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

java.lang.Object
  extended by org.apache.hadoop.mapred.FileOutputFormat<K,V>
All Implemented Interfaces:
OutputFormat<K,V>
Direct Known Subclasses:
MapFileOutputFormat, MultipleOutputFormat, SequenceFileOutputFormat, TextOutputFormat

@InterfaceAudience.Public
@InterfaceStability.Stable
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. ${mapreduce.output.fileoutputformat.outputdir}$

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 ${mapreduce.output.fileoutputformat.outputdir}/_temporary/_${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 ${mapreduce.output.fileoutputformat.outputdir}/_temporary/_${taskid} (only) are promoted to ${mapreduce.output.fileoutputformat.outputdir}. 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 ${mapreduce.task.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 ${mapreduce.task.output.dir} during execution of a particular task-attempt is actually ${mapreduce.output.fileoutputformat.outputdir}/_temporary/_{$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.


Copyright © 2012 Apache Software Foundation. All Rights Reserved.