWeightedOperationRoutingWeightUndefinedForOneZone() throws Excep
         }
     }
 
+    public void testSearchableSnapshotPrimaryDefault() throws Exception {
+        final int numIndices = 1;
+        final int numShards = 2;
+        final int numReplicas = 2;
+        final String[] indexNames = new String[numIndices];
+        for (int i = 0; i < numIndices; i++) {
+            indexNames[i] = "test" + i;
+        }
+        // The first index is a searchable snapshot index
+        final String searchableSnapshotIndex = indexNames[0];
+        ClusterService clusterService = null;
+        ThreadPool threadPool = null;
+
+        try {
+            OperationRouting opRouting = new OperationRouting(
+                Settings.EMPTY,
+                new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS)
+            );
+
+            ClusterState state = ClusterStateCreationUtils.stateWithAssignedPrimariesAndReplicas(indexNames, numShards, numReplicas);
+            threadPool = new TestThreadPool("testSearchableSnapshotPreference");
+            clusterService = ClusterServiceUtils.createClusterService(threadPool);
+
+            // Update the index config within the cluster state to modify the index to a searchable snapshot index
+            IndexMetadata searchableSnapshotIndexMetadata = IndexMetadata.builder(searchableSnapshotIndex)
+                .settings(
+                    Settings.builder()
+                        .put(state.metadata().index(searchableSnapshotIndex).getSettings())
+                        .put(IndexModule.INDEX_STORE_TYPE_SETTING.getKey(), IndexModule.Type.REMOTE_SNAPSHOT.getSettingsKey())
+                        .build()
+                )
+                .build();
+            Metadata.Builder metadataBuilder = Metadata.builder(state.metadata())
+                .put(searchableSnapshotIndexMetadata, false)
+                .generateClusterUuidIfNeeded();
+            state = ClusterState.builder(state).metadata(metadataBuilder.build()).build();
+
+            // Verify default preference is primary only
+            GroupShardsIterator<ShardIterator> groupIterator = opRouting.searchShards(state, indexNames, null, null);
+            assertThat("One group per index shard", groupIterator.size(), equalTo(numIndices * numShards));
+
+            for (ShardIterator shardIterator : groupIterator) {
+                assertThat("Only single shard will be returned with no preference", shardIterator.size(), equalTo(1));
+                assertTrue("Only primary should exist with no preference", shardIterator.nextOrNull().primary());
+            }
+
+            // Verify alternative preference can be applied to a searchable snapshot index
+            groupIterator = opRouting.searchShards(state, indexNames, null, "_replica");
+            assertThat("One group per index shard", groupIterator.size(), equalTo(numIndices * numShards));
+
+            for (ShardIterator shardIterator : groupIterator) {
+                assertThat("Replica shards will be returned", shardIterator.size(), equalTo(numReplicas));
+                assertFalse("Returned shard should be a replica", shardIterator.nextOrNull().primary());
+            }
+        } finally {
+            IOUtils.close(clusterService);
+            terminate(threadPool);
+        }
+    }
+
     private DiscoveryNode[] setupNodes() {
         // Sets up two data nodes in zone-a and one data node in zone-b
         List<String> zones = Arrays.asList("a", "a", "b");

server/src/test/java/org/opensearch/cluster/structure/RoutingIteratorTests.java

@@ -38,6 +38,7 @@
 import org.opensearch.cluster.OpenSearchAllocationTestCase;
 import org.opensearch.cluster.metadata.IndexMetadata;
 import org.opensearch.cluster.metadata.Metadata;
+import org.opensearch.cluster.node.DiscoveryNodeRole;
 import org.opensearch.cluster.node.DiscoveryNodes;
 import org.opensearch.cluster.routing.GroupShardsIterator;
 import org.opensearch.cluster.routing.IndexShardRoutingTable;
@@ -55,6 +56,7 @@
 import org.opensearch.cluster.service.ClusterService;
 import org.opensearch.common.settings.ClusterSettings;
 import org.opensearch.common.settings.Settings;
+import org.opensearch.index.IndexModule;
 import org.opensearch.index.shard.ShardId;
 import org.opensearch.test.ClusterServiceUtils;
 import org.opensearch.threadpool.TestThreadPool;
@@ -562,6 +564,71 @@ public void testReplicaShardPreferenceIters() {
         assertTrue(routing.primary());
     }
 
+    public void testSearchableSnapshotPreference() {
+        AllocationService strategy = createAllocationService(
+            Settings.builder().put("cluster.routing.allocation.node_concurrent_recoveries", 10).build()
+        );
+        OperationRouting operationRouting = new OperationRouting(
+            Settings.EMPTY,
+            new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS)
+        );
+
+        // Modify the index to be a searchable snapshot index
+        Metadata metadata = Metadata.builder()
+            .put(
+                IndexMetadata.builder("test")
+                    .settings(
+                        Settings.builder()
+                            .put(settings(Version.CURRENT).build())
+                            .put(IndexModule.INDEX_STORE_TYPE_SETTING.getKey(), IndexModule.Type.REMOTE_SNAPSHOT.getSettingsKey())
+                    )
+                    .numberOfShards(2)
+                    .numberOfReplicas(2)
+            )
+            .build();
+
+        RoutingTable routingTable = RoutingTable.builder().addAsNew(metadata.index("test")).build();
+
+        ClusterState clusterState = ClusterState.builder(ClusterName.CLUSTER_NAME_SETTING.getDefault(Settings.EMPTY))
+            .metadata(metadata)
+            .routingTable(routingTable)
+            .build();
+
+        clusterState = ClusterState.builder(clusterState)
+            .nodes(
+                DiscoveryNodes.builder()
+                    .add(newNode("node1", Collections.singleton(DiscoveryNodeRole.CLUSTER_MANAGER_ROLE)))
+                    .add(newNode("node2", Collections.singleton(DiscoveryNodeRole.SEARCH_ROLE)))
+                    .add(newNode("node3", Collections.singleton(DiscoveryNodeRole.SEARCH_ROLE)))
+                    .localNodeId("node1")
+            )
+            .build();
+
+        clusterState = strategy.reroute(clusterState, "reroute"); // Move primaries to initializing
+        clusterState = strategy.applyStartedShards(clusterState, clusterState.getRoutingNodes().shardsWithState(INITIALIZING));
+
+        clusterState = strategy.reroute(clusterState, "reroute"); // Move replicas to initializing
+        clusterState = strategy.applyStartedShards(clusterState, clusterState.getRoutingNodes().shardsWithState(INITIALIZING));
+
+        // When replicas haven't initialized, it comes back with the primary first, then initializing replicas
+        GroupShardsIterator<ShardIterator> shardIterators = operationRouting.searchShards(
+            clusterState,
+            new String[] { "test" },
+            null,
+            null
+        );
+        assertThat(shardIterators.size(), equalTo(2)); // two potential shards
+        ShardIterator iter = shardIterators.iterator().next();
+        assertThat(iter.size(), equalTo(1)); // one potential candidate (primary) for the shard
+
+        ShardRouting routing = iter.nextOrNull();
+        assertNotNull(routing);
+        assertTrue(routing.primary()); // Default preference is primary
+        assertTrue(routing.started());
+        routing = iter.nextOrNull();
+        assertNull(routing); // No other candidate apart from primary
+    }
+
     public void testWeightedRoutingWithDifferentWeights() {
         TestThreadPool threadPool = null;
         try {