001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *     http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing, software
013     * distributed under the License is distributed on an "AS IS" BASIS,
014     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015     * See the License for the specific language governing permissions and
016     * limitations under the License.
017     */
018    package org.apache.hadoop.mapred.lib;
019    
020    import org.apache.hadoop.classification.InterfaceAudience;
021    import org.apache.hadoop.classification.InterfaceStability;
022    import org.apache.hadoop.mapred.*;
023    
024    import java.io.IOException;
025    import java.util.Iterator;
026    
027    /**
028     * The ChainReducer class allows to chain multiple Mapper classes after a
029     * Reducer within the Reducer task.
030     * <p/>
031     * For each record output by the Reducer, the Mapper classes are invoked in a
032     * chained (or piped) fashion, the output of the first becomes the input of the
033     * second, and so on until the last Mapper, the output of the last Mapper will
034     * be written to the task's output.
035     * <p/>
036     * The key functionality of this feature is that the Mappers in the chain do not
037     * need to be aware that they are executed after the Reducer or in a chain.
038     * This enables having reusable specialized Mappers that can be combined to
039     * perform composite operations within a single task.
040     * <p/>
041     * Special care has to be taken when creating chains that the key/values output
042     * by a Mapper are valid for the following Mapper in the chain. It is assumed
043     * all Mappers and the Reduce in the chain use maching output and input key and
044     * value classes as no conversion is done by the chaining code.
045     * <p/>
046     * Using the ChainMapper and the ChainReducer classes is possible to compose
047     * Map/Reduce jobs that look like <code>[MAP+ / REDUCE MAP*]</code>. And
048     * immediate benefit of this pattern is a dramatic reduction in disk IO.
049     * <p/>
050     * IMPORTANT: There is no need to specify the output key/value classes for the
051     * ChainReducer, this is done by the setReducer or the addMapper for the last
052     * element in the chain.
053     * <p/>
054     * ChainReducer usage pattern:
055     * <p/>
056     * <pre>
057     * ...
058     * conf.setJobName("chain");
059     * conf.setInputFormat(TextInputFormat.class);
060     * conf.setOutputFormat(TextOutputFormat.class);
061     * <p/>
062     * JobConf mapAConf = new JobConf(false);
063     * ...
064     * ChainMapper.addMapper(conf, AMap.class, LongWritable.class, Text.class,
065     *   Text.class, Text.class, true, mapAConf);
066     * <p/>
067     * JobConf mapBConf = new JobConf(false);
068     * ...
069     * ChainMapper.addMapper(conf, BMap.class, Text.class, Text.class,
070     *   LongWritable.class, Text.class, false, mapBConf);
071     * <p/>
072     * JobConf reduceConf = new JobConf(false);
073     * ...
074     * ChainReducer.setReducer(conf, XReduce.class, LongWritable.class, Text.class,
075     *   Text.class, Text.class, true, reduceConf);
076     * <p/>
077     * ChainReducer.addMapper(conf, CMap.class, Text.class, Text.class,
078     *   LongWritable.class, Text.class, false, null);
079     * <p/>
080     * ChainReducer.addMapper(conf, DMap.class, LongWritable.class, Text.class,
081     *   LongWritable.class, LongWritable.class, true, null);
082     * <p/>
083     * FileInputFormat.setInputPaths(conf, inDir);
084     * FileOutputFormat.setOutputPath(conf, outDir);
085     * ...
086     * <p/>
087     * JobClient jc = new JobClient(conf);
088     * RunningJob job = jc.submitJob(conf);
089     * ...
090     * </pre>
091     */
092    @InterfaceAudience.Public
093    @InterfaceStability.Stable
094    public class ChainReducer implements Reducer {
095    
096      /**
097       * Sets the Reducer class to the chain job's JobConf.
098       * <p/>
099       * It has to be specified how key and values are passed from one element of
100       * the chain to the next, by value or by reference. If a Reducer leverages the
101       * assumed semantics that the key and values are not modified by the collector
102       * 'by value' must be used. If the Reducer does not expect this semantics, as
103       * an optimization to avoid serialization and deserialization 'by reference'
104       * can be used.
105       * <p/>
106       * For the added Reducer the configuration given for it,
107       * <code>reducerConf</code>, have precedence over the job's JobConf. This
108       * precedence is in effect when the task is running.
109       * <p/>
110       * IMPORTANT: There is no need to specify the output key/value classes for the
111       * ChainReducer, this is done by the setReducer or the addMapper for the last
112       * element in the chain.
113       *
114       * @param job              job's JobConf to add the Reducer class.
115       * @param klass            the Reducer class to add.
116       * @param inputKeyClass    reducer input key class.
