[Backport 2.x] Allow changing number of replicas of searchable snapshot index

CHANGELOG.md

@@ -28,7 +28,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add template snippets support for field and target_field in KV ingest processor ([#10040](https://github.com/opensearch-project/OpenSearch/pull/10040))
 - Allowing pipeline processors to access index mapping info by passing ingest service ref as part of the processor factory parameters ([#10307](https://github.com/opensearch-project/OpenSearch/pull/10307))
 - Make number of segment metadata files in remote segment store configurable ([#11329](https://github.com/opensearch-project/OpenSearch/pull/11329))
-
+- Allow changing number of replicas of searchable snapshot index ([#11317](https://github.com/opensearch-project/OpenSearch/pull/11317))
 
 ### Dependencies
 - Bumps jetty version to 9.4.52.v20230823 to fix GMS-2023-1857 ([#9822](https://github.com/opensearch-project/OpenSearch/pull/9822))

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

@@ -458,7 +458,9 @@ private void testUpdateIndexSettingsOnlyNotAllowedSettings(String index) {
 
     private void testUpdateIndexSettingsOnlyAllowedSettings(String index) {
         final UpdateSettingsRequestBuilder builder = client().admin().indices().prepareUpdateSettings(index);
-        builder.setSettings(Map.of("index.max_result_window", 1000, "index.search.slowlog.threshold.query.warn", "10s"));
+        builder.setSettings(
+            Map.of("index.max_result_window", 1000, "index.search.slowlog.threshold.query.warn", "10s", "index.number_of_replicas", 0)
+        );
         AcknowledgedResponse settingsResponse = builder.execute().actionGet();
         assertThat(settingsResponse, notNullValue());
     }

server/src/main/java/org/opensearch/action/admin/indices/settings/put/TransportUpdateSettingsAction.java

@@ -55,6 +55,7 @@
 import org.opensearch.transport.TransportService;
 
 import java.io.IOException;
+import java.util.Arrays;
 import java.util.Set;
 import java.util.stream.Stream;
 
@@ -77,7 +78,8 @@ public class TransportUpdateSettingsAction extends TransportClusterManagerNodeAc
         "index.max_script_fields",
         "index.max_terms_count",
         "index.max_regex_length",
-        "index.highlight.max_analyzed_offset"
+        "index.highlight.max_analyzed_offset",
+        "index.number_of_replicas"
     );
 
     private final static String[] ALLOWLIST_REMOTE_SNAPSHOT_SETTINGS_PREFIXES = { "index.search.slowlog" };
@@ -145,10 +147,10 @@ protected ClusterBlockException checkBlock(UpdateSettingsRequest request, Cluste
             }
         }
 
+        final String[] requestIndexNames = Arrays.stream(requestIndices).map(Index::getName).toArray(String[]::new);
         return allowSearchableSnapshotSettingsUpdate
             ? null
-            : state.blocks()
-                .indicesBlockedException(ClusterBlockLevel.METADATA_WRITE, indexNameExpressionResolver.concreteIndexNames(state, request));
+            : state.blocks().indicesBlockedException(ClusterBlockLevel.METADATA_WRITE, requestIndexNames);
     }
 
     @Override