Package org.apache.hadoop.mapred

Interface Partitioner<K2,V2>

All Superinterfaces:

JobConfigurable

All Known Implementing Classes:

BinaryPartitioner, HashPartitioner, KeyFieldBasedPartitioner, TotalOrderPartitioner

@Public
@Stable
public interface Partitioner<K2,V2>
extends JobConfigurable

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.

See Also:

• Reducer

Method Summary

Modifier and Type	Method	Description
int	getPartition(K2 key, V2 value, int numPartitions)	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.

Methods inherited from interface org.apache.hadoop.mapred.JobConfigurable

configure

Method Details

getPartition

int getPartition(K2 key,
 V2 value,
 int numPartitions)

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.

Parameters:

key - the key to be paritioned.

value - the entry value.

numPartitions - the total number of partitions.

Returns:

the partition number for the key.