FileOutputFormat (Hadoop 0.20.2+737 API)
Date: 2010-10-11

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## org.apache.hadoop.mapred
Class FileOutputFormat<K,V>

    java.lang.Object
      org.apache.hadoop.mapred.FileOutputFormat<K,V>

All Implemented Interfaces:
  OutputFormat<K,V>

Direct Known Subclasses:
  IndexUpdateOutputFormat, MapFileOutputFormat, MultipleOutputFormat, SequenceFileOutputFormat, TextOutputFormat

    public abstract class FileOutputFormat<K,V>

extends Object
implements OutputFormat<K,V>

A base class for `OutputFormat`.

Constructor Summary
`FileOutputFormat()`

Method Summary
`void` | `checkOutputSpecs(FileSystem ignored, JobConf job)` Check for validity of the output-specification for the job.
`static boolean` | `getCompressOutput(JobConf conf)` Is the job output compressed?
`static Class<? extends CompressionCodec>` | `getOutputCompressorClass(JobConf conf, Class<? extends CompressionCodec> defaultValue)` Get the `CompressionCodec` for compressing the job outputs.
`static Path` | `getOutputPath(JobConf conf)` Get the `Path` to the output directory for the map-reduce job.
`static Path` | `getPathForCustomFile(JobConf conf, String name)` Helper function to generate a `Path` for a file that is unique for the task within the job output directory.
`abstract RecordWriter<K,V>` | `getRecordWriter(FileSystem ignored, JobConf job, String name, Progressable progress)` Get the `RecordWriter` for the given job.
`static Path` | `getTaskOutputPath(JobConf conf, String name)` Helper function to create the task's temporary output directory and return the path to the task's output file.
`static String` | `getUniqueName(JobConf conf, String name)` Helper function to generate a name that is unique for the task.
`static Path` | `getWorkOutputPath(JobConf conf)` Get the `Path` to the task's temporary output directory for the map-reduce job #### Tasks' Side-Effect Files
`static void` | `setCompressOutput(JobConf conf, boolean compress)` Set whether the output of the job is compressed.
`static void` | `setOutputCompressorClass(JobConf conf, Class<? extends CompressionCodec> codecClass)` Set the `CompressionCodec` to be used to compress job outputs.
`static void` | `setOutputPath(JobConf conf, Path outputDir)` Set the `Path` of the output directory for the map-reduce job.

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

Constructor Detail

### FileOutputFormat

    public FileOutputFormat()

Method Detail

### setCompressOutput

    public static void setCompressOutput(JobConf conf,
                                         boolean compress)

  Set whether the output of the job is compressed.

  Parameters:
    `conf` - the `JobConf` to modify
    `compress` - should the output of the job be compressed?

### getCompressOutput

    public static boolean getCompressOutput(JobConf conf)

  Is the job output compressed?

  Parameters:
    `conf` - the `JobConf` to look in
  Returns:
    `true` if the job output should be compressed, `false` otherwise

### setOutputCompressorClass

    public static void setOutputCompressorClass(JobConf conf,
                                                Class<? extends CompressionCodec> codecClass)

  Set the `CompressionCodec` to be used to compress job outputs.

  Parameters:
    `conf` - the `JobConf` to modify
    `codecClass` - the `CompressionCodec` to be used to compress the job outputs

### getOutputCompressorClass

    public static Class<? extends CompressionCodec> getOutputCompressorClass(JobConf conf,
                                                                             Class<? extends CompressionCodec> defaultValue)

  Get the `CompressionCodec` for compressing the job outputs.

  Parameters:
    `conf` - the `JobConf` to look in
    `defaultValue` - the `CompressionCodec` to return if not set
  Returns:
    the `CompressionCodec` to be used to compress the job outputs
  Throws:
    `IllegalArgumentException` - if the class was specified, but not found

### getRecordWriter

    public abstract RecordWriter<K,V> getRecordWriter(FileSystem ignored,
                                                      JobConf job,
                                                      String name,
                                                      Progressable progress)
                                               throws IOException

  Description copied from interface: `OutputFormat`
  Get the `RecordWriter` for the given job.

  Specified by:
    `getRecordWriter` in interface `OutputFormat<K,V>`
    `job` - configuration for the job whose output is being written.
    `name` - the unique name for this part of the output.
    `progress` - mechanism for reporting progress while writing to file.
  Returns:
    a `RecordWriter` to write the output for the job.
  Throws:
    `IOException`

### checkOutputSpecs

    public void checkOutputSpecs(FileSystem ignored,
                                 JobConf job)
                          throws FileAlreadyExistsException,
                                 InvalidJobConfException,
                                 IOException

  Description copied from interface: `OutputFormat`
  Check for validity of the output-specification for the job.

