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| <h2 title="Class ChainReducer" class="title">Class ChainReducer</h2> |
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| <li>org.apache.hadoop.mapred.lib.ChainReducer</li> |
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| <dd><a href="https://docs.oracle.com/javase/8/docs/api/java/io/Closeable.html?is-external=true" title="class or interface in java.io">Closeable</a>, <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html?is-external=true" title="class or interface in java.lang">AutoCloseable</a>, <a href="../../../../../org/apache/hadoop/mapred/JobConfigurable.html" title="interface in org.apache.hadoop.mapred">JobConfigurable</a>, <a href="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</a></dd> |
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| <br> |
| <pre>@InterfaceAudience.Public |
| @InterfaceStability.Stable |
| public class <span class="typeNameLabel">ChainReducer</span> |
| extends <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a> |
| implements <a href="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</a></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 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> |
| 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> |
| 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 maching output and input key and |
| value classes as no conversion is done by the chaining code. |
| <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> |
| 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> |
| ... |
| conf.setJobName("chain"); |
| conf.setInputFormat(TextInputFormat.class); |
| conf.setOutputFormat(TextOutputFormat.class); |
| |
| JobConf mapAConf = new JobConf(false); |
| ... |
| ChainMapper.addMapper(conf, AMap.class, LongWritable.class, Text.class, |
| Text.class, Text.class, true, mapAConf); |
| |
| JobConf mapBConf = new JobConf(false); |
| ... |
| ChainMapper.addMapper(conf, BMap.class, Text.class, Text.class, |
| LongWritable.class, Text.class, false, mapBConf); |
| |
| JobConf reduceConf = new JobConf(false); |
| ... |
| ChainReducer.setReducer(conf, XReduce.class, LongWritable.class, Text.class, |
| Text.class, Text.class, true, reduceConf); |
| |
| ChainReducer.addMapper(conf, CMap.class, Text.class, Text.class, |
| LongWritable.class, Text.class, false, null); |
| |
| ChainReducer.addMapper(conf, DMap.class, LongWritable.class, Text.class, |
| LongWritable.class, LongWritable.class, true, null); |
| |
| FileInputFormat.setInputPaths(conf, inDir); |
| FileOutputFormat.setOutputPath(conf, outDir); |
| ... |
| |
| JobClient jc = new JobClient(conf); |
| RunningJob job = jc.submitJob(conf); |
| ... |
| </pre></div> |
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| <td class="colOne"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainReducer.html#ChainReducer--">ChainReducer</a></span>()</code> |
| <div class="block">Constructor.</div> |
| </td> |
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| <th class="colFirst" scope="col">Modifier and Type</th> |
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| <td class="colFirst"><code>static <K1,V1,K2,V2><br>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainReducer.html#addMapper-org.apache.hadoop.mapred.JobConf-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-boolean-org.apache.hadoop.mapred.JobConf-">addMapper</a></span>(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</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/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</a><K1,V1,K2,V2>> 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><? extends K1> 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><? extends V1> 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><? extends K2> 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><? extends V2> outputValueClass, |
| boolean byValue, |
| <a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a> mapperConf)</code> |
| <div class="block">Adds a Mapper class to the chain job's JobConf.</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/mapred/lib/ChainReducer.html#close--">close</a></span>()</code> |
| <div class="block">Closes the ChainReducer, the Reducer and all the Mappers in the chain.</div> |
| </td> |
| </tr> |
| <tr id="i2" class="altColor"> |
| <td class="colFirst"><code>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainReducer.html#configure-org.apache.hadoop.mapred.JobConf-">configure</a></span>(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a> job)</code> |
| <div class="block">Configures the ChainReducer, the Reducer and all the Mappers in the chain.</div> |
| </td> |
| </tr> |
| <tr id="i3" class="rowColor"> |
| <td class="colFirst"><code>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainReducer.html#reduce-java.lang.Object-java.util.Iterator-org.apache.hadoop.mapred.OutputCollector-org.apache.hadoop.mapred.Reporter-">reduce</a></span>(<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a> key, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html?is-external=true" title="class or interface in java.util">Iterator</a> values, |
| <a href="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</a> output, |
| <a href="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</a> reporter)</code> |
| <div class="block">Chains the <code>reduce(...)</code> method of the Reducer with the |
| <code>map(...) </code> methods of the Mappers in the chain.</div> |
| </td> |
| </tr> |
| <tr id="i4" class="altColor"> |
| <td class="colFirst"><code>static <K1,V1,K2,V2><br>void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../org/apache/hadoop/mapred/lib/ChainReducer.html#setReducer-org.apache.hadoop.mapred.JobConf-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-boolean-org.apache.hadoop.mapred.JobConf-">setReducer</a></span>(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</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/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</a><K1,V1,K2,V2>> 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><? extends K1> 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><? extends V1> 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><? extends K2> 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><? extends V2> outputValueClass, |
| boolean byValue, |
| <a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a> reducerConf)</code> |
| <div class="block">Sets the Reducer class to the chain job's JobConf.</div> |
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| </a> |
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| <li class="blockList"> |
| <h4>ChainReducer</h4> |
| <pre>public ChainReducer()</pre> |
| <div class="block">Constructor.</div> |
| </li> |
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| <a name="setReducer-org.apache.hadoop.mapred.JobConf-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-boolean-org.apache.hadoop.mapred.JobConf-"> |
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| <h4>setReducer</h4> |
| <pre>public static <K1,V1,K2,V2> void setReducer(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</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/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</a><K1,V1,K2,V2>> 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><? extends K1> 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><? extends V1> 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><? extends K2> 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><? extends V2> outputValueClass, |
