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| <div class="subTitle">org.apache.hadoop.mapreduce</div> |
| <h2 title="Class Reducer" class="title">Class Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT></h2> |
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| <li>org.apache.hadoop.mapreduce.Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT></li> |
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| <dt>Direct Known Subclasses:</dt> |
| <dd><a href="../../../../org/apache/hadoop/mapreduce/lib/chain/ChainReducer.html" title="class in org.apache.hadoop.mapreduce.lib.chain">ChainReducer</a>, <a href="../../../../org/apache/hadoop/mapreduce/lib/fieldsel/FieldSelectionReducer.html" title="class in org.apache.hadoop.mapreduce.lib.fieldsel">FieldSelectionReducer</a>, <a href="../../../../org/apache/hadoop/mapreduce/lib/reduce/IntSumReducer.html" title="class in org.apache.hadoop.mapreduce.lib.reduce">IntSumReducer</a>, <a href="../../../../org/apache/hadoop/mapreduce/lib/reduce/LongSumReducer.html" title="class in org.apache.hadoop.mapreduce.lib.reduce">LongSumReducer</a>, <a href="../../../../org/apache/hadoop/mapreduce/lib/aggregate/ValueAggregatorCombiner.html" title="class in org.apache.hadoop.mapreduce.lib.aggregate">ValueAggregatorCombiner</a>, <a href="../../../../org/apache/hadoop/mapreduce/lib/aggregate/ValueAggregatorReducer.html" title="class in org.apache.hadoop.mapreduce.lib.aggregate">ValueAggregatorReducer</a>, <a href="../../../../org/apache/hadoop/mapreduce/lib/reduce/WrappedReducer.html" title="class in org.apache.hadoop.mapreduce.lib.reduce">WrappedReducer</a></dd> |
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| <pre><a href="../../../../org/apache/hadoop/mapreduce/task/annotation/Checkpointable.html" title="annotation in org.apache.hadoop.mapreduce.task.annotation">@Checkpointable</a> |
| @InterfaceAudience.Public |
| @InterfaceStability.Stable |
| public class <span class="typeNameLabel">Reducer<KEYIN,VALUEIN,KEYOUT,VALUEOUT></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></pre> |
| <div class="block">Reduces a set of intermediate values which share a key to a smaller set of |
| values. |
| |
| <p><code>Reducer</code> implementations |
| can access the <a href="../../../../org/apache/hadoop/conf/Configuration.html" title="class in org.apache.hadoop.conf"><code>Configuration</code></a> for the job via the |
| <a href="../../../../org/apache/hadoop/mapreduce/JobContext.html#getConfiguration--"><code>JobContext.getConfiguration()</code></a> method.</p> |
| |
| <p><code>Reducer</code> has 3 primary phases:</p> |
| <ol> |
| <li> |
| |
| <b id="Shuffle">Shuffle</b> |
| |
| <p>The <code>Reducer</code> copies the sorted output from each |
| <a href="../../../../org/apache/hadoop/mapreduce/Mapper.html" title="class in org.apache.hadoop.mapreduce"><code>Mapper</code></a> using HTTP across the network.</p> |
| </li> |
| |
| <li> |
| <b id="Sort">Sort</b> |
| |
| <p>The framework merge sorts <code>Reducer</code> inputs by |
| <code>key</code>s |
| (since different <code>Mapper</code>s may have output the same key).</p> |
| |
| <p>The shuffle and sort phases occur simultaneously i.e. while outputs are |
| being fetched they are merged.</p> |
| |
| <b id="SecondarySort">SecondarySort</b> |
| |
| <p>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 |
| <a href="../../../../org/apache/hadoop/mapreduce/Job.html#setGroupingComparatorClass-java.lang.Class-"><code>Job.setGroupingComparatorClass(Class)</code></a>. The sort order is |
| controlled by |
| <a href="../../../../org/apache/hadoop/mapreduce/Job.html#setSortComparatorClass-java.lang.Class-"><code>Job.setSortComparatorClass(Class)</code></a>.</p> |
| |
| |
| 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: |
| <ul> |
| <li>Map Input Key: url</li> |
| <li>Map Input Value: document</li> |
| <li>Map Output Key: document checksum, url pagerank</li> |
| <li>Map Output Value: url</li> |
| <li>Partitioner: by checksum</li> |
| <li>OutputKeyComparator: by checksum and then decreasing pagerank</li> |
| <li>OutputValueGroupingComparator: by checksum</li> |
| </ul> |
| </li> |
| |
| <li> |
| <b id="Reduce">Reduce</b> |
| |
| <p>In this phase the |
| <a href="../../../../org/apache/hadoop/mapreduce/Reducer.html#reduce-KEYIN-java.lang.Iterable-org.apache.hadoop.mapreduce.Reducer.Context-"><code>reduce(Object, Iterable, org.apache.hadoop.mapreduce.Reducer.Context)</code></a> |
| method is called for each <code><key, (collection of values)></code> in |
| the sorted inputs.</p> |
| <p>The output of the reduce task is typically written to a |
| <a href="../../../../org/apache/hadoop/mapreduce/RecordWriter.html" title="class in org.apache.hadoop.mapreduce"><code>RecordWriter</code></a> via |
| <a href="../../../../org/apache/hadoop/mapreduce/TaskInputOutputContext.html#write-KEYOUT-VALUEOUT-"><code>TaskInputOutputContext.write(Object, Object)</code></a>.</p> |
| </li> |
| </ol> |
| |
| <p>The output of the <code>Reducer</code> is <b>not re-sorted</b>.</p> |
| |
| <p>Example:</p> |
| <p><blockquote><pre> |
