[FEATURE] Create ReloadableExtension interface

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

@@ -0,0 +1,46 @@
+/*
+ * 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.common.settings.Settings;
+
+/**
+* An extension point for {@link Plugin}s that can be reloaded. There is no
+* clear definition about what reloading a plugin actually means. When a plugin
+* is reloaded it might rebuild any internal members. Plugins usually implement
+* this interface in order to reread the values of {@code SecureSetting}s and
+* then rebuild any dependent internal members.
+*/
+public interface ReloadableExtension {
+
+    /**
+     * Called to trigger the rebuilt of the plugin's internal members. The reload
+     * operation <b>is required to have been completed</b> when the method returns.
+     * Strictly speaking, the <code>settings</code> argument should not be accessed
+     * outside of this method's call stack, as any values stored in the node's
+     * keystore (see {@code SecureSetting}) will not otherwise be retrievable. The
+     * setting values do not follow dynamic updates, i.e. the values are identical
+     * to the ones during the initial plugin loading, barring the keystore file on
+     * disk changes. Any failure during the operation should be signaled by raising
+     * an exception, but the plugin should otherwise continue to function
+     * unperturbed.
+     *
+     * @param settings
+     *            Settings used while reloading the plugin. All values are
+     *            retrievable, including the values stored in the node's keystore.
+     *            The setting values are the initial ones, from when the node has be
+     *            started, i.e. they don't follow dynamic updates.
+     * @throws Exception
+     *             if the operation failed. The plugin should continue to operate as
+     *             if the offending call didn't happen.
+     */
+
+    void reload(Settings settings) throws Exception;
+}