117       * @param inputValueClass  reducer input value class.
118       * @param outputKeyClass   reducer output key class.
119       * @param outputValueClass reducer output value class.
120       * @param byValue          indicates if key/values should be passed by value
121       * to the next Mapper in the chain, if any.
122       * @param reducerConf      a JobConf with the configuration for the Reducer
123       * class. It is recommended to use a JobConf without default values using the
124       * <code>JobConf(boolean loadDefaults)</code> constructor with FALSE.
125       */
126      public static <K1, V1, K2, V2> void setReducer(JobConf job,
127                               Class<? extends Reducer<K1, V1, K2, V2>> klass,
128                               Class<? extends K1> inputKeyClass,
129                               Class<? extends V1> inputValueClass,
130                               Class<? extends K2> outputKeyClass,
131                               Class<? extends V2> outputValueClass,
132                               boolean byValue, JobConf reducerConf) {
133        job.setReducerClass(ChainReducer.class);
134        job.setOutputKeyClass(outputKeyClass);
135        job.setOutputValueClass(outputValueClass);
136        Chain.setReducer(job, klass, inputKeyClass, inputValueClass, outputKeyClass,
137                         outputValueClass, byValue, reducerConf);
138      }
139    
140      /**
141       * Adds a Mapper class to the chain job's JobConf.
142       * <p/>
143       * It has to be specified how key and values are passed from one element of
144       * the chain to the next, by value or by reference. If a Mapper leverages the
145       * assumed semantics that the key and values are not modified by the collector
146       * 'by value' must be used. If the Mapper does not expect this semantics, as
147       * an optimization to avoid serialization and deserialization 'by reference'
148       * can be used.
149       * <p/>
150       * For the added Mapper the configuration given for it,
151       * <code>mapperConf</code>, have precedence over the job's JobConf. This
152       * precedence is in effect when the task is running.
153       * <p/>
154       * IMPORTANT: There is no need to specify the output key/value classes for the
155       * ChainMapper, this is done by the addMapper for the last mapper in the chain
156       * .
157       *
158       * @param job              chain job's JobConf to add the Mapper class.
159       * @param klass            the Mapper class to add.
160       * @param inputKeyClass    mapper input key class.
161       * @param inputValueClass  mapper input value class.
162       * @param outputKeyClass   mapper output key class.
163       * @param outputValueClass mapper output value class.
164       * @param byValue          indicates if key/values should be passed by value
165       * to the next Mapper in the chain, if any.
166       * @param mapperConf       a JobConf with the configuration for the Mapper
167       * class. It is recommended to use a JobConf without default values using the
168       * <code>JobConf(boolean loadDefaults)</code> constructor with FALSE.
169       */
170      public static <K1, V1, K2, V2> void addMapper(JobConf job,
171                               Class<? extends Mapper<K1, V1, K2, V2>> klass,
172                               Class<? extends K1> inputKeyClass,
173                               Class<? extends V1> inputValueClass,
174                               Class<? extends K2> outputKeyClass,
175                               Class<? extends V2> outputValueClass,
176                               boolean byValue, JobConf mapperConf) {
177        job.setOutputKeyClass(outputKeyClass);
178        job.setOutputValueClass(outputValueClass);
179        Chain.addMapper(false, job, klass, inputKeyClass, inputValueClass,
180                        outputKeyClass, outputValueClass, byValue, mapperConf);
181      }
182    
183      private Chain chain;
184    
185      /**
186       * Constructor.
187       */
188      public ChainReducer() {
189        chain = new Chain(false);
190      }
191    
192      /**
193       * Configures the ChainReducer, the Reducer and all the Mappers in the chain.
194       * <p/>
195       * If this method is overriden <code>super.configure(...)</code> should be
196       * invoked at the beginning of the overwriter method.
197       */
198      public void configure(JobConf job) {
199        chain.configure(job);
200      }
201    
202      /**
203       * Chains the <code>reduce(...)</code> method of the Reducer with the
204       * <code>map(...) </code> methods of the Mappers in the chain.
205       */
206      @SuppressWarnings({"unchecked"})
207      public void reduce(Object key, Iterator values, OutputCollector output,
208                         Reporter reporter) throws IOException {
209        Reducer reducer = chain.getReducer();
210        if (reducer != null) {
211          reducer.reduce(key, values, chain.getReducerCollector(output, reporter),
212                         reporter);
213        }
214      }
215    
216      /**
217       * Closes  the ChainReducer, the Reducer and all the Mappers in the chain.
218       * <p/>
219       * If this method is overriden <code>super.close()</code> should be
220       * invoked at the end of the overwriter method.
221       */
222      public void close() throws IOException {
223        chain.close();
224      }
225    
226    }