org.apache.hadoop.mapreduce.Partitioner.java Source code

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Here is the source code for org.apache.hadoop.mapreduce.Partitioner.java

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/**
 * 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 org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configurable;

/** 
 * Partitions the key space.
 * 
 * <p><code>Partitioner</code> controls the partitioning of the keys of the 
 * intermediate map-outputs. The key (or a subset of the key) is used to derive
 * the partition, typically by a hash function. The total number of partitions
 * is the same as the number of reduce tasks for the job. Hence this controls
 * which of the <code>m</code> reduce tasks the intermediate key (and hence the 
 * record) is sent for reduction.</p>
 *
 * <p>Note: A <code>Partitioner</code> is created only when there are multiple
 * reducers.</p>
 *
 * <p>Note: If you require your Partitioner class to obtain the Job's
 * configuration object, implement the {@link Configurable} interface.</p>
 * 
 * @see Reducer
 */
@InterfaceAudience.Public
@InterfaceStability.Stable
public abstract class Partitioner<KEY, VALUE> {

    /** 
     * Get the partition number for a given key (hence record) given the total 
     * number of partitions i.e. number of reduce-tasks for the job.
     *   
     * <p>Typically a hash function on a all or a subset of the key.</p>
     *
     * @param key the key to be partioned.
     * @param value the entry value.
     * @param numPartitions the total number of partitions.
     * @return the partition number for the <code>key</code>.
     */
    public abstract int getPartition(KEY key, VALUE value, int numPartitions);

}