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.JobConf;
023 import org.apache.hadoop.mapred.Mapper;
024 import org.apache.hadoop.mapred.OutputCollector;
025 import org.apache.hadoop.mapred.Reporter;
026
027 import java.io.IOException;
028
029 /**
030 * The ChainMapper class allows to use multiple Mapper classes within a single
031 * Map task.
032 * <p/>
033 * The Mapper classes are invoked in a chained (or piped) fashion, the output of
034 * the first becomes the input of the second, and so on until the last Mapper,
035 * the output of the last Mapper will be written to the task's output.
036 * <p/>
037 * The key functionality of this feature is that the Mappers in the chain do not
038 * need to be aware that they are executed in a chain. This enables having
039 * reusable specialized Mappers that can be combined to perform composite
040 * operations within a single task.
041 * <p/>
042 * Special care has to be taken when creating chains that the key/values output
043 * by a Mapper are valid for the following Mapper in the chain. It is assumed
044 * all Mappers and the Reduce in the chain use maching output and input key and
045 * value classes as no conversion is done by the chaining code.
046 * <p/>
047 * Using the ChainMapper and the ChainReducer classes is possible to compose
048 * Map/Reduce jobs that look like <code>[MAP+ / REDUCE MAP*]</code>. And
049 * immediate benefit of this pattern is a dramatic reduction in disk IO.
050 * <p/>
051 * IMPORTANT: There is no need to specify the output key/value classes for the
052 * ChainMapper, this is done by the addMapper for the last mapper in the chain.
053 * <p/>
054 * ChainMapper 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 ChainMapper implements Mapper {
095
096 /**
097 * Adds a Mapper 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 Mapper leverages the
101 * assumed semantics that the key and values are not modified by the collector
102 * 'by value' must be used. If the Mapper 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 Mapper the configuration given for it,
107 * <code>mapperConf</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 * ChainMapper, this is done by the addMapper for the last mapper in the chain
112 * <p/>
113 *
114 * @param job job's JobConf to add the Mapper class.
115 * @param klass the Mapper class to add.
116 * @param inputKeyClass mapper input key class.
117 * @param inputValueClass mapper input value class.
118 * @param outputKeyClass mapper output key class.
119 * @param outputValueClass mapper 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 mapperConf a JobConf with the configuration for the Mapper
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 addMapper(JobConf job,
127 Class<? extends Mapper<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 mapperConf) {
133 job.setMapperClass(ChainMapper.class);
134 job.setMapOutputKeyClass(outputKeyClass);
135 job.setMapOutputValueClass(outputValueClass);
136 Chain.addMapper(true, job, klass, inputKeyClass, inputValueClass,
137 outputKeyClass, outputValueClass, byValue, mapperConf);
138 }
139
140 private Chain chain;
141
142 /**
143 * Constructor.
144 */
145 public ChainMapper() {
146 chain = new Chain(true);
147 }
148
149 /**
150 * Configures the ChainMapper and all the Mappers in the chain.
151 * <p/>
152 * If this method is overriden <code>super.configure(...)</code> should be
153 * invoked at the beginning of the overwriter method.
154 */
155 public void configure(JobConf job) {
156 chain.configure(job);
157 }
158
159 /**
160 * Chains the <code>map(...)</code> methods of the Mappers in the chain.
161 */
162 @SuppressWarnings({"unchecked"})
163 public void map(Object key, Object value, OutputCollector output,
164 Reporter reporter) throws IOException {
165 Mapper mapper = chain.getFirstMap();
166 if (mapper != null) {
167 mapper.map(key, value, chain.getMapperCollector(0, output, reporter),
168 reporter);
169 }
170 }
171
172 /**
173 * Closes the ChainMapper and all the Mappers in the chain.
174 * <p/>
175 * If this method is overriden <code>super.close()</code> should be
176 * invoked at the end of the overwriter method.
177 */
178 public void close() throws IOException {
179 chain.close();
180 }
181
182 }