mapreduce/src/java/org/apache/hadoop/mapreduce/OutputCommitter.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF 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.
 */

package org.apache.hadoop.mapreduce;

import java.io.IOException;

/**
 * <code>OutputCommitter</code> describes the commit of task output for a 
 * Map-Reduce job.
 *
 * <p>The Map-Reduce framework relies on the <code>OutputCommitter</code> of 
 * the job to:<p>
 * <ol>
 *   <li>
 *   Setup the job during initialization. For example, create the temporary 
 *   output directory for the job during the initialization of the job.
 *   </li>
 *   <li>
 *   Cleanup the job after the job completion. For example, remove the
 *   temporary output directory after the job completion. 
 *   </li>
 *   <li>
 *   Setup the task temporary output.
 *   </li> 
 *   <li>
 *   Check whether a task needs a commit. This is to avoid the commit
 *   procedure if a task does not need commit.
 *   </li>
 *   <li>
 *   Commit of the task output.
 *   </li>  
 *   <li>
 *   Discard the task commit.
 *   </li>
 * </ol>
 * 
 * @see org.apache.hadoop.mapreduce.lib.output.FileOutputCommitter 
 * @see JobContext
 * @see TaskAttemptContext 
 *
 */
public abstract class OutputCommitter {
  /**
   * For the framework to setup the job output during initialization
   * 
   * @param jobContext Context of the job whose output is being written.
   * @throws IOException if temporary output could not be created
   */
  public abstract void setupJob(JobContext jobContext) throws IOException;

  /**
   * For cleaning up the job's output after job completion
   * 
   * @param jobContext Context of the job whose output is being written.
   * @throws IOException
   */
  public abstract void cleanupJob(JobContext jobContext) throws IOException;

  /**
   * Sets up output for the task.
   * 
   * @param taskContext Context of the task whose output is being written.
   * @throws IOException
   */
  public abstract void setupTask(TaskAttemptContext taskContext)
  throws IOException;
  
  /**
   * Check whether task needs a commit
   * 
   * @param taskContext
   * @return true/false
   * @throws IOException
   */
  public abstract boolean needsTaskCommit(TaskAttemptContext taskContext)
  throws IOException;

  /**
   * To promote the task's temporary output to final output location
   * 
   * The task's output is moved to the job's output directory.
   * 
   * @param taskContext Context of the task whose output is being written.
   * @throws IOException if commit is not 
   */
  public abstract void commitTask(TaskAttemptContext taskContext)
  throws IOException;
  
  /**
   * Discard the task output
   * 
   * @param taskContext
   * @throws IOException
   */
  public abstract void abortTask(TaskAttemptContext taskContext)
  throws IOException;
}