/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.hadoop.mapred.lib;
import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapred.Mapper;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reporter;
import java.io.IOException;
/**
* The ChainMapper class allows to use multiple Mapper classes within a single
* Map task.
* <p/>
* The Mapper classes are invoked in a chained (or piped) fashion, the output of
* the first becomes the input of the second, and so on until the last Mapper,
* the output of the last Mapper will be written to the task's output.
* <p/>
* The key functionality of this feature is that the Mappers in the chain do not
* need to be aware that they are executed in a chain. This enables having
* reusable specialized Mappers that can be combined to perform composite
* operations within a single task.
* <p/>
* Special care has to be taken when creating chains that the key/values output
* by a Mapper are valid for the following Mapper in the chain. It is assumed
* all Mappers and the Reduce in the chain use maching output and input key and
* value classes as no conversion is done by the chaining code.
* <p/>
* Using the ChainMapper and the ChainReducer classes is possible to compose
* Map/Reduce jobs that look like <code>[MAP+ / REDUCE MAP*]</code>. And
* immediate benefit of this pattern is a dramatic reduction in disk IO.
* <p/>
* IMPORTANT: There is no need to specify the output key/value classes for the
* ChainMapper, this is done by the addMapper for the last mapper in the chain.
* <p/>
* ChainMapper usage pattern:
* <p/>
* <pre>
* ...
* conf.setJobName("chain");
* conf.setInputFormat(TextInputFormat.class);
* conf.setOutputFormat(TextOutputFormat.class);
* <p/>
* JobConf mapAConf = new JobConf(false);
* ...
* ChainMapper.addMapper(conf, AMap.class, LongWritable.class, Text.class,
* Text.class, Text.class, true, mapAConf);
* <p/>
* JobConf mapBConf = new JobConf(false);
* ...
* ChainMapper.addMapper(conf, BMap.class, Text.class, Text.class,
* LongWritable.class, Text.class, false, mapBConf);
* <p/>
* JobConf reduceConf = new JobConf(false);
* ...
* ChainReducer.setReducer(conf, XReduce.class, LongWritable.class, Text.class,
* Text.class, Text.class, true, reduceConf);
* <p/>
* ChainReducer.addMapper(conf, CMap.class, Text.class, Text.class,
* LongWritable.class, Text.class, false, null);
* <p/>
* ChainReducer.addMapper(conf, DMap.class, LongWritable.class, Text.class,
* LongWritable.class, LongWritable.class, true, null);
* <p/>
* FileInputFormat.setInputPaths(conf, inDir);
* FileOutputFormat.setOutputPath(conf, outDir);
* ...
* <p/>
* JobClient jc = new JobClient(conf);
* RunningJob job = jc.submitJob(conf);
* ...
* </pre>
*/
public class ChainMapper implements Mapper {
/**
* Adds a Mapper class to the chain job's JobConf.
* <p/>
* It has to be specified how key and values are passed from one element of
* the chain to the next, by value or by reference. If a Mapper leverages the
* assumed semantics that the key and values are not modified by the collector
* 'by value' must be used. If the Mapper does not expect this semantics, as
* an optimization to avoid serialization and deserialization 'by reference'
* can be used.
* <p/>
* For the added Mapper the configuration given for it,
* <code>mapperConf</code>, have precedence over the job's JobConf. This
* precedence is in effect when the task is running.
* <p/>
* IMPORTANT: There is no need to specify the output key/value classes for the
* ChainMapper, this is done by the addMapper for the last mapper in the chain
* <p/>
*
* @param job job's JobConf to add the Mapper class.
* @param klass the Mapper class to add.
* @param inputKeyClass mapper input key class.
* @param inputValueClass mapper input value class.
* @param outputKeyClass mapper output key class.
* @param outputValueClass mapper output value class.
* @param byValue indicates if key/values should be passed by value
* to the next Mapper in the chain, if any.
* @param mapperConf a JobConf with the configuration for the Mapper
* class. It is recommended to use a JobConf without default values using the
* <code>JobConf(boolean loadDefaults)</code> constructor with FALSE.
*/
public static <K1, V1, K2, V2> void addMapper(JobConf job,
Class<? extends Mapper<K1, V1, K2, V2>> klass,
Class<? extends K1> inputKeyClass,
Class<? extends V1> inputValueClass,
Class<? extends K2> outputKeyClass,
Class<? extends V2> outputValueClass,
boolean byValue, JobConf mapperConf) {
job.setMapperClass(ChainMapper.class);
job.setMapOutputKeyClass(outputKeyClass);
job.setMapOutputValueClass(outputValueClass);
Chain.addMapper(true, job, klass, inputKeyClass, inputValueClass,
outputKeyClass, outputValueClass, byValue, mapperConf);
}
private Chain chain;
/**
* Constructor.
*/
public ChainMapper() {
chain = new Chain(true);
}
/**
* Configures the ChainMapper and all the Mappers in the chain.
* <p/>
* If this method is overriden <code>super.configure(...)</code> should be
* invoked at the beginning of the overwriter method.
*/
public void configure(JobConf job) {
chain.configure(job);
}
/**
* Chains the <code>map(...)</code> methods of the Mappers in the chain.
*/
@SuppressWarnings({"unchecked"})
public void map(Object key, Object value, OutputCollector output,
Reporter reporter) throws IOException {
Mapper mapper = chain.getFirstMap();
if (mapper != null) {
mapper.map(key, value, chain.getMapperCollector(0, output, reporter),
reporter);
}
}
/**
* Closes the ChainMapper and all the Mappers in the chain.
* <p/>
* If this method is overriden <code>super.close()</code> should be
* invoked at the end of the overwriter method.
*/
public void close() throws IOException {
chain.close();
}
}