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7Reducer (Hadoop 0.20.1 API)
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91org.apache.hadoop.mapred</FONT>
92<BR>
93Interface Reducer&lt;K2,V2,K3,V3&gt;</H2>
94<DL>
95<DT><B>All Superinterfaces:</B> <DD><A HREF="http://java.sun.com/javase/6/docs/api/java/io/Closeable.html?is-external=true" title="class or interface in java.io">Closeable</A>, <A HREF="../../../../org/apache/hadoop/mapred/JobConfigurable.html" title="interface in org.apache.hadoop.mapred">JobConfigurable</A></DD>
96</DL>
97<DL>
98<DT><B>All Known Implementing Classes:</B> <DD><A HREF="../../../../org/apache/hadoop/mapred/lib/ChainReducer.html" title="class in org.apache.hadoop.mapred.lib">ChainReducer</A>, <A HREF="../../../../org/apache/hadoop/contrib/utils/join/DataJoinMapperBase.html" title="class in org.apache.hadoop.contrib.utils.join">DataJoinMapperBase</A>, <A HREF="../../../../org/apache/hadoop/contrib/utils/join/DataJoinReducerBase.html" title="class in org.apache.hadoop.contrib.utils.join">DataJoinReducerBase</A>, <A HREF="../../../../org/apache/hadoop/mapred/lib/FieldSelectionMapReduce.html" title="class in org.apache.hadoop.mapred.lib">FieldSelectionMapReduce</A>, <A HREF="../../../../org/apache/hadoop/mapred/lib/IdentityReducer.html" title="class in org.apache.hadoop.mapred.lib">IdentityReducer</A>, <A HREF="../../../../org/apache/hadoop/contrib/index/mapred/IndexUpdateCombiner.html" title="class in org.apache.hadoop.contrib.index.mapred">IndexUpdateCombiner</A>, <A HREF="../../../../org/apache/hadoop/contrib/index/mapred/IndexUpdateReducer.html" title="class in org.apache.hadoop.contrib.index.mapred">IndexUpdateReducer</A>, <A HREF="../../../../org/apache/hadoop/contrib/utils/join/JobBase.html" title="class in org.apache.hadoop.contrib.utils.join">JobBase</A>, <A HREF="../../../../org/apache/hadoop/mapred/lib/LongSumReducer.html" title="class in org.apache.hadoop.mapred.lib">LongSumReducer</A>, <A HREF="../../../../org/apache/hadoop/examples/PiEstimator.PiReducer.html" title="class in org.apache.hadoop.examples">PiEstimator.PiReducer</A>, <A HREF="../../../../org/apache/hadoop/streaming/PipeReducer.html" title="class in org.apache.hadoop.streaming">PipeReducer</A>, <A HREF="../../../../org/apache/hadoop/examples/SleepJob.html" title="class in org.apache.hadoop.examples">SleepJob</A>, <A HREF="../../../../org/apache/hadoop/mapred/lib/aggregate/ValueAggregatorCombiner.html" title="class in org.apache.hadoop.mapred.lib.aggregate">ValueAggregatorCombiner</A>, <A HREF="../../../../org/apache/hadoop/mapred/lib/aggregate/ValueAggregatorJobBase.html" title="class in org.apache.hadoop.mapred.lib.aggregate">ValueAggregatorJobBase</A>, <A HREF="../../../../org/apache/hadoop/mapred/lib/aggregate/ValueAggregatorMapper.html" title="class in org.apache.hadoop.mapred.lib.aggregate">ValueAggregatorMapper</A>, <A HREF="../../../../org/apache/hadoop/mapred/lib/aggregate/ValueAggregatorReducer.html" title="class in org.apache.hadoop.mapred.lib.aggregate">ValueAggregatorReducer</A></DD>
99</DL>
100<HR>
101<B>Deprecated.</B>&nbsp;<I>Use <A HREF="../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce"><CODE>Reducer</CODE></A> instead.</I>
102<P>
103<DL>
104<DT><PRE><FONT SIZE="-1"><A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Deprecated.html?is-external=true" title="class or interface in java.lang">@Deprecated</A>
105</FONT>public interface <B>Reducer&lt;K2,V2,K3,V3&gt;</B><DT>extends <A HREF="../../../../org/apache/hadoop/mapred/JobConfigurable.html" title="interface in org.apache.hadoop.mapred">JobConfigurable</A>, <A HREF="../../../../org/apache/hadoop/io/Closeable.html" title="interface in org.apache.hadoop.io">Closeable</A></DL>
106</PRE>
107
108<P>
109Reduces a set of intermediate values which share a key to a smaller set of
110 values. 
