org.apache.hadoop.mapred.lib
Class FieldSelectionMapReduce

java.lang.Object
  org.apache.hadoop.mapred.lib.FieldSelectionMapReduce

All Implemented Interfaces:

Closeable, JobConfigurable, Mapper, Reducer

public class FieldSelectionMapReduce
extends Object
implements Mapper, Reducer

This class implements a mapper/reducer class that can be used to perform field selections in a manner similar to unix cut. The input data is treated as fields separated by a user specified separator (the default value is "\t"). The user can specify a list of fields that form the map output keys, and a list of fields that form the map output values. If the inputformat is TextInputFormat, the mapper will ignore the key to the map function. and the fields are from the value only. Otherwise, the fields are the union of those from the key and those from the value. The field separator is under attribute "mapred.data.field.separator" The map output field list spec is under attribute "map.output.key.value.fields.spec". The value is expected to be like "keyFieldsSpec:valueFieldsSpec" key/valueFieldsSpec are comma (,) separated field spec: fieldSpec,fieldSpec,fieldSpec ... Each field spec can be a simple number (e.g. 5) specifying a specific field, or a range (like 2-5) to specify a range of fields, or an open range (like 3-) specifying all the fields starting from field 3. The open range field spec applies value fields only. They have no effect on the key fields. Here is an example: "4,3,0,1:6,5,1-3,7-". It specifies to use fields 4,3,0 and 1 for keys, and use fields 6,5,1,2,3,7 and above for values. The reduce output field list spec is under attribute "reduce.output.key.value.fields.spec". The reducer extracts output key/value pairs in a similar manner, except that the key is never ignored.

Field Summary

static org.apache.commons.logging.Log

LOG

Constructor Summary

FieldSelectionMapReduce()

Method Summary

void	close() Called after the last call to any other method on this object to free and/or flush resources.
void	configure(JobConf job) Initializes a new instance from a JobConf.
void	map(WritableComparable key, Writable val, OutputCollector output, Reporter reporter) The identify function.
void	reduce(WritableComparable key, Iterator values, OutputCollector output, Reporter reporter) Combines values for a given key.

Methods inherited from class java.lang.Object

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

Field Detail

LOG

public static final org.apache.commons.logging.Log LOG

Constructor Detail

FieldSelectionMapReduce

public FieldSelectionMapReduce()

Method Detail

map

public void map(WritableComparable key,
                Writable val,
                OutputCollector output,
                Reporter reporter)
         throws IOException

The identify function. Input key/value pair is written directly to output.

Specified by:

map in interface Mapper

Parameters:

key - the key

val - the values

output - collects mapped keys and values

Throws:

IOException

configure

public void configure(JobConf job)

Description copied from interface: JobConfigurable

Initializes a new instance from a JobConf.

Specified by:

configure in interface JobConfigurable

Parameters:

job - the configuration

close

public void close()
           throws IOException

Description copied from interface: Closeable

Called after the last call to any other method on this object to free and/or flush resources. Typical implementations do nothing.

Specified by:

close in interface Closeable

Throws:

IOException

reduce

public void reduce(WritableComparable key,
                   Iterator values,
                   OutputCollector output,
                   Reporter reporter)
            throws IOException

Description copied from interface: Reducer

Combines values for a given key. Output values must be of the same type as input values. Input keys must not be altered. Typically all values are combined into zero or one value. Output pairs are collected with calls to OutputCollector.collect(WritableComparable,Writable).

Specified by:

reduce in interface Reducer

Parameters:

key - the key

values - the values to combine

output - to collect combined values

Throws:

IOException