/**
* 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.mapred;
/**
* 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>
*
* @see Reducer
*/
public interface Partitioner<K2, V2> extends JobConfigurable {
/**
* Get the paritition 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 paritioned.
* @param value the entry value.
* @param numPartitions the total number of partitions.
* @return the partition number for the <code>key</code>.
*/
int getPartition(K2 key, V2 value, int numPartitions);
}