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| <div class="subTitle">org.apache.hadoop.mapreduce.lib.chain</div> |
| <h2 title="Class ChainReducer" class="title">Class ChainReducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT></h2> |
| </div> |
| <div class="contentContainer"> |
| <ul class="inheritance"> |
| <li><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">java.lang.Object</a></li> |
| <li> |
| <ul class="inheritance"> |
| <li><a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce">org.apache.hadoop.mapreduce.Reducer</a><KEYIN,VALUEIN,KEYOUT,VALUEOUT></li> |
| <li> |
| <ul class="inheritance"> |
| <li>org.apache.hadoop.mapreduce.lib.chain.ChainReducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT></li> |
| </ul> |
| </li> |
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| <pre>@InterfaceAudience.Public |
| @InterfaceStability.Stable |
| public class <span class="typeNameLabel">ChainReducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT></span> |
| extends <a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce">Reducer</a><KEYIN,VALUEIN,KEYOUT,VALUEOUT></pre> |
| <div class="block">The ChainReducer class allows to chain multiple Mapper classes after a |
| Reducer within the Reducer task. |
| |
| <p> |
| For each record output by the Reducer, the Mapper classes are invoked in a |
| chained (or piped) fashion. The output of the reducer becomes the input of |
| the first mapper and output of first becomes the input of the second, and so |
| on until the last Mapper, the output of the last Mapper will be written to |
| the task's output. |
| </p> |
| <p> |
| The key functionality of this feature is that the Mappers in the chain do not |
| need to be aware that they are executed after the Reducer or in a chain. This |
| enables having reusable specialized Mappers that can be combined to perform |
| composite operations within a single task. |
| </p> |
| <p> |
| Special care has to be taken when creating chains that the key/values output |
| by a Mapper are valid for the following Mapper in the chain. It is assumed |
| all Mappers and the Reduce in the chain use matching output and input key and |
| value classes as no conversion is done by the chaining code. |
| </p> |
| <p> Using the ChainMapper and the ChainReducer classes is possible to |
| compose Map/Reduce jobs that look like <code>[MAP+ / REDUCE MAP*]</code>. And |
| immediate benefit of this pattern is a dramatic reduction in disk IO. </p> |
| <p> |
| IMPORTANT: There is no need to specify the output key/value classes for the |
| ChainReducer, this is done by the setReducer or the addMapper for the last |
| element in the chain. |
| </p> |
| ChainReducer usage pattern: |
| <p> |
| |
| <pre> |
| ... |
| Job = new Job(conf); |
| .... |
| |
| Configuration reduceConf = new Configuration(false); |
| ... |
| ChainReducer.setReducer(job, XReduce.class, LongWritable.class, Text.class, |
| Text.class, Text.class, true, reduceConf); |
| |
| ChainReducer.addMapper(job, CMap.class, Text.class, Text.class, |
| LongWritable.class, Text.class, false, null); |
| |
| ChainReducer.addMapper(job, DMap.class, LongWritable.class, Text.class, |
| LongWritable.class, LongWritable.class, true, null); |
| |
| ... |
| |
| job.waitForCompletion(true); |
| ... |
| </pre></div> |
| </li> |
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| <th class="colOne" scope="col">Constructor and Description</th> |
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| <tr class="altColor"> |
| <td class="colOne"><code><span class="memberNameLink"><a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html#ChainReducer--">ChainReducer</a></span>()</code> </td> |
| </tr> |
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| <tr id="i0" class="altColor"> |
| <td class="colFirst"><code>static void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html#addMapper-org.apache.hadoop.mapreduce.Job-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-org.apache.hadoop.conf.Configuration-">addMapper</a></span>(<a href="../../../../../../org/apache/hadoop/mapreduce/Job.html" title="class in org.apache.hadoop.mapreduce">Job</a> job, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><? extends <a href="../../../../../../org/apache/hadoop/mapreduce/Mapper.html" title="class in org.apache.hadoop.mapreduce">Mapper</a>> klass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> inputKeyClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> inputValueClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> outputKeyClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> outputValueClass, |
| <a href="../../../../../../org/apache/hadoop/conf/Configuration.html" title="class in org.apache.hadoop.conf">Configuration</a> mapperConf)</code> |
| <div class="block">Adds a <a href="../../../../../../org/apache/hadoop/mapreduce/Mapper.html" title="class in org.apache.hadoop.mapreduce"><code>Mapper</code></a> class to the chain reducer.</div> |
| </td> |
| </tr> |
| <tr id="i1" class="rowColor"> |
| <td class="colFirst"><code>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html#run-org.apache.hadoop.mapreduce.Reducer.Context-">run</a></span>(org.apache.hadoop.mapreduce.Reducer.Context context)</code> |
| <div class="block">Advanced application writers can use the |
| <a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html#run-org.apache.hadoop.mapreduce.Reducer.Context-"><code>Reducer.run(org.apache.hadoop.mapreduce.Reducer.Context)</code></a> method to |
