ChainMapper (Hadoop 1.2.1 API)
Date: 2013-07-22

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## org.apache.hadoop.mapred.lib
Class ChainMapper

    java.lang.Object
      org.apache.hadoop.mapred.lib.ChainMapper

All Implemented Interfaces:
  Closeable, JobConfigurable, Mapper

    public class ChainMapper

extends Object
implements Mapper

The ChainMapper class allows to use multiple Mapper classes within a single Map task.

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.

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.

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.

Using the ChainMapper and the ChainReducer classes is possible to compose Map/Reduce jobs that look like `[MAP+ / REDUCE MAP*]`. And immediate benefit of this pattern is a dramatic reduction in disk IO.

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.

ChainMapper usage pattern:

     ...
     conf.setJobName("chain");
     conf.setInputFormat(TextInputFormat.class);
     conf.setOutputFormat(TextOutputFormat.class);

     JobConf mapAConf = new JobConf(false);
     ...
     ChainMapper.addMapper(conf, AMap.class, LongWritable.class, Text.class,
       Text.class, Text.class, true, mapAConf);

     JobConf mapBConf = new JobConf(false);
     ...
     ChainMapper.addMapper(conf, BMap.class, Text.class, Text.class,
       LongWritable.class, Text.class, false, mapBConf);

     JobConf reduceConf = new JobConf(false);
     ...
     ChainReducer.setReducer(conf, XReduce.class, LongWritable.class, Text.class,
       Text.class, Text.class, true, reduceConf);

     ChainReducer.addMapper(conf, CMap.class, Text.class, Text.class,
       LongWritable.class, Text.class, false, null);

     ChainReducer.addMapper(conf, DMap.class, LongWritable.class, Text.class,
       LongWritable.class, LongWritable.class, true, null);

     FileInputFormat.setInputPaths(conf, inDir);
     FileOutputFormat.setOutputPath(conf, outDir);
     ...

     JobClient jc = new JobClient(conf);
     RunningJob job = jc.submitJob(conf);
     ...

Constructor Summary
`ChainMapper()` Constructor.

Method Summary
`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)` Adds a Mapper class to the chain job's JobConf.
`void` | `close()` Closes the ChainMapper and all the Mappers in the chain.
`void` | `configure(JobConf job)` Configures the ChainMapper and all the Mappers in the chain.
`void` | `map(Object key, Object value, OutputCollector output, Reporter reporter)` Chains the `map(...)` methods of the Mappers in the chain.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### ChainMapper

    public ChainMapper()

  Constructor.

Method Detail

### addMapper

    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)

  Adds a Mapper class to the chain job's JobConf.

  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.

  For the added Mapper the configuration given for it, `mapperConf`, have precedence over the job's JobConf. This precedence is in effect when the task is running.

  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

  Parameters:
    `job` - job's JobConf to add the Mapper class.
    `klass` - the Mapper class to add.
    `inputKeyClass` - mapper input key class.
    `inputValueClass` - mapper input value class.
    `outputKeyClass` - mapper output key class.
    `outputValueClass` - mapper output value class.
    `byValue` - indicates if key/values should be passed by value to the next Mapper in the chain, if any.
    `mapperConf` - a JobConf with the configuration for the Mapper class. It is recommended to use a JobConf without default values using the `JobConf(boolean loadDefaults)` constructor with FALSE.

### configure

    public void configure(JobConf job)

  Configures the ChainMapper and all the Mappers in the chain.

  If this method is overriden `super.configure(...)` should be invoked at the beginning of the overwriter method.

  Specified by:
    `configure` in interface `JobConfigurable`
  Parameters:
    `job` - the configuration

### map

    public void map(Object key,
                    Object value,
                    OutputCollector output,
                    Reporter reporter)
             throws IOException

  Chains the `map(...)` methods of the Mappers in the chain.

  Specified by:
    `map` in interface `Mapper`
  Parameters:
    `key` - the input key.
    `value` - the input value.
    `output` - collects mapped keys and values.
    `reporter` - facility to report progress.
  Throws:
    `IOException`

### close

    public void close()
               throws IOException

  Closes the ChainMapper and all the Mappers in the chain.

  If this method is overriden `super.close()` should be invoked at the end of the overwriter method.

  Specified by:
    `close` in interface `Closeable`
  Throws:
    `IOException`

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