Add javadocs

Signed-off-by: Shivansh Arora <hishiv@amazon.com>

server/src/main/java/org/opensearch/common/remote/AbstractRemoteEntitiesManager.java

@@ -18,9 +18,22 @@
 import java.util.HashMap;
 import java.util.Map;
 
+/**
+ * An abstract class that provides a base implementation for managing remote entities in the remote store.
+ */
 public abstract class AbstractRemoteEntitiesManager implements RemoteEntitiesManager {
+    /**
+     * A map that stores the remote writable entity stores, keyed by the entity type.
+     */
     protected final Map<String, RemoteWritableEntityStore> remoteWritableEntityStores = new HashMap<>();
 
+    /**
+     * Retrieves the remote writable entity store for the given entity.
+     *
+     * @param entity the entity for which the store is requested
+     * @return the remote writable entity store for the given entity
+     * @throws IllegalArgumentException if the entity type is unknown
+     */
     protected RemoteWritableEntityStore getStore(AbstractRemoteWritableBlobEntity entity) {
         RemoteWritableEntityStore remoteStore = remoteWritableEntityStores.get(entity.getType());
         if (remoteStore == null) {
@@ -29,12 +42,29 @@ protected RemoteWritableEntityStore getStore(AbstractRemoteWritableBlobEntity en
         return remoteStore;
     }
 
+    /**
+     * Returns an ActionListener for handling the write operation for the specified component, remote object, and latched action listener.
+     *
+     * @param component the component for which the write operation is performed
+     * @param remoteObject the remote object to be written
+     * @param latchedActionListener the latched action listener to be notified when the write operation completes
+     * @return an ActionListener for handling the write operation
+     */
     protected abstract ActionListener<Void> getWriteActionListener(
         String component,
         AbstractRemoteWritableBlobEntity remoteObject,
         LatchedActionListener<ClusterMetadataManifest.UploadedMetadata> latchedActionListener
     );
 
+    /**
+     * Returns an ActionListener for handling the read operation for the specified component,
+     * remote object, and latched action listener.
+     *
+     * @param component the component for which the read operation is performed
+     * @param remoteObject the remote object to be read
+     * @param latchedActionListener the latched action listener to be notified when the read operation completes
+     * @return an ActionListener for handling the read operation
+     */
     protected abstract ActionListener<Object> getReadActionListener(
         String component,
         AbstractRemoteWritableBlobEntity remoteObject,

server/src/main/java/org/opensearch/common/remote/RemoteEntitiesManager.java

@@ -15,13 +15,33 @@
 
 import java.io.IOException;
 
+/**
+ * The RemoteEntitiesManager interface provides async read and write methods for managing remote entities in the remote store
+ */
 public interface RemoteEntitiesManager {
+
+    /**
+     * Returns a CheckedRunnable that performs an asynchronous read operation for the specified component and entity.
+     *
+     * @param component the component for which the read operation is performed
+     * @param entity the entity to be read
+     * @param latchedActionListener the listener to be notified when the read operation completes
+     * @return a CheckedRunnable that performs the asynchronous read operation
+     */
     CheckedRunnable<IOException> getAsyncReadRunnable(
         String component,
         AbstractRemoteWritableBlobEntity entity,
         LatchedActionListener<RemoteReadResult> latchedActionListener
     );
 
+    /**
+     * Returns a CheckedRunnable that performs an asynchronous write operation for the specified component and entity.
+     *
+     * @param component the component for which the write operation is performed
+     * @param entity the entity to be written
+     * @param latchedActionListener the listener to be notified when the write operation completes
+     * @return a CheckedRunnable that performs the asynchronous write operation
+     */
     CheckedRunnable<IOException> getAsyncWriteRunnable(
         String component,
         AbstractRemoteWritableBlobEntity entity,

server/src/test/java/org/opensearch/common/remote/AbstractRemoteEntitiesManagerTests.java

@@ -0,0 +1,57 @@
+/*
+ * 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.remote;
+
+import org.opensearch.action.LatchedActionListener;
+import org.opensearch.core.action.ActionListener;
+import org.opensearch.gateway.remote.ClusterMetadataManifest;
+import org.opensearch.gateway.remote.model.RemoteReadResult;
+import org.opensearch.test.OpenSearchTestCase;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class AbstractRemoteEntitiesManagerTests extends OpenSearchTestCase {
+    public void testGetStoreWithKnownEntityType() {
+        AbstractRemoteEntitiesManager manager = new ConcreteRemoteEntitiesManager();
+        String knownEntityType = "knownType";
+        RemoteWritableEntityStore mockStore = mock(RemoteWritableEntityStore.class);
+        manager.remoteWritableEntityStores.put(knownEntityType, mockStore);
+        AbstractRemoteWritableBlobEntity mockEntity = mock(AbstractRemoteWritableBlobEntity.class);
+        when(mockEntity.getType()).thenReturn(knownEntityType);
+
+        RemoteWritableEntityStore store = manager.getStore(mockEntity);
+        verify(mockEntity).getType();
+        assertEquals(mockStore, store);
+    }
+
+    public void testGetStoreWithUnknownEntityType() {
+        AbstractRemoteEntitiesManager manager = new ConcreteRemoteEntitiesManager();
+        String unknownEntityType = "unknownType";
+        AbstractRemoteWritableBlobEntity mockEntity = mock(AbstractRemoteWritableBlobEntity.class);
+        when(mockEntity.getType()).thenReturn(unknownEntityType);
+
+        assertThrows(IllegalArgumentException.class, () -> manager.getStore(mockEntity));
+        verify(mockEntity, times(2)).getType();
+    }
+
+    private static class ConcreteRemoteEntitiesManager extends AbstractRemoteEntitiesManager {
+        @Override
+        protected ActionListener<Void> getWriteActionListener(String component, AbstractRemoteWritableBlobEntity remoteObject, LatchedActionListener<ClusterMetadataManifest.UploadedMetadata> latchedActionListener) {
+            return null;
+        }
+
+        @Override
+        protected ActionListener<Object> getReadActionListener(String component, AbstractRemoteWritableBlobEntity remoteObject, LatchedActionListener<RemoteReadResult> latchedActionListener) {
+            return null;
+        }
+    }
+}