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7ChainReducer (Hadoop 0.20.1 API)
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91org.apache.hadoop.mapred.lib</FONT>
92<BR>
93Class ChainReducer</H2>
94<PRE>
95<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">java.lang.Object</A>
96  <IMG SRC="../../../../../resources/inherit.gif" ALT="extended by "><B>org.apache.hadoop.mapred.lib.ChainReducer</B>
97</PRE>
98<DL>
99<DT><B>All Implemented Interfaces:</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>, <A HREF="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</A></DD>
100</DL>
101<HR>
102<DL>
103<DT><PRE>public class <B>ChainReducer</B><DT>extends <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</A><DT>implements <A HREF="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</A></DL>
104</PRE>
105
106<P>
107The ChainReducer class allows to chain multiple Mapper classes after a
108 Reducer within the Reducer task.
109 <p/>
110 For each record output by the Reducer, the Mapper classes are invoked in a
111 chained (or piped) fashion, the output of the first becomes the input of the
112 second, and so on until the last Mapper, the output of the last Mapper will
113 be written to the task's output.
114 <p/>
115 The key functionality of this feature is that the Mappers in the chain do not
116 need to be aware that they are executed after the Reducer or in a chain.
117 This enables having reusable specialized Mappers that can be combined to
118 perform composite operations within a single task.
119 <p/>
120 Special care has to be taken when creating chains that the key/values output
121 by a Mapper are valid for the following Mapper in the chain. It is assumed
122 all Mappers and the Reduce in the chain use maching output and input key and
123 value classes as no conversion is done by the chaining code.
124 <p/>
125 Using the ChainMapper and the ChainReducer classes is possible to compose
126 Map/Reduce jobs that look like <code>[MAP+ / REDUCE MAP*]</code>. And
127 immediate benefit of this pattern is a dramatic reduction in disk IO.
128 <p/>
129 IMPORTANT: There is no need to specify the output key/value classes for the
130 ChainReducer, this is done by the setReducer or the addMapper for the last
131 element in the chain.
132 <p/>
133 ChainReducer usage pattern:
134 <p/>
135 <pre>
136 ...
137 conf.setJobName("chain");
138 conf.setInputFormat(TextInputFormat.class);
139 conf.setOutputFormat(TextOutputFormat.class);
140 <p/>
141 JobConf mapAConf = new JobConf(false);
142 ...
143 ChainMapper.addMapper(conf, AMap.class, LongWritable.class, Text.class,
144   Text.class, Text.class, true, mapAConf);
145 <p/>
146 JobConf mapBConf = new JobConf(false);
147 ...
148 ChainMapper.addMapper(conf, BMap.class, Text.class, Text.class,
149   LongWritable.class, Text.class, false, mapBConf);
150 <p/>
151 JobConf reduceConf = new JobConf(false);
152 ...
153 ChainReducer.setReducer(conf, XReduce.class, LongWritable.class, Text.class,
154   Text.class, Text.class, true, reduceConf);
155 <p/>
156 ChainReducer.addMapper(conf, CMap.class, Text.class, Text.class,
157   LongWritable.class, Text.class, false, null);
158 <p/>
159 ChainReducer.addMapper(conf, DMap.class, LongWritable.class, Text.class,
160   LongWritable.class, LongWritable.class, true, null);
161 <p/>
162 FileInputFormat.setInputPaths(conf, inDir);
163 FileOutputFormat.setOutputPath(conf, outDir);
164 ...
165 <p/>
166 JobClient jc = new JobClient(conf);
167 RunningJob job = jc.submitJob(conf);
168 ...
