Package org.apache.hadoop.mapreduce.lib.fieldsel

Class FieldSelectionReducer<K,V>

java.lang.Object

org.apache.hadoop.mapreduce.Reducer<Text,Text,Text,Text>

org.apache.hadoop.mapreduce.lib.fieldsel.FieldSelectionReducer<K,V>

@Public
@Stable
public class FieldSelectionReducer<K,V>
extends Reducer<Text,Text,Text,Text>

This class implements a 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 reduce output keys, and a list of fields that form the reduce output values. The fields are the union of those from the key and those from the value. The field separator is under attribute "mapreduce.fieldsel.data.field.separator" The reduce output field list spec is under attribute "mapreduce.fieldsel.reduce.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.

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.hadoop.mapreduce.Reducer

org.apache.hadoop.mapreduce.Reducer.Context

Field Summary

Fields

Modifier and Type	Field	Description
static final org.slf4j.Logger	LOG

Constructor Summary

Constructors

Constructor	Description
FieldSelectionReducer()

Method Summary

Modifier and Type	Method	Description
void	reduce(Text key, Iterable<Text> values, Reducer<Text,Text,Text,Text>.org.apache.hadoop.mapreduce.Reducer.Context context)	This method is called once for each key.
void	setup(Reducer<Text,Text,Text,Text>.org.apache.hadoop.mapreduce.Reducer.Context context)	Called once at the start of the task.

Methods inherited from class org.apache.hadoop.mapreduce.Reducer

cleanup, run

Methods inherited from class java.lang.Object

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

Field Details

LOG

public static final org.slf4j.Logger LOG

Constructor Details

FieldSelectionReducer

public FieldSelectionReducer()

Method Details

setup

public void setup(Reducer<Text,Text,Text,Text>.org.apache.hadoop.mapreduce.Reducer.Context context)
           throws IOException,
InterruptedException

Description copied from class: Reducer

Called once at the start of the task.

Overrides:

setup in class Reducer<Text,Text,Text,Text>

Throws:

IOException

InterruptedException

reduce

public void reduce(Text key,
 Iterable<Text> values,
 Reducer<Text,Text,Text,Text>.org.apache.hadoop.mapreduce.Reducer.Context context)
            throws IOException,
InterruptedException

Description copied from class: Reducer

This method is called once for each key. Most applications will define their reduce class by overriding this method. The default implementation is an identity function.

Overrides:

reduce in class Reducer<Text,Text,Text,Text>

Throws:

IOException

InterruptedException