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MAPREDUCE-4616. Improve javadoc for MultipleOutputs. Contributed by Tony Burton. 

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1397182 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
Arun Murthy 2012-10-11 17:21:45 +00:00
parent 3d76be910d
commit f174adfc15
3 changed files with 69 additions and 1 deletions
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3
hadoop-mapreduce-project/CHANGES.txt
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@ -155,6 +155,9 @@ Release 2.0.3-alpha - Unreleased
MAPREDUCE-3678. The Map tasks logs should have the value of input
split it processed. (harsh)
MAPREDUCE-4616. Improve javadoc for MultipleOutputs. (Tony Burton via
acmurthy)
OPTIMIZATIONS
BUG FIXES

5
hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/lib/output/LazyOutputFormat.java
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@ -32,7 +32,10 @@ import org.apache.hadoop.mapreduce.TaskAttemptContext;
import org.apache.hadoop.util.ReflectionUtils;
/**
* A Convenience class that creates output lazily.
* A Convenience class that creates output lazily.
* Use in conjuction with org.apache.hadoop.mapreduce.lib.output.MultipleOutputs to recreate the
* behaviour of org.apache.hadoop.mapred.lib.MultipleTextOutputFormat (etc) of the old Hadoop API.
* See {@link MultipleOutputs} documentation for more information.
*/
@InterfaceAudience.Public
@InterfaceStability.Stable

62
hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/lib/output/MultipleOutputs.java
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@ -20,7 +20,10 @@ package org.apache.hadoop.mapreduce.lib.output;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.*;
import org.apache.hadoop.mapreduce.Reducer.Context;
import org.apache.hadoop.mapreduce.lib.output.MultipleOutputs;
import org.apache.hadoop.mapreduce.task.TaskAttemptContextImpl;
import org.apache.hadoop.util.ReflectionUtils;
@ -37,6 +40,7 @@ import java.util.*;
* Each additional output, or named output, may be configured with its own
* <code>OutputFormat</code>, with its own key class and with its own value
* class.
* </p>
*
* <p>
* Case two: to write data to different files provided by user
@ -107,6 +111,64 @@ import java.util.*;
*
* }
* </pre>
*
* <p>
* When used in conjuction with org.apache.hadoop.mapreduce.lib.output.LazyOutputFormat,
* MultipleOutputs can mimic the behaviour of MultipleTextOutputFormat and MultipleSequenceFileOutputFormat
* from the old Hadoop API - ie, output can be written from the Reducer to more than one location.
* </p>
*
* <p>
* Use <code>MultipleOutputs.write(KEYOUT key, VALUEOUT value, String baseOutputPath)</code> to write key and
* value to a path specified by <code>baseOutputPath</code>, with no need to specify a named output:
* </p>
*
* <pre>
* private MultipleOutputs<Text, Text> out;
*
* public void setup(Context context) {
* out = new MultipleOutputs<Text, Text>(context);
* ...
* }
*
* public void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
* for (Text t : values) {
* out.write(key, t, generateFileName(<<i>parameter list...</i>>));
* }
* }
*
* protected void cleanup(Context context) throws IOException, InterruptedException {
* out.close();
* }
* </pre>
*
* <p>
* Use your own code in <code>generateFileName()</code> to create a custom path to your results.
* '/' characters in <code>baseOutputPath</code> will be translated into directory levels in your file system.
* Also, append your custom-generated path with "part" or similar, otherwise your output will be -00000, -00001 etc.
* No call to <code>context.write()</code> is necessary. See example <code>generateFileName()</code> code below.
* </p>
*
* <pre>
* private String generateFileName(Text k) {
* // expect Text k in format "Surname|Forename"
* String[] kStr = k.toString().split("\\|");
*
* String sName = kStr[0];
* String fName = kStr[1];
*
* // example for k = Smith|John
* // output written to /user/hadoop/path/to/output/Smith/John-r-00000 (etc)
* return sName + "/" + fName;
* }
* </pre>
*
* <p>
* Using MultipleOutputs in this way will still create zero-sized default output, eg part-00000.
* To prevent this use <code>LazyOutputFormat.setOutputFormatClass(job, TextOutputFormat.class);</code>
* instead of <code>job.setOutputFormatClass(TextOutputFormat.class);</code> in your Hadoop job configuration.
* </p>
*
*/
@InterfaceAudience.Public
@InterfaceStability.Stable