Class PipeReducer

  extended by org.apache.hadoop.streaming.PipeMapRed
      extended by org.apache.hadoop.streaming.PipeReducer
All Implemented Interfaces:
Closeable, JobConfigurable, Reducer
Direct Known Subclasses:

public class PipeReducer
extends PipeMapRed
implements Reducer

A generic Reducer bridge. It delegates operations to an external program via stdin and stdout.

Field Summary
Fields inherited from class org.apache.hadoop.streaming.PipeMapRed
LOG, outerrThreadsThrowable
Constructor Summary
Method Summary
 void close()
 void configure(JobConf job)
          Initializes a new instance from a JobConf.
 byte[] getFieldSeparator()
          Returns the field separator to be used.
 byte[] getInputSeparator()
          Returns the input separator to be used.
 int getNumOfKeyFields()
          Returns the number of key fields.
 void reduce(Object key, Iterator values, OutputCollector output, Reporter reporter)
          Reduces values for a given key.
Methods inherited from class org.apache.hadoop.streaming.PipeMapRed
getClientInput, getClientOutput, getConfiguration, getContext, mapRedFinished
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail


public PipeReducer()
Method Detail


public void configure(JobConf job)
Description copied from interface: JobConfigurable
Initializes a new instance from a JobConf.

Specified by:
configure in interface JobConfigurable
configure in class PipeMapRed
job - the configuration


public void reduce(Object key,
                   Iterator values,
                   OutputCollector output,
                   Reporter reporter)
            throws IOException
Description copied from interface: Reducer
Reduces values for a given key.

The framework calls this method for each <key, (list of values)> pair in the grouped inputs. Output values must be of the same type as input values. Input keys must not be altered. The framework will reuse the key and value objects that are passed into the reduce, therefore the application should clone the objects they want to keep a copy of. In many cases, all values are combined into zero or one value.

Output pairs are collected with calls to OutputCollector.collect(Object,Object).

Applications can use the Reporter provided to report progress or just indicate that they are alive. In scenarios where the application takes an insignificant amount of time to process individual key/value pairs, this is crucial since the framework might assume that the task has timed-out and kill that task. The other way of avoiding this is to set mapred.task.timeout to a high-enough value (or even zero for no time-outs).

Specified by:
reduce in interface Reducer
key - the key.
values - the list of values to reduce.
output - to collect keys and combined values.
reporter - facility to report progress.


public void close()
Specified by:
close in interface Closeable


public byte[] getInputSeparator()
Description copied from class: PipeMapRed
Returns the input separator to be used.

Specified by:
getInputSeparator in class PipeMapRed


public byte[] getFieldSeparator()
Description copied from class: PipeMapRed
Returns the field separator to be used.

Specified by:
getFieldSeparator in class PipeMapRed


public int getNumOfKeyFields()
Description copied from class: PipeMapRed
Returns the number of key fields.

Specified by:
getNumOfKeyFields in class PipeMapRed

Copyright © 2009 The Apache Software Foundation