  This is to validate the output specification for the job when it is a job is submitted. Typically checks that it does not already exist, throwing an exception when it already exists, so that output is not overwritten.

  Specified by:
    `checkOutputSpecs` in interface `OutputFormat<K,V>`
    `job` - job configuration.
  Throws:
    `IOException` - when output should not be attempted
    `FileAlreadyExistsException`
    `InvalidJobConfException`

### setOutputPath

    public static void setOutputPath(JobConf conf,
                                     Path outputDir)

  Set the `Path` of the output directory for the map-reduce job.

  Parameters:
    `conf` - The configuration of the job.
    `outputDir` - the `Path` of the output directory for the map-reduce job.

### getOutputPath

    public static Path getOutputPath(JobConf conf)

  Get the `Path` to the output directory for the map-reduce job.

  Returns:
    the `Path` to the output directory for the map-reduce job.
  See Also:
    `getWorkOutputPath(JobConf)`

### getWorkOutputPath

    public static Path getWorkOutputPath(JobConf conf)

  Get the `Path` to the task's temporary output directory for the map-reduce job

  #### Tasks' Side-Effect Files

  Note: The following is valid only if the `OutputCommitter` is `FileOutputCommitter`. If `OutputCommitter` is not a `FileOutputCommitter`, the task's temporary output directory is same as `getOutputPath(JobConf)` i.e. `${mapred.output.dir}$`

  Some applications need to create/write-to side-files, which differ from the actual job-outputs.

  In such cases there could be issues with 2 instances of the same TIP (running simultaneously e.g. speculative tasks) trying to open/write-to the same file (path) on HDFS. Hence the application-writer will have to pick unique names per task-attempt (e.g. using the attemptid, say `attempt_200709221812_0001_m_000000_0`), not just per TIP.

  To get around this the Map-Reduce framework helps the application-writer out by maintaining a special `${mapred.output.dir}/_temporary/_${taskid}` sub-directory for each task-attempt on HDFS where the output of the task-attempt goes. On successful completion of the task-attempt the files in the `${mapred.output.dir}/_temporary/_${taskid}` (only) are promoted to `${mapred.output.dir}`. Of course, the framework discards the sub-directory of unsuccessful task-attempts. This is completely transparent to the application.

  The application-writer can take advantage of this by creating any side-files required in `${mapred.work.output.dir}` during execution of his reduce-task i.e. via `getWorkOutputPath(JobConf)`, and the framework will move them out similarly - thus she doesn't have to pick unique paths per task-attempt.

  Note: the value of `${mapred.work.output.dir}` during execution of a particular task-attempt is actually `${mapred.output.dir}/_temporary/_{$taskid}`, and this value is set by the map-reduce framework. So, just create any side-files in the path returned by `getWorkOutputPath(JobConf)` from map/reduce task to take advantage of this feature.

  The entire discussion holds true for maps of jobs with reducer=NONE (i.e. 0 reduces) since output of the map, in that case, goes directly to HDFS.

  Returns:
    the `Path` to the task's temporary output directory for the map-reduce job.

### getTaskOutputPath

    public static Path getTaskOutputPath(JobConf conf,
                                         String name)
                                  throws IOException

  Helper function to create the task's temporary output directory and return the path to the task's output file.

  Parameters:
    `conf` - job-configuration
    `name` - temporary task-output filename
  Returns:
    path to the task's temporary output file
  Throws:
    `IOException`

### getUniqueName

    public static String getUniqueName(JobConf conf,
                                       String name)

  Helper function to generate a name that is unique for the task.

  The generated name can be used to create custom files from within the different tasks for the job, the names for different tasks will not collide with each other.

  The given name is postfixed with the task type, 'm' for maps, 'r' for reduces and the task partition number. For example, give a name 'test' running on the first map o the job the generated name will be 'test-m-00000'.

  Parameters:
    `conf` - the configuration for the job.
    `name` - the name to make unique.
  Returns:
    a unique name accross all tasks of the job.

### getPathForCustomFile

    public static Path getPathForCustomFile(JobConf conf,
                                            String name)

  Helper function to generate a `Path` for a file that is unique for the task within the job output directory.

  The path can be used to create custom files from within the map and reduce tasks. The path name will be unique for each task. The path parent will be the job output directory.

  ls

  This method uses the `getUniqueName(org.apache.hadoop.mapred.JobConf, java.lang.String)` method to make the file name unique for the task.

  Parameters:
    `conf` - the configuration for the job.
    `name` - the name for the file.
  Returns:
    a unique path accross all tasks of the job.

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