Created EngineExtension with default implementation from EnginePlugin (…

…opensearch-project#393)

* Added EngineExtension Interface
Signed-off-by: varuntumbe <[email protected]>

* deleting the depreciated method
Signed-off-by: varuntumbe <[email protected]>

* spotlessApply has been done
Signed-off-by: varuntumbe <[email protected]>

src/main/java/org/opensearch/sdk/EngineExtension.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright OpenSearch Contributors
+ * 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.sdk;
+
+import org.opensearch.index.IndexSettings;
+import org.opensearch.index.codec.CodecServiceFactory;
+import org.opensearch.index.engine.EngineFactory;
+import org.opensearch.index.seqno.RetentionLeases;
+import org.opensearch.index.translog.TranslogDeletionPolicy;
+import org.opensearch.index.translog.TranslogDeletionPolicyFactory;
+
+import java.util.Optional;
+import java.util.function.Supplier;
+
+/**
+ * A Extension that provides alternative engine implementations.
+ *
+ */
+public interface EngineExtension {
+
+    /**
+     * When an index is created this method is invoked for each engine Extension. Engine Extensions can inspect the index settings to determine
+     * whether or not to provide an engine factory for the given index. A Extension that is not overriding the default engine should return
+     * {@link Optional#empty()}. If multiple Extensions return an engine factory for a given index the index will not be created and an
+     * {@link IllegalStateException} will be thrown during index creation.
+     *
+     * @return an optional engine factory
+     */
+    default Optional<EngineFactory> getEngineFactory(IndexSettings indexSettings) {
+        return Optional.empty();
+    }
+
+    /**
+     * EXPERT:
+     * When an index is created this method is invoked for each engine Extension. Engine Extensions can inspect the index settings
+     * to determine if a custom {@link CodecServiceFactory} should be provided for the given index. A Extension that is not overriding
+     * the {@link CodecServiceFactory} through the Extension can ignore this method and the default Codec specified in the
+     * {@link IndexSettings} will be used.
+     */
+    default Optional<CodecServiceFactory> getCustomCodecServiceFactory(IndexSettings indexSettings) {
+        return Optional.empty();
+    }
+
+    /**
+     * When an index is created this method is invoked for each engine Extension. Engine Extensions that need to provide a
+     * custom {@link TranslogDeletionPolicy} can override this method to return a function that takes the {@link IndexSettings}
+     * and a {@link Supplier} for {@link RetentionLeases} and returns a custom {@link TranslogDeletionPolicy}.
+     *
+     * Only one of the installed Engine Extensions can override this otherwise {@link IllegalStateException} will be thrown.
+     *
+     * @return a function that returns an instance of {@link TranslogDeletionPolicy}
+     */
+    default Optional<TranslogDeletionPolicyFactory> getCustomTranslogDeletionPolicyFactory() {
+        return Optional.empty();
+    }
+}