| public class IntSumReducer<Key> extends Reducer<Key,IntWritable, |
| Key,IntWritable> { |
| private IntWritable result = new IntWritable(); |
| |
| public void reduce(Key key, Iterable<IntWritable> values, |
| Context context) throws IOException, InterruptedException { |
| int sum = 0; |
| for (IntWritable val : values) { |
| sum += val.get(); |
| } |
| result.set(sum); |
| context.write(key, result); |
| } |
| } |
| </pre></blockquote></div> |
| <dl> |
| <dt><span class="seeLabel">See Also:</span></dt> |
| <dd><a href="../../../../org/apache/hadoop/mapreduce/Mapper.html" title="class in org.apache.hadoop.mapreduce"><code>Mapper</code></a>, |
| <a href="../../../../org/apache/hadoop/mapreduce/Partitioner.html" title="class in org.apache.hadoop.mapreduce"><code>Partitioner</code></a></dd> |
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| <td class="colOne"><code><span class="memberNameLink"><a href="../../../../org/apache/hadoop/mapreduce/Reducer.html#Reducer--">Reducer</a></span>()</code> </td> |
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| <tr> |
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| <td class="colFirst"><code>protected void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../org/apache/hadoop/mapreduce/Reducer.html#cleanup-org.apache.hadoop.mapreduce.Reducer.Context-">cleanup</a></span>(org.apache.hadoop.mapreduce.Reducer.Context context)</code> |
| <div class="block">Called once at the end of the task.</div> |
| </td> |
| </tr> |
| <tr id="i1" class="rowColor"> |
| <td class="colFirst"><code>protected void</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../org/apache/hadoop/mapreduce/Reducer.html#reduce-KEYIN-java.lang.Iterable-org.apache.hadoop.mapreduce.Reducer.Context-">reduce</a></span>(<a href="../../../../org/apache/hadoop/mapreduce/Reducer.html" title="type parameter in Reducer">KEYIN</a> key, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Iterable.html?is-external=true" title="class or interface in java.lang">Iterable</a><<a href="../../../../org/apache/hadoop/mapreduce/Reducer.html" title="type parameter in Reducer">VALUEIN</a>> values, |
| org.apache.hadoop.mapreduce.Reducer.Context context)</code> |
| <div class="block">This method is called once for each key.</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/mapreduce/Reducer.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>run(org.apache.hadoop.mapreduce.Reducer.Context)</code></a> method to |
| control how the reduce task works.</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/Reducer.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> |
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| <h3>Constructor Detail</h3> |
| <a name="Reducer--"> |
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| <h4>Reducer</h4> |
| <pre>public Reducer()</pre> |
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| <h3>Method Detail</h3> |
| <a name="setup-org.apache.hadoop.mapreduce.Reducer.Context-"> |
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| <ul class="blockList"> |
| <li class="blockList"> |
| <h4>setup</h4> |
| <pre>protected void setup(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">Called once at the start of the task.</div> |
| <dl> |
| <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> |
| </dl> |
| </li> |
| </ul> |
| <a name="reduce-java.lang.Object-java.lang.Iterable-org.apache.hadoop.mapreduce.Reducer.Context-"> |
| <!-- --> |
| </a><a name="reduce-KEYIN-java.lang.Iterable-org.apache.hadoop.mapreduce.Reducer.Context-"> |
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| <h4>reduce</h4> |
| <pre>protected void reduce(<a href="../../../../org/apache/hadoop/mapreduce/Reducer.html" title="type parameter in Reducer">KEYIN</a> key, |
| <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/Iterable.html?is-external=true" title="class or interface in java.lang">Iterable</a><<a href="../../../../org/apache/hadoop/mapreduce/Reducer.html" title="type parameter in Reducer">VALUEIN</a>> values, |
| 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">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.</div> |
| <dl> |
| <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> |
| </dl> |
| </li> |
| </ul> |
| <a name="cleanup-org.apache.hadoop.mapreduce.Reducer.Context-"> |
| <!-- --> |
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| <h4>cleanup</h4> |
| <pre>protected void cleanup(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">Called once at the end of the task.</div> |
| <dl> |
| <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> |
| </dl> |
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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">Advanced application writers can use the |
| <a href="../../../../org/apache/hadoop/mapreduce/Reducer.html#run-org.apache.hadoop.mapreduce.Reducer.Context-"><code>run(org.apache.hadoop.mapreduce.Reducer.Context)</code></a> method to |
| control how the reduce task works.</div> |
| <dl> |
| <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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