public class Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT> extends Object
Reducer
implementations
can access the Configuration
for the job via the
JobContext.getConfiguration()
method.
Reducer
has 3 primary phases:
The Reducer
copies the sorted output from each
Mapper
using HTTP across the network.
The framework merge sorts Reducer
inputs by
key
s
(since different Mapper
s may have output the same key).
The shuffle and sort phases occur simultaneously i.e. while outputs are being fetched they are merged.
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)
.
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 IntSumReducerextends 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); } }
Mapper
,
Partitioner
限定符和类型 | 类和说明 |
---|---|
class |
Reducer.Context |
构造器和说明 |
---|
Reducer() |
限定符和类型 | 方法和说明 |
---|---|
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.
|
protected void setup(Reducer.Context context) throws IOException, InterruptedException
protected void reduce(KEYIN key, Iterable<VALUEIN> values, Reducer.Context context) throws IOException, InterruptedException
protected void cleanup(Reducer.Context context) throws IOException, InterruptedException
public void run(Reducer.Context context) throws IOException, InterruptedException
run(org.apache.hadoop.mapreduce.Reducer.Context)
method to
control how the reduce task works.Copyright © 2009 The Apache Software Foundation