org.apache.hadoop.mapred.Partitioner Maven / Gradle / Ivy
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* to you under the Apache License, Version 2.0 (the
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*
* http://www.apache.org/licenses/LICENSE-2.0
*
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*/
package org.apache.hadoop.mapred;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
/**
* 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
*/
@InterfaceAudience.Public
@InterfaceStability.Stable
public interface Partitioner 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.
*
* Typically a hash function on a all or a subset of the key.
*
* @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 key.
*/
int getPartition(K2 key, V2 value, int numPartitions);
}