111 
112 <p>The number of <code>Reducer</code>s for the job is set by the user via
113 <A HREF="../../../../org/apache/hadoop/mapred/JobConf.html#setNumReduceTasks(int)"><CODE>JobConf.setNumReduceTasks(int)</CODE></A>. <code>Reducer</code> implementations
114 can access the <A HREF="../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred"><CODE>JobConf</CODE></A> for the job via the
115 <A HREF="../../../../org/apache/hadoop/mapred/JobConfigurable.html#configure(org.apache.hadoop.mapred.JobConf)"><CODE>JobConfigurable.configure(JobConf)</CODE></A> method and initialize themselves.
116 Similarly they can use the <A HREF="http://java.sun.com/javase/6/docs/api/java/io/Closeable.html?is-external=true#close()" title="class or interface in java.io"><CODE>Closeable.close()</CODE></A> method for
117 de-initialization.</p>
118
119 <p><code>Reducer</code> has 3 primary phases:</p>
120 <ol>
121   <li>
122   
123   <h4 id="Shuffle">Shuffle</h4>
124   
125   <p><code>Reducer</code> is input the grouped output of a <A HREF="../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred"><CODE>Mapper</CODE></A>.
126   In the phase the framework, for each <code>Reducer</code>, fetches the
127   relevant partition of the output of all the <code>Mapper</code>s, via HTTP.
128   </p>
129   </li>
130   
131   <li>
132   <h4 id="Sort">Sort</h4>
133   
134   <p>The framework groups <code>Reducer</code> inputs by <code>key</code>s
135   (since different <code>Mapper</code>s may have output the same key) in this
136   stage.</p>
137   
138   <p>The shuffle and sort phases occur simultaneously i.e. while outputs are
139   being fetched they are merged.</p>
140     
141   <h5 id="SecondarySort">SecondarySort</h5>
142   
143   <p>If equivalence rules for keys while grouping the intermediates are
144   different from those for grouping keys before reduction, then one may
145   specify a <code>Comparator</code> via
146   <A HREF="../../../../org/apache/hadoop/mapred/JobConf.html#setOutputValueGroupingComparator(java.lang.Class)"><CODE>JobConf.setOutputValueGroupingComparator(Class)</CODE></A>.Since
147   <A HREF="../../../../org/apache/hadoop/mapred/JobConf.html#setOutputKeyComparatorClass(java.lang.Class)"><CODE>JobConf.setOutputKeyComparatorClass(Class)</CODE></A> can be used to
148   control how intermediate keys are grouped, these can be used in conjunction
149   to simulate <i>secondary sort on values</i>.</p>
150   
151   
152   For example, say that you want to find duplicate web pages and tag them
153   all with the url of the "best" known example. You would set up the job
154   like:
155   <ul>
156     <li>Map Input Key: url</li>
157     <li>Map Input Value: document</li>
158     <li>Map Output Key: document checksum, url pagerank</li>
159     <li>Map Output Value: url</li>
160     <li>Partitioner: by checksum</li>
161     <li>OutputKeyComparator: by checksum and then decreasing pagerank</li>
162     <li>OutputValueGroupingComparator: by checksum</li>
163   </ul>
164   </li>
165   
166   <li>   
167   <h4 id="Reduce">Reduce</h4>
168   
169   <p>In this phase the
170   <A HREF="../../../../org/apache/hadoop/mapred/Reducer.html#reduce(K2, java.util.Iterator, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)"><CODE>reduce(Object, Iterator, OutputCollector, Reporter)</CODE></A>
171   method is called for each <code>&lt;key, (list of values)></code> pair in
172   the grouped inputs.</p>
173   <p>The output of the reduce task is typically written to the
174   <A HREF="../../../../org/apache/hadoop/fs/FileSystem.html" title="class in org.apache.hadoop.fs"><CODE>FileSystem</CODE></A> via
175   <A HREF="../../../../org/apache/hadoop/mapred/OutputCollector.html#collect(K, V)"><CODE>OutputCollector.collect(Object, Object)</CODE></A>.</p>
176   </li>
177 </ol>
178 
179 <p>The output of the <code>Reducer</code> is <b>not re-sorted</b>.</p>
180 
181 <p>Example:</p>
182 <p><blockquote><pre>
183     public class MyReducer&lt;K extends WritableComparable, V extends Writable&gt; 
184     extends MapReduceBase implements Reducer&lt;K, V, K, V&gt; {
185     
186       static enum MyCounters { NUM_RECORDS }
187       
188       private String reduceTaskId;
189       private int noKeys = 0;
190       
191       public void configure(JobConf job) {
192         reduceTaskId = job.get("mapred.task.id");
193       }
194       
195       public void reduce(K key, Iterator&lt;V&gt; values,
196                          OutputCollector&lt;K, V&gt; output,
197                          Reporter reporter)
198       throws IOException {
199       
200         // Process
201         int noValues = 0;
202         while (values.hasNext()) {
203           V value = values.next();
204           
205           // Increment the no. of values for this key
206           ++noValues;
207           
208           // Process the &lt;key, value&gt; pair (assume this takes a while)
209           // ...
