org.apache.hadoop.mapred

Interface Partitioner<K2,V2>

All Superinterfaces:

JobConfigurable

All Known Implementing Classes:

BinaryPartitioner, HashPartitioner, KeyFieldBasedPartitioner, TotalOrderPartitioner

@InterfaceAudience.Public
 @InterfaceStability.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 and 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.

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

configure

Method Detail

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.