Added support for feature flags in opensearch.yml

This change introduces a static store "settings"
in FeatureFlags.java file to enable isEnabled method to
fetch flag settings defined in opensearch.yml.

distribution/src/config/opensearch.yml

@@ -86,3 +86,26 @@ ${path.logs}
 # Require explicit names when deleting indices:
 #
 #action.destructive_requires_name: true
+#
+# ---------------------------------- Experimental Features -----------------------------------
+#
+# Gates the visibility of the index setting that allows changing of replication type.
+# Once the feature is ready for production release, this feature flag can be removed.
+#
+#opensearch.experimental.feature.replication_type.enabled: false
+#
+#
+# Gates the visibility of the index setting that allows persisting data to remote store along with local disk.
+# Once the feature is ready for production release, this feature flag can be removed.
+#
+#opensearch.experimental.feature.remote_store.enabled: false
+#
+#
+# Gates the functionality of a new parameter to the snapshot restore API
+# that allows for creation of a new index type that searches a snapshot
+# directly in a remote repository without restoring all index data to disk
+# ahead of time.
+#
+#opensearch.experimental.feature.searchable_snapshot.enabled: false
+#
+#

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

@@ -8,6 +8,8 @@
 
 package org.opensearch.common.util;
 
+import org.opensearch.common.settings.Settings;
+
 /**
  * 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
@@ -37,12 +39,28 @@ public class FeatureFlags {
      */
     public static final String SEARCHABLE_SNAPSHOT = "opensearch.experimental.feature.searchable_snapshot.enabled";
 
+    private static Settings settings;
+
+    /**
+     * This method is responsible to map settings from opensearch.yml to local stored
+     * settings value. That is used for the existing isEnabled method.
+     *
+     * @param openSearchSettings The settings stored in opensearch.yml.
+     */
+    public static void initialiseFeatureFlags(Settings openSearchSettings) {
+        settings = openSearchSettings;
+    }
+
     /**
      * 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));
+        if ("true".equalsIgnoreCase(System.getProperty(featureFlagName))) {
+            // TODO: Remove the if condition once FeatureFlags are only supported via opensearch.yml
+            return true;
+        }
+        return settings != null && settings.getAsBoolean(featureFlagName, false);
     }
 }

server/src/main/java/org/opensearch/node/Node.java

@@ -426,6 +426,9 @@ protected Node(
             );
             final Settings settings = pluginsService.updatedSettings();
 
+            // Ensure to initialise Feature Flags via the settings from opensearch.yml
+            FeatureFlags.initialiseFeatureFlags(settings);
+
             final Set<DiscoveryNodeRole> additionalRoles = pluginsService.filterPlugins(Plugin.class)
                 .stream()
                 .map(Plugin::getRoles)