com.google.appengine.tools.mapreduce

Class Reducer<K,V,O>

java.lang.Object

com.google.appengine.tools.mapreduce.Worker<ReducerContext<O>>

com.google.appengine.tools.mapreduce.Reducer<K,V,O>

Type Parameters:

K - type of intermediate keys received

V - type of intermediate values received

O - type of output values produced

All Implemented Interfaces:

Serializable

Direct Known Subclasses:

IdentityReducer, KeyProjectionReducer, NoReducer, ValueProjectionReducer

public abstract class Reducer<K,V,O>
extends Worker<ReducerContext<O>>

Reduce function for use in MapReduce. Called once for each key, together with the sequence of all values for that key that the map phase produced. Can emit output values through the context.

This class is really an interface that might be evolving. In order to avoid breaking users when we change the interface, we made it an abstract class.

Author:

ohler@google.com (Christian Ohler)

See Also:

Serialized Form

Constructor Summary

Constructors

Constructor and Description
Reducer()

Method Summary

Methods

Modifier and Type	Method and Description
protected void	emit(O value) Syntactic sugar for getContext().emit(value)
abstract void	reduce(K key, ReducerInput<V> values) Processes the values for a given key, using the context returned by Worker.getContext() to emit output to the Output of the MapReduce.

Methods inherited from class com.google.appengine.tools.mapreduce.Worker

allowSliceRetry, beginShard, beginSlice, endShard, endSlice, estimateMemoryRequirement, getContext, setContext

Methods inherited from class java.lang.Object

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

Constructor Detail

Reducer

public Reducer()

Method Detail

reduce

public abstract void reduce(K key,
          ReducerInput<V> values)

Processes the values for a given key, using the context returned by Worker.getContext() to emit output to the Output of the MapReduce. values enumerates all values that the map phase produced for the key key. It will always contain at least one value.

emit

protected void emit(O value)

Syntactic sugar for getContext().emit(value)