org.apache.hadoop.mapreduce
Class Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT>

java.lang.Object
  org.apache.hadoop.mapreduce.Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT>

Direct Known Subclasses:

FieldSelectionReducer, IntSumReducer, LongSumReducer, SecondarySort.Reduce, WordCount.IntSumReducer

public class Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT>
extends Object

Reduces a set of intermediate values which share a key to a smaller set of values.

Reducer implementations can access the Configuration for the job via the JobContext.getConfiguration() method.

Reducer has 3 primary phases:

1. Shuffle

   The Reducer copies the sorted output from each Mapper using HTTP across the network.

2. Sort

   The framework merge sorts Reducer inputs by keys (since different Mappers may have output the same key).

   The shuffle and sort phases occur simultaneously i.e. while outputs are being fetched they are merged.

   SecondarySort

   To achieve a secondary sort on the values returned by the value iterator, the application should extend the key with the secondary key and define a grouping comparator. The keys will be sorted using the entire key, but will be grouped using the grouping comparator to decide which keys and values are sent in the same call to reduce.The grouping comparator is specified via Job.setGroupingComparatorClass(Class). The sort order is controlled by Job.setSortComparatorClass(Class).

   For example, say that you want to find duplicate web pages and tag them all with the url of the "best" known example. You would set up the job like:
   • Map Input Key: url
   • Map Input Value: document
   • Map Output Key: document checksum, url pagerank
   • Map Output Value: url
   • Partitioner: by checksum
   • OutputKeyComparator: by checksum and then decreasing pagerank
   • OutputValueGroupingComparator: by checksum

3. Reduce

   In this phase the reduce(Object, Iterable, Context) method is called for each <key, (collection of values)> in the sorted inputs.

   The output of the reduce task is typically written to a RecordWriter via TaskInputOutputContext.write(Object, Object).

The output of the Reducer is not re-sorted.

Example:

 public class IntSumReducer extends Reducer {
   private IntWritable result = new IntWritable();
 
   public void reduce(Key key, Iterable values, 
                      Context context) throws IOException {
     int sum = 0;
     for (IntWritable val : values) {
       sum += val.get();
     }
     result.set(sum);
     context.collect(key, result);
   }
 }
 

See Also:

Mapper, Partitioner

Nested Class Summary

class

Reducer.Context

Constructor Summary

Reducer()

Method Summary

protected void	cleanup(Reducer.Context context) Called once at the end of the task.
protected void	reduce(KEYIN key, Iterable<VALUEIN> values, Reducer.Context context) This method is called once for each key.
void	run(Reducer.Context context) Advanced application writers can use the run(org.apache.hadoop.mapreduce.Reducer.Context) method to control how the reduce task works.
protected void	setup(Reducer.Context context) Called once at the start of the task.

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

setup

protected void setup(Reducer.Context context)
              throws IOException,
                     InterruptedException

Called once at the start of the task.

Throws:

IOException

InterruptedException

reduce

protected void reduce(KEYIN key,
                      Iterable<VALUEIN> values,
                      Reducer.Context context)
               throws IOException,
                      InterruptedException

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.

Throws:

IOException

InterruptedException

cleanup

protected void cleanup(Reducer.Context context)
                throws IOException,
                       InterruptedException

Called once at the end of the task.

Throws:

IOException

InterruptedException

run

public void run(Reducer.Context context)
         throws IOException,
                InterruptedException

Advanced application writers can use the run(org.apache.hadoop.mapreduce.Reducer.Context) method to control how the reduce task works.

Throws:

IOException

InterruptedException