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/*
* 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
* specific language governing permissions and limitations
* under the License.
*/
/**
* The Persistent Tasks Executors are responsible for executing restartable tasks that can survive disappearance of a
* coordinating and executor nodes.
*
* 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:
*
* 1. The coordinating node sends an ordinary transport request to the master node to start a new persistent task. This task is handled
* by the {@link org.elasticsearch.persistent.PersistentTasksService}, which is using
* {@link org.elasticsearch.persistent.PersistentTasksClusterService} to update cluster state with the record about running persistent
* task.
*
* 2. The master node updates the {@link org.elasticsearch.persistent.PersistentTasksCustomMetaData} in the cluster state to indicate
* that there is a new persistent task running in the system.
*
* 3. The {@link org.elasticsearch.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.
*
* 4. If the task fails to start on the node, the {@link org.elasticsearch.persistent.PersistentTasksNodeService} uses the
* {@link org.elasticsearch.persistent.PersistentTasksCustomMetaData} to notify the
* {@link org.elasticsearch.persistent.PersistentTasksService}, which reassigns the action to another node in the cluster.
*
* 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.
*
* 6. The {@link org.elasticsearch.persistent.RemovePersistentTaskAction} action can be also used to remove the persistent task.
*/
package org.elasticsearch.persistent;