| control how the reduce task works.</div> |
| </td> |
| </tr> |
| <tr id="i2" class="altColor"> |
| <td class="colFirst"><code>static void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html#setReducer-org.apache.hadoop.mapreduce.Job-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-org.apache.hadoop.conf.Configuration-">setReducer</a></span>(<a href="../../../../../../org/apache/hadoop/mapreduce/Job.html" title="class in org.apache.hadoop.mapreduce">Job</a> job, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><? extends <a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce">Reducer</a>> klass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> inputKeyClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> inputValueClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> outputKeyClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> outputValueClass, |
| <a href="../../../../../../org/apache/hadoop/conf/Configuration.html" title="class in org.apache.hadoop.conf">Configuration</a> reducerConf)</code> |
| <div class="block">Sets the <a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce"><code>Reducer</code></a> class to the chain job.</div> |
| </td> |
| </tr> |
| <tr id="i3" class="rowColor"> |
| <td class="colFirst"><code>protected void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html#setup-org.apache.hadoop.mapreduce.Reducer.Context-">setup</a></span>(org.apache.hadoop.mapreduce.Reducer.Context context)</code> |
| <div class="block">Called once at the start of the task.</div> |
| </td> |
| </tr> |
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| <h3>Methods inherited from class org.apache.hadoop.mapreduce.<a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce">Reducer</a></h3> |
| <code><a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html#cleanup-org.apache.hadoop.mapreduce.Reducer.Context-">cleanup</a>, <a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html#reduce-KEYIN-java.lang.Iterable-org.apache.hadoop.mapreduce.Reducer.Context-">reduce</a></code></li> |
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| <h3>Constructor Detail</h3> |
| <a name="ChainReducer--"> |
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| </a> |
| <ul class="blockListLast"> |
| <li class="blockList"> |
| <h4>ChainReducer</h4> |
| <pre>public ChainReducer()</pre> |
| </li> |
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| <a name="setReducer-org.apache.hadoop.mapreduce.Job-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-org.apache.hadoop.conf.Configuration-"> |
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| <h4>setReducer</h4> |
| <pre>public static void setReducer(<a href="../../../../../../org/apache/hadoop/mapreduce/Job.html" title="class in org.apache.hadoop.mapreduce">Job</a> job, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><? extends <a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce">Reducer</a>> klass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> inputKeyClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> inputValueClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> outputKeyClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> outputValueClass, |
| <a href="../../../../../../org/apache/hadoop/conf/Configuration.html" title="class in org.apache.hadoop.conf">Configuration</a> reducerConf)</pre> |
| <div class="block">Sets the <a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce"><code>Reducer</code></a> class to the chain job. |
| |
| <p> |
| The key and values are passed from one element of the chain to the next, by |
| value. For the added Reducer the configuration given for it, |
| <code>reducerConf</code>, have precedence over the job's Configuration. |
| This precedence is in effect when the task is running. |
| </p> |
| <p> |
| IMPORTANT: There is no need to specify the output key/value classes for the |
| ChainReducer, this is done by the setReducer or the addMapper for the last |
| element in the chain. |
| </p></div> |
| <dl> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>job</code> - the job</dd> |
| <dd><code>klass</code> - the Reducer class to add.</dd> |
| <dd><code>inputKeyClass</code> - reducer input key class.</dd> |
| <dd><code>inputValueClass</code> - reducer input value class.</dd> |
| <dd><code>outputKeyClass</code> - reducer output key class.</dd> |
| <dd><code>outputValueClass</code> - reducer output value class.</dd> |
| <dd><code>reducerConf</code> - a configuration for the Reducer class. It is recommended to use a |
| Configuration without default values using the |
| <code>Configuration(boolean loadDefaults)</code> constructor with |
| FALSE.</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="addMapper-org.apache.hadoop.mapreduce.Job-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-org.apache.hadoop.conf.Configuration-"> |
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| <h4>addMapper</h4> |
| <pre>public static void addMapper(<a href="../../../../../../org/apache/hadoop/mapreduce/Job.html" title="class in org.apache.hadoop.mapreduce">Job</a> job, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><? extends <a href="../../../../../../org/apache/hadoop/mapreduce/Mapper.html" title="class in org.apache.hadoop.mapreduce">Mapper</a>> klass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> inputKeyClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> inputValueClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> outputKeyClass, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</a><?> outputValueClass, |