169 </pre>
170<P>
171
172<P>
173<HR>
174
175<P>
176
177<!-- ======== CONSTRUCTOR SUMMARY ======== -->
178
179<A NAME="constructor_summary"><!-- --></A>
180<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
181<TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor">
182<TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2">
183<B>Constructor Summary</B></FONT></TH>
184</TR>
185<TR BGCOLOR="white" CLASS="TableRowColor">
186<TD><CODE><B><A HREF="../../../../../org/apache/hadoop/mapred/lib/ChainReducer.html#ChainReducer()">ChainReducer</A></B>()</CODE>
187
188<BR>
189&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Constructor.</TD>
190</TR>
191</TABLE>
192&nbsp;
193<!-- ========== METHOD SUMMARY =========== -->
194
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198<TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2">
199<B>Method Summary</B></FONT></TH>
200</TR>
201<TR BGCOLOR="white" CLASS="TableRowColor">
202<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
203<CODE>static
204<TABLE BORDER="0" CELLPADDING="0" CELLSPACING="0" SUMMARY="">
205<TR ALIGN="right" VALIGN="">
206<TD NOWRAP><FONT SIZE="-1">
207<CODE>&lt;K1,V1,K2,V2&gt; 
208<BR>
209void</CODE></FONT></TD>
210</TR>
211</TABLE>
212</CODE></FONT></TD>
213<TD><CODE><B><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></B>(<A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;job,
214          <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends <A HREF="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</A>&lt;K1,V1,K2,V2&gt;&gt;&nbsp;klass,
215          <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends K1&gt;&nbsp;inputKeyClass,
216          <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends V1&gt;&nbsp;inputValueClass,
217          <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends K2&gt;&nbsp;outputKeyClass,
218          <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends V2&gt;&nbsp;outputValueClass,
219          boolean&nbsp;byValue,
220          <A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;mapperConf)</CODE>
221
222<BR>
223&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Adds a Mapper class to the chain job's JobConf.</TD>
224</TR>
225<TR BGCOLOR="white" CLASS="TableRowColor">
226<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
227<CODE>&nbsp;void</CODE></FONT></TD>
228<TD><CODE><B><A HREF="../../../../../org/apache/hadoop/mapred/lib/ChainReducer.html#close()">close</A></B>()</CODE>
229
230<BR>
231&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Closes  the ChainReducer, the Reducer and all the Mappers in the chain.</TD>
232</TR>
233<TR BGCOLOR="white" CLASS="TableRowColor">
234<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
235<CODE>&nbsp;void</CODE></FONT></TD>
236<TD><CODE><B><A HREF="../../../../../org/apache/hadoop/mapred/lib/ChainReducer.html#configure(org.apache.hadoop.mapred.JobConf)">configure</A></B>(<A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;job)</CODE>
237
238<BR>
239&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Configures the ChainReducer, the Reducer and all the Mappers in the chain.</TD>
240</TR>
241<TR BGCOLOR="white" CLASS="TableRowColor">
242<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
243<CODE>&nbsp;void</CODE></FONT></TD>
244<TD><CODE><B><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></B>(<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</A>&nbsp;key,
245       <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>&nbsp;values,
246       <A HREF="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</A>&nbsp;output,
247       <A HREF="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</A>&nbsp;reporter)</CODE>
248
249<BR>
250&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Chains the <code>reduce(...)</code> method of the Reducer with the
251 <code>map(...) </code> methods of the Mappers in the chain.</TD>
252</TR>
253<TR BGCOLOR="white" CLASS="TableRowColor">
254<TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1">
255<CODE>static
256<TABLE BORDER="0" CELLPADDING="0" CELLSPACING="0" SUMMARY="">
257<TR ALIGN="right" VALIGN="">
258<TD NOWRAP><FONT SIZE="-1">
259<CODE>&lt;K1,V1,K2,V2&gt; 
260<BR>
261void</CODE></FONT></TD>
262</TR>
263</TABLE>
264</CODE></FONT></TD>
265<TD><CODE><B><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></B>(<A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;job,
266           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends <A HREF="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</A>&lt;K1,V1,K2,V2&gt;&gt;&nbsp;klass,
267           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends K1&gt;&nbsp;inputKeyClass,
268           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends V1&gt;&nbsp;inputValueClass,
269           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends K2&gt;&nbsp;outputKeyClass,
270           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends V2&gt;&nbsp;outputValueClass,
271           boolean&nbsp;byValue,
272           <A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;reducerConf)</CODE>
273
274<BR>
275&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Sets the Reducer class to the chain job's JobConf.</TD>
276</TR>
277</TABLE>
278&nbsp;<A NAME="methods_inherited_from_class_java.lang.Object"><!-- --></A>
279<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
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281<TH ALIGN="left"><B>Methods inherited from class java.lang.<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</A></B></TH>
282</TR>
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285</TR>
286</TABLE>
287&nbsp;
288<P>
289
290<!-- ========= CONSTRUCTOR DETAIL ======== -->
291
292<A NAME="constructor_detail"><!-- --></A>
293<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
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295<TH ALIGN="left" COLSPAN="1"><FONT SIZE="+2">
296<B>Constructor Detail</B></FONT></TH>
297</TR>
298</TABLE>
299
300<A NAME="ChainReducer()"><!-- --></A><H3>
301ChainReducer</H3>
302<PRE>
303public <B>ChainReducer</B>()</PRE>
304<DL>
305<DD>Constructor.
