Introduce Remote translog feature flag

server/src/main/java/org/opensearch/cluster/metadata/IndexMetadata.java

@@ -284,6 +284,8 @@ public Iterator<Setting<?>> settings() {
     );
 
     public static final String SETTING_REMOTE_STORE_ENABLED = "index.remote_store.enabled";
+
+    public static final String SETTING_REMOTE_TRANSLOG_STORE_ENABLED = "index.remote.translog_store.enabled";
     /**
      * Used to specify if the index data should be persisted in the remote store.
      */
@@ -294,6 +296,16 @@ public Iterator<Setting<?>> settings() {
         Property.Final
     );
 
+    /**
+     * Used to specify if the index translog operations should be persisted in the remote store.
+     */
+    public static final Setting<Boolean> INDEX_REMOTE_TRANSLOG_STORE_ENABLED_SETTING = Setting.boolSetting(
+        SETTING_REMOTE_TRANSLOG_STORE_ENABLED,
+        false,
+        Property.IndexScope,
+        Property.Final
+    );
+
     public static final String SETTING_AUTO_EXPAND_REPLICAS = "index.auto_expand_replicas";
     public static final Setting<AutoExpandReplicas> INDEX_AUTO_EXPAND_REPLICAS_SETTING = AutoExpandReplicas.SETTING;
 

server/src/main/java/org/opensearch/common/settings/IndexScopedSettings.java

@@ -222,7 +222,9 @@ public final class IndexScopedSettings extends AbstractScopedSettings {
         FeatureFlags.REPLICATION_TYPE,
         IndexMetadata.INDEX_REPLICATION_TYPE_SETTING,
         FeatureFlags.REMOTE_STORE,
-        IndexMetadata.INDEX_REMOTE_STORE_ENABLED_SETTING
+        IndexMetadata.INDEX_REMOTE_STORE_ENABLED_SETTING,
+        FeatureFlags.REMOTE_TRANSLOG_STORE,
+        IndexMetadata.INDEX_REMOTE_TRANSLOG_STORE_ENABLED_SETTING
     );
 
     public static final IndexScopedSettings DEFAULT_SCOPED_SETTINGS = new IndexScopedSettings(Settings.EMPTY, BUILT_IN_INDEX_SETTINGS);

server/src/main/java/org/opensearch/common/util/FeatureFlags.java

@@ -29,6 +29,12 @@ public class FeatureFlags {
      */
     public static final String REMOTE_STORE = "opensearch.experimental.feature.remote_store.enabled";
 
+    /**
+     * Gates the visibility of the index setting that allows persisting operations to remote translog store.
+     * Once the feature is ready for production release, this feature flag can be removed.
+     */
+    public static final String REMOTE_TRANSLOG_STORE = "opensearch.experimental.feature.remote.translog_store.enabled";
+
     /**
      * Used to test feature flags whose values are expected to be booleans.
      * This method returns true if the value is "true" (case-insensitive),

server/src/main/java/org/opensearch/index/IndexSettings.java

@@ -559,6 +559,7 @@ public final class IndexSettings {
     private final int numberOfShards;
     private final ReplicationType replicationType;
     private final boolean isRemoteStoreEnabled;
+    private final boolean isRemoteTranslogStoreEnabled;
     // volatile fields are updated via #updateIndexMetadata(IndexMetadata) under lock
     private volatile Settings settings;
     private volatile IndexMetadata indexMetadata;
@@ -719,7 +720,7 @@ public IndexSettings(final IndexMetadata indexMetadata, final Settings nodeSetti
         numberOfShards = settings.getAsInt(IndexMetadata.SETTING_NUMBER_OF_SHARDS, null);
         replicationType = ReplicationType.parseString(settings.get(IndexMetadata.SETTING_REPLICATION_TYPE));
         isRemoteStoreEnabled = settings.getAsBoolean(IndexMetadata.SETTING_REMOTE_STORE_ENABLED, false);
-
+        isRemoteTranslogStoreEnabled = settings.getAsBoolean(IndexMetadata.SETTING_REMOTE_TRANSLOG_STORE_ENABLED, false);
         this.searchThrottled = INDEX_SEARCH_THROTTLED.get(settings);
         this.queryStringLenient = QUERY_STRING_LENIENT_SETTING.get(settings);
         this.queryStringAnalyzeWildcard = QUERY_STRING_ANALYZE_WILDCARD.get(nodeSettings);
@@ -971,6 +972,13 @@ public boolean isRemoteStoreEnabled() {
         return isRemoteStoreEnabled;
     }
 
+    /**
+     * Returns if remote translog store is enabled for this index.
+     */
+    public boolean isRemoteTranslogStoreEnabled() {
+        return isRemoteTranslogStoreEnabled;
+    }
+
     /**
      * Returns the node settings. The settings returned from {@link #getSettings()} are a merged version of the
      * index settings and the node settings where node settings are overwritten by index settings.

server/src/test/java/org/opensearch/index/IndexSettingsTests.java

@@ -777,6 +777,27 @@ public void testRemoteStoreExplicitSetting() {
         assertTrue(settings.isRemoteStoreEnabled());
     }
 
+    public void testRemoteTranslogStoreDefaultSetting() {
+        IndexMetadata metadata = newIndexMeta(
+            "index",
+            Settings.builder().put(IndexMetadata.SETTING_VERSION_CREATED, Version.CURRENT).build()
+        );
+        IndexSettings settings = new IndexSettings(metadata, Settings.EMPTY);
+        assertFalse(settings.isRemoteTranslogStoreEnabled());
+    }
+
+    public void testRemoteTranslogStoreExplicitSetting() {
+        IndexMetadata metadata = newIndexMeta(
+            "index",
+            Settings.builder()
+                .put(IndexMetadata.SETTING_VERSION_CREATED, Version.CURRENT)
+                .put(IndexMetadata.SETTING_REMOTE_TRANSLOG_STORE_ENABLED, true)
+                .build()
+        );
+        IndexSettings settings = new IndexSettings(metadata, Settings.EMPTY);
+        assertTrue(settings.isRemoteTranslogStoreEnabled());
+    }
+
     public void testUpdateRemoteStoreFails() {
         Set<Setting<?>> remoteStoreSettingSet = new HashSet<>();
         remoteStoreSettingSet.add(FEATURE_FLAGGED_INDEX_SETTINGS.get(FeatureFlags.REMOTE_STORE));
@@ -792,4 +813,20 @@ public void testUpdateRemoteStoreFails() {
         );
         assertEquals(error.getMessage(), "final index setting [index.remote_store.enabled], not updateable");
     }
+
+    public void testUpdateRemoteTranslogStoreFails() {
+        Set<Setting<?>> remoteStoreSettingSet = new HashSet<>();
+        remoteStoreSettingSet.add(FEATURE_FLAGGED_INDEX_SETTINGS.get(FeatureFlags.REMOTE_TRANSLOG_STORE));
+        IndexScopedSettings settings = new IndexScopedSettings(Settings.EMPTY, remoteStoreSettingSet);
+        IllegalArgumentException error = expectThrows(
+            IllegalArgumentException.class,
+            () -> settings.updateSettings(
+                Settings.builder().put("index.remote.translog_store.enabled", randomBoolean()).build(),
+                Settings.builder(),
+                Settings.builder(),
+                "index"
+            )
+        );
+        assertEquals(error.getMessage(), "final index setting [index.remote.translog_store.enabled], not updateable");
+    }
 }