Add default for EnginePlugin.getEngineFactory

server/src/main/java/org/opensearch/plugins/EnginePlugin.java

@@ -56,7 +56,9 @@ public interface EnginePlugin {
      *
      * @return an optional engine factory
      */
-    Optional<EngineFactory> getEngineFactory(IndexSettings indexSettings);
+    default Optional<EngineFactory> getEngineFactory(IndexSettings indexSettings) {
+        return Optional.empty();
+    }
 
     /**
      * EXPERT:

server/src/test/java/org/opensearch/plugins/EnginePluginTests.java

@@ -0,0 +1,33 @@
+/*
+ * 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.plugins;
+
+import org.opensearch.index.IndexSettings;
+import org.opensearch.index.engine.EngineFactory;
+import org.opensearch.test.OpenSearchTestCase;
+
+import java.util.Optional;
+
+public class EnginePluginTests extends OpenSearchTestCase {
+
+    public void testGetEngineFactory() {
+        final EngineFactory engineFactory = config -> null;
+        EnginePlugin enginePluginThatImplementsGetEngineFactory = new EnginePlugin() {
+            @Override
+            public Optional<EngineFactory> getEngineFactory(IndexSettings indexSettings) {
+                return Optional.of(engineFactory);
+            }
+        };
+        assertEquals(engineFactory, enginePluginThatImplementsGetEngineFactory.getEngineFactory(null).orElse(null));
+
+        EnginePlugin enginePluginThatDoesNotImplementsGetEngineFactory = new EnginePlugin() {
+        };
+        assertFalse(enginePluginThatDoesNotImplementsGetEngineFactory.getEngineFactory(null).isPresent());
+    }
+}