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

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

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

    public class MultipleOutputs

extends Object

The MultipleOutputs class simplifies writting to additional outputs other than the job default output via the `OutputCollector` passed to the `map()` and `reduce()` methods of the `Mapper` and `Reducer` implementations.

Each additional output, or named output, may be configured with its own `OutputFormat`, with its own key class and with its own value class.

A named output can be a single file or a multi file. The later is refered as a multi named output.

A multi named output is an unbound set of files all sharing the same `OutputFormat`, key class and value class configuration.

When named outputs are used within a `Mapper` implementation, key/values written to a name output are not part of the reduce phase, only key/values written to the job `OutputCollector` are part of the reduce phase.

MultipleOutputs supports counters, by default the are disabled. The counters group is the `MultipleOutputs` class name. The names of the counters are the same as the named outputs. For multi named outputs the name of the counter is the concatenation of the named output, and underscore '_' and the multiname.

Job configuration usage pattern is:

     JobConf conf = new JobConf();

     conf.setInputPath(inDir);
     FileOutputFormat.setOutputPath(conf, outDir);

     conf.setMapperClass(MOMap.class);
     conf.setReducerClass(MOReduce.class);
     ...

     // Defines additional single text based output 'text' for the job
     MultipleOutputs.addNamedOutput(conf, "text", TextOutputFormat.class,
     LongWritable.class, Text.class);

     // Defines additional multi sequencefile based output 'sequence' for the
     // job
     MultipleOutputs.addMultiNamedOutput(conf, "seq",
       SequenceFileOutputFormat.class,
       LongWritable.class, Text.class);
     ...

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

     ...

Job configuration usage pattern is:

     public class MOReduce implements
       Reducer<WritableComparable, Writable> {
     private MultipleOutputs mos;

     public void configure(JobConf conf) {
     ...
     mos = new MultipleOutputs(conf);
     }

     public void reduce(WritableComparable key, Iterator<Writable> values,
     OutputCollector output, Reporter reporter)
     throws IOException {
     ...
     mos.getCollector("text", reporter).collect(key, new Text("Hello"));
     mos.getCollector("seq", "A", reporter).collect(key, new Text("Bye"));
     mos.getCollector("seq", "B", reporter).collect(key, new Text("Chau"));
     ...
     }

     public void close() throws IOException {
     mos.close();
     ...
     }

     }

Constructor Summary
`MultipleOutputs(JobConf job)` Creates and initializes multiple named outputs support, it should be instantiated in the Mapper/Reducer configure method.

Method Summary
`static void` | `addMultiNamedOutput(JobConf conf, String namedOutput, Class<? extends OutputFormat> outputFormatClass, Class<?> keyClass, Class<?> valueClass)` Adds a multi named output for the job.
`static void` | `addNamedOutput(JobConf conf, String namedOutput, Class<? extends OutputFormat> outputFormatClass, Class<?> keyClass, Class<?> valueClass)` Adds a named output for the job.
`void` | `close()` Closes all the opened named outputs.
`OutputCollector` | `getCollector(String namedOutput, Reporter reporter)` Gets the output collector for a named output.
`OutputCollector` | `getCollector(String namedOutput, String multiName, Reporter reporter)` Gets the output collector for a multi named output.
`static boolean` | `getCountersEnabled(JobConf conf)` Returns if the counters for the named outputs are enabled or not.
`static Class<? extends OutputFormat>` | `getNamedOutputFormatClass(JobConf conf, String namedOutput)` Returns the named output OutputFormat.
`static Class<? extends WritableComparable>` | `getNamedOutputKeyClass(JobConf conf, String namedOutput)` Returns the key class for a named output.
`Iterator<String>` | `getNamedOutputs()` Returns iterator with the defined name outputs.
`static List<String>` | `getNamedOutputsList(JobConf conf)` Returns list of channel names.
`static Class<? extends Writable>` | `getNamedOutputValueClass(JobConf conf, String namedOutput)` Returns the value class for a named output.
`static boolean` | `isMultiNamedOutput(JobConf conf, String namedOutput)` Returns if a named output is multiple.
`static void` | `setCountersEnabled(JobConf conf, boolean enabled)` Enables or disables counters for the named outputs.

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

Constructor Detail

### MultipleOutputs

    public MultipleOutputs(JobConf job)

  Creates and initializes multiple named outputs support, it should be instantiated in the Mapper/Reducer configure method.

  Parameters:
    `job` - the job configuration object

Method Detail

### getNamedOutputsList

    public static List<String> getNamedOutputsList(JobConf conf)

  Returns list of channel names.