306<P>
307</DL>
308
309<!-- ============ METHOD DETAIL ========== -->
310
311<A NAME="method_detail"><!-- --></A>
312<TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY="">
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314<TH ALIGN="left" COLSPAN="1"><FONT SIZE="+2">
315<B>Method Detail</B></FONT></TH>
316</TR>
317</TABLE>
318
319<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)"><!-- --></A><H3>
320setReducer</H3>
321<PRE>
322public static &lt;K1,V1,K2,V2&gt; void <B>setReducer</B>(<A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;job,
323                                            <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends <A HREF="../../../../../org/apache/hadoop/mapred/Reducer.html" title="interface in org.apache.hadoop.mapred">Reducer</A>&lt;K1,V1,K2,V2&gt;&gt;&nbsp;klass,
324                                            <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends K1&gt;&nbsp;inputKeyClass,
325                                            <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends V1&gt;&nbsp;inputValueClass,
326                                            <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends K2&gt;&nbsp;outputKeyClass,
327                                            <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends V2&gt;&nbsp;outputValueClass,
328                                            boolean&nbsp;byValue,
329                                            <A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;reducerConf)</PRE>
330<DL>
331<DD>Sets the Reducer class to the chain job's JobConf.
332 <p/>
333 It has to be specified how key and values are passed from one element of
334 the chain to the next, by value or by reference. If a Reducer leverages the
335 assumed semantics that the key and values are not modified by the collector
336 'by value' must be used. If the Reducer does not expect this semantics, as
337 an optimization to avoid serialization and deserialization 'by reference'
338 can be used.
339 <p/>
340 For the added Reducer the configuration given for it,
341 <code>reducerConf</code>, have precedence over the job's JobConf. This
342 precedence is in effect when the task is running.
343 <p/>
344 IMPORTANT: There is no need to specify the output key/value classes for the
345 ChainReducer, this is done by the setReducer or the addMapper for the last
346 element in the chain.
347<P>
348<DD><DL>
349</DL>
350</DD>
351<DD><DL>
352<DT><B>Parameters:</B><DD><CODE>job</CODE> - job's JobConf to add the Reducer class.<DD><CODE>klass</CODE> - the Reducer class to add.<DD><CODE>inputKeyClass</CODE> - reducer input key class.<DD><CODE>inputValueClass</CODE> - reducer input value class.<DD><CODE>outputKeyClass</CODE> - reducer output key class.<DD><CODE>outputValueClass</CODE> - reducer output value class.<DD><CODE>byValue</CODE> - indicates if key/values should be passed by value
353 to the next Mapper in the chain, if any.<DD><CODE>reducerConf</CODE> - a JobConf with the configuration for the Reducer
354 class. It is recommended to use a JobConf without default values using the
355 <code>JobConf(boolean loadDefaults)</code> constructor with FALSE.</DL>
356</DD>
357</DL>
358<HR>
359
360<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)"><!-- --></A><H3>
361addMapper</H3>
362<PRE>
363public static &lt;K1,V1,K2,V2&gt; void <B>addMapper</B>(<A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;job,
364                                           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends <A HREF="../../../../../org/apache/hadoop/mapred/Mapper.html" title="interface in org.apache.hadoop.mapred">Mapper</A>&lt;K1,V1,K2,V2&gt;&gt;&nbsp;klass,
365                                           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends K1&gt;&nbsp;inputKeyClass,
366                                           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends V1&gt;&nbsp;inputValueClass,
367                                           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends K2&gt;&nbsp;outputKeyClass,
368                                           <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Class.html?is-external=true" title="class or interface in java.lang">Class</A>&lt;? extends V2&gt;&nbsp;outputValueClass,
369                                           boolean&nbsp;byValue,
370                                           <A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;mapperConf)</PRE>
371<DL>
372<DD>Adds a Mapper class to the chain job's JobConf.
