org.apache.hadoop.streaming
Class PipeReducer

java.lang.Object
  org.apache.hadoop.streaming.PipeMapRed
      org.apache.hadoop.streaming.PipeReducer

All Implemented Interfaces:

Closeable, JobConfigurable, Reducer

Direct Known Subclasses:

PipeCombiner

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

PipeReducer()

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

PipeReducer

public PipeReducer()

Method Detail

configure

public void configure(JobConf job)

Description copied from interface: JobConfigurable

Initializes a new instance from a JobConf.

Specified by:

configure in interface JobConfigurable

Overrides:

configure in class PipeMapRed

Parameters:

job - the configuration

reduce

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

Parameters:

key - the key.

values - the list of values to reduce.

output - to collect keys and combined values.

reporter - facility to report progress.

Throws:

IOException

close

public void close()

Specified by:

close in interface Closeable

getInputSeparator

public byte[] getInputSeparator()

Description copied from class: PipeMapRed

Returns the input separator to be used.

Specified by:

getInputSeparator in class PipeMapRed

getFieldSeparator

public byte[] getFieldSeparator()

Description copied from class: PipeMapRed

Returns the field separator to be used.

Specified by:

getFieldSeparator in class PipeMapRed

getNumOfKeyFields

public int getNumOfKeyFields()

Description copied from class: PipeMapRed

Returns the number of key fields.

Specified by:

getNumOfKeyFields in class PipeMapRed