210           // ...
211           
212           // Let the framework know that we are alive, and kicking!
213           if ((noValues%10) == 0) {
214             reporter.progress();
215           }
216         
217           // Process some more
218           // ...
219           // ...
220           
221           // Output the &lt;key, value&gt; 
222           output.collect(key, value);
223         }
224         
225         // Increment the no. of &lt;key, list of values&gt; pairs processed
226         ++noKeys;
227         
228         // Increment counters
229         reporter.incrCounter(NUM_RECORDS, 1);
230         
231         // Every 100 keys update application-level status
232         if ((noKeys%100) == 0) {
233           reporter.setStatus(reduceTaskId + " processed " + noKeys);
234         }
235       }
236     }
237 </pre></blockquote></p>
238<P>
239
240<P>
241<DL>
242<DT><B>See Also:</B><DD><A HREF="../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred"><CODE>Mapper</CODE></A>,
243<A HREF="../../../../org/apache/hadoop/mapred/Partitioner.html" title="interface in org.apache.hadoop.mapred"><CODE>Partitioner</CODE></A>,
244<A HREF="../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred"><CODE>Reporter</CODE></A>,
245<A HREF="../../../../org/apache/hadoop/mapred/MapReduceBase.html" title="class in org.apache.hadoop.mapred"><CODE>MapReduceBase</CODE></A></DL>
246<HR>
247
248<P>
249
250<!-- ========== METHOD SUMMARY =========== -->
251
252<A NAME="method_summary"><!-- --></A>
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255<TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2">
256<B>Method Summary</B></FONT></TH>
257</TR>
258<TR BGCOLOR="white" CLASS="TableRowColor">
259<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
260<CODE>&nbsp;void</CODE></FONT></TD>
261<TD><CODE><B><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></B>(<A HREF="../../../../org/apache/hadoop/mapred/Reducer.html" title="type parameter in Reducer">K2</A>&nbsp;key,
262       <A HREF="http://java.sun.com/javase/6/docs/api/java/util/Iterator.html?is-external=true" title="class or interface in java.util">Iterator</A>&lt;<A HREF="../../../../org/apache/hadoop/mapred/Reducer.html" title="type parameter in Reducer">V2</A>&gt;&nbsp;values,
263       <A HREF="../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</A>&lt;<A HREF="../../../../org/apache/hadoop/mapred/Reducer.html" title="type parameter in Reducer">K3</A>,<A HREF="../../../../org/apache/hadoop/mapred/Reducer.html" title="type parameter in Reducer">V3</A>&gt;&nbsp;output,
264       <A HREF="../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</A>&nbsp;reporter)</CODE>
265
266<BR>
267&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<B>Deprecated.</B>&nbsp;<i>Reduces</i> values for a given key.</TD>
268</TR>
269</TABLE>
270&nbsp;<A NAME="methods_inherited_from_class_org.apache.hadoop.mapred.JobConfigurable"><!-- --></A>
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272<TR BGCOLOR="#EEEEFF" CLASS="TableSubHeadingColor">
273<TH ALIGN="left"><B>Methods inherited from interface org.apache.hadoop.mapred.<A HREF="../../../../org/apache/hadoop/mapred/JobConfigurable.html" title="interface in org.apache.hadoop.mapred">JobConfigurable</A></B></TH>
274</TR>
275<TR BGCOLOR="white" CLASS="TableRowColor">
276<TD><CODE><A HREF="../../../../org/apache/hadoop/mapred/JobConfigurable.html#configure(org.apache.hadoop.mapred.JobConf)">configure</A></CODE></TD>
277</TR>
278</TABLE>
279&nbsp;<A NAME="methods_inherited_from_class_java.io.Closeable"><!-- --></A>
280<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
281<TR BGCOLOR="#EEEEFF" CLASS="TableSubHeadingColor">
