/** * 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; /** * Partitions the key space. * *

Partitioner 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 m reduce tasks the intermediate key (and hence the * record) is sent for reduction.

* * @see Reducer */ public abstract class Partitioner { /** * 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. * *

Typically a hash function on a all or a subset of the key.

* * @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 key. */ public abstract int getPartition(KEY key, VALUE value, int numPartitions); }