Package org.apache.hadoop.mapreduce

Class Partitioner<KEY,VALUE>

java.lang.Object

org.apache.hadoop.mapreduce.Partitioner<KEY,VALUE>

Direct Known Subclasses:

BinaryPartitioner, HashPartitioner, KeyFieldBasedPartitioner, RehashPartitioner, TotalOrderPartitioner

@Public
@Stable
public abstract class Partitioner<KEY,VALUE>
extends Object

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.

Note: A Partitioner is created only when there are multiple reducers.

Note: If you require your Partitioner class to obtain the Job's configuration object, implement the Configurable interface.

See Also:

• Reducer

Constructor Summary

Constructors

Constructor	Description
Partitioner()

Method Summary

Modifier and Type	Method	Description
abstract int	getPartition(KEY key, VALUE value, int numPartitions)	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.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

Partitioner

public Partitioner()

Method Details

getPartition

public abstract int getPartition(KEY key,
 VALUE value,
 int numPartitions)

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.

Parameters:

key - the key to be partioned.

value - the entry value.

numPartitions - the total number of partitions.

Returns:

the partition number for the key.