org.apache.hadoop.mapreduce.Partitioner Maven / Gradle / Ivy
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* 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
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*/
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);
}