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 */ 018package org.apache.hadoop.mapred.lib; 019 020import org.apache.hadoop.classification.InterfaceAudience; 021import org.apache.hadoop.classification.InterfaceStability; 022import org.apache.hadoop.mapred.JobConf; 023import org.apache.hadoop.mapred.Mapper; 024import org.apache.hadoop.mapred.OutputCollector; 025import org.apache.hadoop.mapred.Reporter; 026 027import 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 * 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 * 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 * 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 * 077 * ChainReducer.addMapper(conf, CMap.class, Text.class, Text.class, 078 * LongWritable.class, Text.class, false, null); 079 * 080 * ChainReducer.addMapper(conf, DMap.class, LongWritable.class, Text.class, 081 * LongWritable.class, LongWritable.class, true, null); 082 * 083 * FileInputFormat.setInputPaths(conf, inDir); 084 * FileOutputFormat.setOutputPath(conf, outDir); 085 * ... 086 * 087 * JobClient jc = new JobClient(conf); 088 * RunningJob job = jc.submitJob(conf); 089 * ... 090 * </pre> 091 */ 092@InterfaceAudience.Public 093@InterfaceStability.Stable 094public 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}