Adds the replication type index setting, alongside a formal notion of feature flags

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

@@ -260,6 +260,17 @@ public Iterator<Setting<?>> settings() {
         Property.IndexScope
     );
 
+    public static final String SETTING_SEGMENT_REPLICATION = "index.replication.segment_replication";
+    /**
+     * Used to specify if the index should use segment replication. If false, document replication is used.
+     */
+    public static final Setting<Boolean> INDEX_SEGMENT_REPLICATION_SETTING = Setting.boolSetting(
+        SETTING_SEGMENT_REPLICATION,
+        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

@@ -40,6 +40,7 @@
 import org.opensearch.cluster.routing.allocation.decider.ShardsLimitAllocationDecider;
 import org.opensearch.common.logging.Loggers;
 import org.opensearch.common.settings.Setting.Property;
+import org.opensearch.common.util.FeatureFlags;
 import org.opensearch.index.IndexModule;
 import org.opensearch.index.IndexSettings;
 import org.opensearch.index.IndexSortConfig;
@@ -207,6 +208,16 @@ public final class IndexScopedSettings extends AbstractScopedSettings {
         )
     );
 
+    /**
+     * Map of feature flag name to feature-flagged index setting. Once each feature
+     * is ready for production release, the feature flag can be removed, and the
+     * setting should be moved to {@link #BUILT_IN_INDEX_SETTINGS}.
+     */
+    public static final Map<String, Setting> FEATURE_FLAGGED_INDEX_SETTINGS = Map.of(
+        FeatureFlags.SEGREP_FEATURE_FLAG,
+        IndexMetadata.INDEX_SEGMENT_REPLICATION_SETTING
+    );
+
     public static final IndexScopedSettings DEFAULT_SCOPED_SETTINGS = new IndexScopedSettings(Settings.EMPTY, BUILT_IN_INDEX_SETTINGS);
 
     public IndexScopedSettings(Settings settings, Set<Setting<?>> settingsSet) {

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

@@ -37,6 +37,7 @@
 import org.opensearch.common.Strings;
 import org.opensearch.common.inject.Binder;
 import org.opensearch.common.inject.Module;
+import org.opensearch.common.util.FeatureFlags;
 import org.opensearch.common.xcontent.ToXContent;
 import org.opensearch.common.xcontent.XContentBuilder;
 import org.opensearch.common.xcontent.XContentType;
@@ -85,6 +86,12 @@ public SettingsModule(
             registerSetting(setting);
         }
 
+        for (Map.Entry<String, Setting> featureFlaggedSetting : IndexScopedSettings.FEATURE_FLAGGED_INDEX_SETTINGS.entrySet()) {
+            if (FeatureFlags.isEnabled(featureFlaggedSetting.getKey())) {
+                registerSetting(featureFlaggedSetting.getValue());
+            }
+        }
+
         for (Setting<?> setting : additionalSettings) {
             registerSetting(setting);
         }

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

@@ -0,0 +1,32 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.common.util;
+
+/**
+ * Utility class to manage feature flags. Feature flags are system properties that must be set on the JVM.
+ * These are used to gate the visibility/availability of incomplete features. Fore more information, see
+ * https://featureflags.io/feature-flag-introduction/
+ */
+public class FeatureFlags {
+
+    /**
+     * Gates the visibility of the index setting that enables segment replication.
+     * Once the feature is ready for production release, this feature flag can be removed.
+     */
+    public static final String SEGREP_FEATURE_FLAG = "opensearch.segment_replication_feature_flag_enabled";
+
+    /**
+     * Used to test feature flags whose values are expected to be booleans.
+     * This method returns true if the value is "true" (case-insensitive),
+     * and false otherwise.
+     */
+    public static boolean isEnabled(String featureFlagName) {
+        return "true".equalsIgnoreCase(System.getProperty(featureFlagName));
+    }
+}

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

@@ -530,6 +530,7 @@ public final class IndexSettings {
     private final String nodeName;
     private final Settings nodeSettings;
     private final int numberOfShards;
+    private final Boolean isSegRepEnabled;
     // volatile fields are updated via #updateIndexMetadata(IndexMetadata) under lock
     private volatile Settings settings;
     private volatile IndexMetadata indexMetadata;
@@ -681,6 +682,7 @@ public IndexSettings(final IndexMetadata indexMetadata, final Settings nodeSetti
         nodeName = Node.NODE_NAME_SETTING.get(settings);
         this.indexMetadata = indexMetadata;
         numberOfShards = settings.getAsInt(IndexMetadata.SETTING_NUMBER_OF_SHARDS, null);
+        isSegRepEnabled = settings.getAsBoolean(IndexMetadata.SETTING_SEGMENT_REPLICATION, false);
 
         this.searchThrottled = INDEX_SEARCH_THROTTLED.get(settings);
         this.queryStringLenient = QUERY_STRING_LENIENT_SETTING.get(settings);
@@ -915,6 +917,13 @@ public int getNumberOfReplicas() {
         return settings.getAsInt(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, null);
     }
 
+    /**
+     * Returns true if segment replication is enabled on the index.
+     */
+    public boolean isSegrepEnabled() {
+        return Boolean.TRUE.equals(isSegRepEnabled);
+    }
+
     /**
      * 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.