  Parameters:
    `conf` - job conf
  Returns:
    List of channel Names

### isMultiNamedOutput

    public static boolean isMultiNamedOutput(JobConf conf,
                                             String namedOutput)

  Returns if a named output is multiple.

  Parameters:
    `conf` - job conf
    `namedOutput` - named output
  Returns:
    `true` if the name output is multi, `false` if it is single. If the name output is not defined it returns `false`

### getNamedOutputFormatClass

    public static Class<? extends OutputFormat> getNamedOutputFormatClass(JobConf conf,
                                                                          String namedOutput)

  Returns the named output OutputFormat.

  Parameters:
    `conf` - job conf
    `namedOutput` - named output
  Returns:
    namedOutput OutputFormat

### getNamedOutputKeyClass

    public static Class<? extends WritableComparable> getNamedOutputKeyClass(JobConf conf,
                                                                             String namedOutput)

  Returns the key class for a named output.

  Parameters:
    `conf` - job conf
    `namedOutput` - named output
  Returns:
    class for the named output key

### getNamedOutputValueClass

    public static Class<? extends Writable> getNamedOutputValueClass(JobConf conf,
                                                                     String namedOutput)

  Returns the value class for a named output.

  Parameters:
    `conf` - job conf
    `namedOutput` - named output
  Returns:
    class of named output value

### addNamedOutput

    public static void addNamedOutput(JobConf conf,
                                      String namedOutput,
                                      Class<? extends OutputFormat> outputFormatClass,
                                      Class<?> keyClass,
                                      Class<?> valueClass)

  Adds a named output for the job.

  Parameters:
    `conf` - job conf to add the named output
    `namedOutput` - named output name, it has to be a word, letters and numbers only, cannot be the word 'part' as that is reserved for the default output.
    `outputFormatClass` - OutputFormat class.
    `keyClass` - key class
    `valueClass` - value class

### addMultiNamedOutput

    public static void addMultiNamedOutput(JobConf conf,
                                           String namedOutput,
                                           Class<? extends OutputFormat> outputFormatClass,
                                           Class<?> keyClass,
                                           Class<?> valueClass)

  Adds a multi named output for the job.

  Parameters:
    `conf` - job conf to add the named output
    `namedOutput` - named output name, it has to be a word, letters and numbers only, cannot be the word 'part' as that is reserved for the default output.
    `outputFormatClass` - OutputFormat class.
    `keyClass` - key class
    `valueClass` - value class

### setCountersEnabled

    public static void setCountersEnabled(JobConf conf,
                                          boolean enabled)

  Enables or disables counters for the named outputs.

  By default these counters are disabled.

  MultipleOutputs supports counters, by default the are disabled. The counters group is the `MultipleOutputs` class name. The names of the counters are the same as the named outputs. For multi named outputs the name of the counter is the concatenation of the named output, and underscore '_' and the multiname.

  Parameters:
    `conf` - job conf to enableadd the named output.
    `enabled` - indicates if the counters will be enabled or not.

### getCountersEnabled

    public static boolean getCountersEnabled(JobConf conf)

  Returns if the counters for the named outputs are enabled or not.

  By default these counters are disabled.

  MultipleOutputs supports counters, by default the are disabled. The counters group is the `MultipleOutputs` class name. The names of the counters are the same as the named outputs. For multi named outputs the name of the counter is the concatenation of the named output, and underscore '_' and the multiname.

  Parameters:
    `conf` - job conf to enableadd the named output.
  Returns:
    TRUE if the counters are enabled, FALSE if they are disabled.

### getNamedOutputs

    public Iterator<String> getNamedOutputs()

  Returns iterator with the defined name outputs.

  Returns:
    iterator with the defined named outputs

### getCollector

    public OutputCollector getCollector(String namedOutput,
                                        Reporter reporter)
                                 throws IOException

  Gets the output collector for a named output.

  Parameters:
    `namedOutput` - the named output name
    `reporter` - the reporter
  Returns:
    the output collector for the given named output
  Throws:
    `IOException` - thrown if output collector could not be created

### getCollector

    public OutputCollector getCollector(String namedOutput,
                                        String multiName,
                                        Reporter reporter)
                                 throws IOException

  Gets the output collector for a multi named output.

  Parameters:
    `namedOutput` - the named output name
    `multiName` - the multi name part
    `reporter` - the reporter
  Returns:
    the output collector for the given named output
  Throws:
    `IOException` - thrown if output collector could not be created

### close

    public void close()
               throws IOException

  Closes all the opened named outputs.

  If overriden subclasses must invoke `super.close()` at the end of their `close()`

  Throws:
    `IOException` - thrown if any of the MultipleOutput files could not be closed properly.

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