282<TH ALIGN="left"><B>Methods inherited from interface java.io.<A HREF="http://java.sun.com/javase/6/docs/api/java/io/Closeable.html?is-external=true" title="class or interface in java.io">Closeable</A></B></TH>
283</TR>
284<TR BGCOLOR="white" CLASS="TableRowColor">
285<TD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/Closeable.html?is-external=true#close()" title="class or interface in java.io">close</A></CODE></TD>
286</TR>
287</TABLE>
288&nbsp;
289<P>
290
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292
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296<TH ALIGN="left" COLSPAN="1"><FONT SIZE="+2">
297<B>Method Detail</B></FONT></TH>
298</TR>
299</TABLE>
300
301<A NAME="reduce(java.lang.Object,java.util.Iterator,org.apache.hadoop.mapred.OutputCollector,org.apache.hadoop.mapred.Reporter)"><!-- --></A><A NAME="reduce(K2, java.util.Iterator, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)"><!-- --></A><H3>
302reduce</H3>
303<PRE>
304void <B>reduce</B>(<A HREF="../../../../org/apache/hadoop/mapred/Reducer.html" title="type parameter in Reducer">K2</A>&nbsp;key,
305            <A HREF="http://java.sun.com/javase/6/docs/api/java/util/Iterator.html?is-external=true" title="class or interface in java.util">Iterator</A>&lt;<A HREF="../../../../org/apache/hadoop/mapred/Reducer.html" title="type parameter in Reducer">V2</A>&gt;&nbsp;values,
306            <A HREF="../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</A>&lt;<A HREF="../../../../org/apache/hadoop/mapred/Reducer.html" title="type parameter in Reducer">K3</A>,<A HREF="../../../../org/apache/hadoop/mapred/Reducer.html" title="type parameter in Reducer">V3</A>&gt;&nbsp;output,
307            <A HREF="../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</A>&nbsp;reporter)
308            throws <A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></PRE>
309<DL>
310<DD><B>Deprecated.</B>&nbsp;<DD><i>Reduces</i> values for a given key. 
311 
312 <p>The framework calls this method for each
313 <code>&lt;key, (list of values)></code> pair in the grouped inputs.
314 Output values must be of the same type as input values.  Input keys must
315 not be altered. The framework will <b>reuse</b> the key and value objects
316 that are passed into the reduce, therefore the application should clone
317 the objects they want to keep a copy of. In many cases, all values are
318 combined into zero or one value.
319 </p>
320   
321 <p>Output pairs are collected with calls to 
322 <A HREF="../../../../org/apache/hadoop/mapred/OutputCollector.html#collect(K, V)"><CODE>OutputCollector.collect(Object,Object)</CODE></A>.</p>
323
324 <p>Applications can use the <A HREF="../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred"><CODE>Reporter</CODE></A> provided to report progress
325 or just indicate that they are alive. In scenarios where the application
326 takes an insignificant amount of time to process individual key/value
327 pairs, this is crucial since the framework might assume that the task has
328 timed-out and kill that task. The other way of avoiding this is to set
329 <a href="../../../../../mapred-default.html#mapred.task.timeout">
330 mapred.task.timeout</a> to a high-enough value (or even zero for no
331 time-outs).</p>
332<P>
333<DD><DL>
334</DL>
335</DD>
336<DD><DL>
337<DT><B>Parameters:</B><DD><CODE>key</CODE> - the key.<DD><CODE>values</CODE> - the list of values to reduce.<DD><CODE>output</CODE> - to collect keys and combined values.<DD><CODE>reporter</CODE> - facility to report progress.
338<DT><B>Throws:</B>
339<DD><CODE><A HREF="http://java.sun.com/javase/6/docs/api/java/io/IOException.html?is-external=true" title="class or interface in java.io">IOException</A></CODE></DL>
340</DD>
341</DL>
342<!-- ========= END OF CLASS DATA ========= -->
343<HR>
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