373 <p/>
374 It has to be specified how key and values are passed from one element of
375 the chain to the next, by value or by reference. If a Mapper leverages the
376 assumed semantics that the key and values are not modified by the collector
377 'by value' must be used. If the Mapper does not expect this semantics, as
378 an optimization to avoid serialization and deserialization 'by reference'
379 can be used.
380 <p/>
381 For the added Mapper the configuration given for it,
382 <code>mapperConf</code>, have precedence over the job's JobConf. This
383 precedence is in effect when the task is running.
384 <p/>
385 IMPORTANT: There is no need to specify the output key/value classes for the
386 ChainMapper, this is done by the addMapper for the last mapper in the chain
387 .
388<P>
389<DD><DL>
390</DL>
391</DD>
392<DD><DL>
393<DT><B>Parameters:</B><DD><CODE>job</CODE> - chain job's JobConf to add the Mapper class.<DD><CODE>klass</CODE> - the Mapper class to add.<DD><CODE>inputKeyClass</CODE> - mapper input key class.<DD><CODE>inputValueClass</CODE> - mapper input value class.<DD><CODE>outputKeyClass</CODE> - mapper output key class.<DD><CODE>outputValueClass</CODE> - mapper output value class.<DD><CODE>byValue</CODE> - indicates if key/values should be passed by value
394 to the next Mapper in the chain, if any.<DD><CODE>mapperConf</CODE> - a JobConf with the configuration for the Mapper
395 class. It is recommended to use a JobConf without default values using the
396 <code>JobConf(boolean loadDefaults)</code> constructor with FALSE.</DL>
397</DD>
398</DL>
399<HR>
400
401<A NAME="configure(org.apache.hadoop.mapred.JobConf)"><!-- --></A><H3>
402configure</H3>
403<PRE>
404public void <B>configure</B>(<A HREF="../../../../../org/apache/hadoop/mapred/JobConf.html" title="class in org.apache.hadoop.mapred">JobConf</A>&nbsp;job)</PRE>
405<DL>
406<DD>Configures the ChainReducer, the Reducer and all the Mappers in the chain.
407 <p/>
408 If this method is overriden <code>super.configure(...)</code> should be
409 invoked at the beginning of the overwriter method.
410<P>
411<DD><DL>
412<DT><B>Specified by:</B><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></DL>
413</DD>
414<DD><DL>
415<DT><B>Parameters:</B><DD><CODE>job</CODE> - the configuration</DL>
416</DD>
417</DL>
418<HR>
419
420<A NAME="reduce(java.lang.Object, java.util.Iterator, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)"><!-- --></A><H3>
421reduce</H3>
422<PRE>
423public void <B>reduce</B>(<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</A>&nbsp;key,
424                   <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>&nbsp;values,
425                   <A HREF="../../../../../org/apache/hadoop/mapred/OutputCollector.html" title="interface in org.apache.hadoop.mapred">OutputCollector</A>&nbsp;output,
426                   <A HREF="../../../../../org/apache/hadoop/mapred/Reporter.html" title="interface in org.apache.hadoop.mapred">Reporter</A>&nbsp;reporter)
427            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>
428<DL>
429<DD>Chains the <code>reduce(...)</code> method of the Reducer with the
430 <code>map(...) </code> methods of the Mappers in the chain.
431<P>
432<DD><DL>
433<DT><B>Specified by:</B><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></DL>
434</DD>
435<DD><DL>
436<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.
437<DT><B>Throws:</B>
438<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>
439</DD>
440</DL>
441<HR>
442
443<A NAME="close()"><!-- --></A><H3>
444close</H3>
445<PRE>
446public void <B>close</B>()
447           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>
448<DL>
449<DD>Closes  the ChainReducer, the Reducer and all the Mappers in the chain.
450 <p/>
451 If this method is overriden <code>super.close()</code> should be
452 invoked at the end of the overwriter method.
453<P>
454<DD><DL>
455<DT><B>Specified by:</B><DD><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> in interface <CODE><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></CODE></DL>
456</DD>
457<DD><DL>
458
459<DT><B>Throws:</B>
460<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>
461</DD>
462</DL>
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