| boolean byValue, |
| <a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a> reducerConf)</pre> |
| <div class="block">Sets the Reducer class to the chain job's JobConf. |
| <p> |
| It has to be specified how key and values are passed from one element of |
| the chain to the next, by value or by reference. If a Reducer leverages the |
| assumed semantics that the key and values are not modified by the collector |
| 'by value' must be used. If the Reducer does not expect this semantics, as |
| an optimization to avoid serialization and deserialization 'by reference' |
| can be used. |
| <p> |
| For the added Reducer the configuration given for it, |
| <code>reducerConf</code>, have precedence over the job's JobConf. This |
| precedence is in effect when the task is running. |
| <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.</div> |
| <dl> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>job</code> - job's JobConf to add the Reducer class.</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>byValue</code> - indicates if key/values should be passed by value |
| to the next Mapper in the chain, if any.</dd> |
| <dd><code>reducerConf</code> - a JobConf with the configuration for the Reducer |
| class. It is recommended to use a JobConf without default values using the |
| <code>JobConf(boolean loadDefaults)</code> constructor with FALSE.</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="addMapper-org.apache.hadoop.mapred.JobConf-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-java.lang.Class-boolean-org.apache.hadoop.mapred.JobConf-"> |
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| <h4>addMapper</h4> |
| <pre>public static <K1,V1,K2,V2> void addMapper(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</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/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</a><K1,V1,K2,V2>> 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><? extends K1> inputKeyClass, |
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| boolean byValue, |
| <a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a> mapperConf)</pre> |
| <div class="block">Adds a Mapper class to the chain job's JobConf. |
| <p> |
| It has to be specified how key and values are passed from one element of |
| the chain to the next, by value or by reference. If a Mapper leverages the |
| assumed semantics that the key and values are not modified by the collector |
| 'by value' must be used. If the Mapper does not expect this semantics, as |
| an optimization to avoid serialization and deserialization 'by reference' |
| can be used. |
| <p> |
| For the added Mapper the configuration given for it, |
| <code>mapperConf</code>, have precedence over the job's JobConf. This |
| precedence is in effect when the task is running. |
| <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 |
| .</div> |
| <dl> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>job</code> - chain job's JobConf to add the Mapper class.</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>byValue</code> - indicates if key/values should be passed by value |
| to the next Mapper in the chain, if any.</dd> |
| <dd><code>mapperConf</code> - a JobConf with the configuration for the Mapper |
| class. It is recommended to use a JobConf without default values using the |
| <code>JobConf(boolean loadDefaults)</code> constructor with FALSE.</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="configure-org.apache.hadoop.mapred.JobConf-"> |
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| <h4>configure</h4> |
| <pre>public void configure(<a href="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</a> job)</pre> |
| <div class="block">Configures the ChainReducer, the Reducer and all the Mappers in the chain. |
| <p> |
| If this method is overriden <code>super.configure(...)</code> should be |
| invoked at the beginning of the overwriter method.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Specified by:</span></dt> |
| <dd><code><a href="../../../../../org/apache/hadoop/mapred/JobConfigurable.html#configure-org.apache.hadoop.mapred.JobConf-">configure</a></code> in interface <code><a href="../../../../../org/apache/hadoop/mapred/JobConfigurable.html" title="interface in org.apache.hadoop.mapred">JobConfigurable</a></code></dd> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>job</code> - the configuration</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="reduce-java.lang.Object-java.util.Iterator-org.apache.hadoop.mapred.OutputCollector-org.apache.hadoop.mapred.Reporter-"> |
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| <h4>reduce</h4> |
| <pre>public void reduce(<a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a> key, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html?is-external=true" title="class or interface in java.util">Iterator</a> values, |
| <a href="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</a> output, |
| <a href="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</a> reporter) |
| 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">Chains the <code>reduce(...)</code> method of the Reducer with the |
| <code>map(...) </code> methods of the Mappers in the chain.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Specified by:</span></dt> |
| <dd><code><a href="../../../../../org/apache/hadoop/mapred/Reducer.html#reduce-K2-java.util.Iterator-org.apache.hadoop.mapred.OutputCollector-org.apache.hadoop.mapred.Reporter-">reduce</a></code> in interface <code><a href="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</a></code></dd> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>key</code> - the key.</dd> |
| <dd><code>values</code> - the list of values to reduce.</dd> |
| <dd><code>output</code> - to collect keys and combined values.</dd> |
| <dd><code>reporter</code> - facility to report progress.</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> |
| </li> |
| </ul> |
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| <h4>close</h4> |
| <pre>public void close() |
| 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">Closes the ChainReducer, the Reducer and all the Mappers in the chain. |
| <p> |
| If this method is overriden <code>super.close()</code> should be |
| invoked at the end of the overwriter method.</div> |
| <dl> |
| <dt><span class="overrideSpecifyLabel">Specified by:</span></dt> |
| <dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/io/Closeable.html?is-external=true#close--" title="class or interface in java.io">close</a></code> in interface <code><a href="https://docs.oracle.com/javase/8/docs/api/java/io/Closeable.html?is-external=true" title="class or interface in java.io">Closeable</a></code></dd> |
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| <dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html?is-external=true#close--" title="class or interface in java.lang">close</a></code> in interface <code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/AutoCloseable.html?is-external=true" title="class or interface in java.lang">AutoCloseable</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> |
| </dl> |
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