/*
 * 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.
 */

/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch licenses this file to you under
 * the Apache License, Version 2.0 (the "License"); you may
 * not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
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/**
 * The Persistent Tasks Executors are responsible for executing restartable tasks that can survive disappearance of a
 * coordinating and executor nodes.
 * <p>
 * In order to be resilient to node restarts, the persistent tasks are using the cluster state instead of a transport service to send
 * requests and responses. The execution is done in six phases:
 * <p>
 * 1. The coordinating node sends an ordinary transport request to the cluster-manager node to start a new persistent task. This task is handled
 * by the {@link org.opensearch.persistent.PersistentTasksService}, which is using
 * {@link org.opensearch.persistent.PersistentTasksClusterService} to update cluster state with the record about running persistent
 * task.
 * <p>
 * 2. The cluster-manager node updates the {@link org.opensearch.persistent.PersistentTasksCustomMetadata} in the cluster state to indicate
 * that there is a new persistent task running in the system.
 * <p>
 * 3. The {@link org.opensearch.persistent.PersistentTasksNodeService} running on every node in the cluster monitors changes in
 * the cluster state and starts execution of all new tasks assigned to the node it is running on.
 * <p>
 * 4. If the task fails to start on the node, the {@link org.opensearch.persistent.PersistentTasksNodeService} uses the
 * {@link org.opensearch.persistent.PersistentTasksCustomMetadata} to notify the
 * {@link org.opensearch.persistent.PersistentTasksService}, which reassigns the action to another node in the cluster.
 * <p>
 * 5. If a task finishes successfully on the node and calls listener.onResponse(), the corresponding persistent action is removed from the
 * cluster state unless removeOnCompletion flag for this task is set to false.
 * <p>
 * 6. The {@link org.opensearch.persistent.RemovePersistentTaskAction} action can be also used to remove the persistent task.
 */
/*
 * Modifications Copyright OpenSearch Contributors. See
 * GitHub history for details.
 */

package org.opensearch.persistent;