| <a href="../../../../../../org/apache/hadoop/conf/Configuration.html" title="class in org.apache.hadoop.conf">Configuration</a> mapperConf) |
| throws <a href="https://docs.oracle.com/javase/8/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</a></pre> |
| <div class="block">Adds a <a href="../../../../../../org/apache/hadoop/mapreduce/Mapper.html" title="class in org.apache.hadoop.mapreduce"><code>Mapper</code></a> class to the chain reducer. |
| |
| <p> |
| The key and values are passed from one element of the chain to the next, by |
| value For the added Mapper the configuration given for it, |
| <code>mapperConf</code>, have precedence over the job's Configuration. This |
| precedence is in effect when the task is running. |
| </p> |
| <p> |
| IMPORTANT: There is no need to specify the output key/value classes for the |
| ChainMapper, this is done by the addMapper for the last mapper in the |
| chain. |
| </p></div> |
| <dl> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>job</code> - The job.</dd> |
| <dd><code>klass</code> - the Mapper class to add.</dd> |
| <dd><code>inputKeyClass</code> - mapper input key class.</dd> |
| <dd><code>inputValueClass</code> - mapper input value class.</dd> |
| <dd><code>outputKeyClass</code> - mapper output key class.</dd> |
| <dd><code>outputValueClass</code> - mapper output value class.</dd> |
| <dd><code>mapperConf</code> - a configuration for the Mapper class. It is recommended to use a |
| Configuration without default values using the |
| <code>Configuration(boolean loadDefaults)</code> constructor with |
| FALSE.</dd> |
| <dt><span class="throwsLabel">Throws:</span></dt> |
| <dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</a></code></dd> |
| </dl> |
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| <a name="setup-org.apache.hadoop.mapreduce.Reducer.Context-"> |
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| <h4>setup</h4> |
| <pre>protected void setup(org.apache.hadoop.mapreduce.Reducer.Context context)</pre> |
| <div class="block"><span class="descfrmTypeLabel">Description copied from class: <code><a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html#setup-org.apache.hadoop.mapreduce.Reducer.Context-">Reducer</a></code></span></div> |
| <div class="block">Called once at the start of the task.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Overrides:</span></dt> |
| <dd><code><a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html#setup-org.apache.hadoop.mapreduce.Reducer.Context-">setup</a></code> in class <code><a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce">Reducer</a><<a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html" title="type parameter in ChainReducer">KEYIN</a>,<a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html" title="type parameter in ChainReducer">VALUEIN</a>,<a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html" title="type parameter in ChainReducer">KEYOUT</a>,<a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html" title="type parameter in ChainReducer">VALUEOUT</a>></code></dd> |
| </dl> |
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| <a name="run-org.apache.hadoop.mapreduce.Reducer.Context-"> |
| <!-- --> |
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| <h4>run</h4> |
| <pre>public void run(org.apache.hadoop.mapreduce.Reducer.Context context) |
| throws <a href="https://docs.oracle.com/javase/8/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</a>, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</a></pre> |
| <div class="block"><span class="descfrmTypeLabel">Description copied from class: <code><a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html#run-org.apache.hadoop.mapreduce.Reducer.Context-">Reducer</a></code></span></div> |
| <div class="block">Advanced application writers can use the |
| <a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html#run-org.apache.hadoop.mapreduce.Reducer.Context-"><code>Reducer.run(org.apache.hadoop.mapreduce.Reducer.Context)</code></a> method to |
| control how the reduce task works.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Overrides:</span></dt> |
| <dd><code><a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html#run-org.apache.hadoop.mapreduce.Reducer.Context-">run</a></code> in class <code><a href="../../../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce">Reducer</a><<a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html" title="type parameter in ChainReducer">KEYIN</a>,<a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html" title="type parameter in ChainReducer">VALUEIN</a>,<a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html" title="type parameter in ChainReducer">KEYOUT</a>,<a href="../../../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html" title="type parameter in ChainReducer">VALUEOUT</a>></code></dd> |
| <dt><span class="throwsLabel">Throws:</span></dt> |
| <dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</a></code></dd> |
| <dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/InterruptedException.html?is-external=true" title="class or interface in java.lang">InterruptedException</a></code></dd> |
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