org.apache.hadoop.mapreduce.lib.output

Class MultipleOutputs<KEYOUT,VALUEOUT>

java.lang.Object

org.apache.hadoop.mapreduce.lib.output.MultipleOutputs<KEYOUT,VALUEOUT>

@InterfaceAudience.Public
@InterfaceStability.Stable
public class MultipleOutputs<KEYOUT,VALUEOUT>
extends Object

The MultipleOutputs class simplifies writing output data to multiple outputs

Case one: writing to additional outputs other than the job default output. Each additional output, or named output, may be configured with its own OutputFormat, with its own key class and with its own value class.

Case two: to write data to different files provided by user

MultipleOutputs supports counters, by default they are disabled. The counters group is the MultipleOutputs class name. The names of the counters are the same as the output name. These count the number records written to each output name.

Usage pattern for job submission:

 Job job = new Job();

 FileInputFormat.setInputPath(job, inDir);
 FileOutputFormat.setOutputPath(job, outDir);

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

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

 // Defines additional sequence-file based output 'sequence' for the job
 MultipleOutputs.addNamedOutput(job, "seq",
   SequenceFileOutputFormat.class,
   LongWritable.class, Text.class);
 ...

 job.waitForCompletion(true);
 ...
 

Usage in Reducer:

 <K, V> String generateFileName(K k, V v) {
   return k.toString() + "_" + v.toString();
 }
 
 public class MOReduce extends
   Reducer<WritableComparable, Writable,WritableComparable, Writable> {
 private MultipleOutputs mos;
 public void setup(Context context) {
 ...
 mos = new MultipleOutputs(context);
 }

 public void reduce(WritableComparable key, Iterator<Writable> values,
 Context context)
 throws IOException {
 ...
 mos.write("text", , key, new Text("Hello"));
 mos.write("seq", LongWritable(1), new Text("Bye"), "seq_a");
 mos.write("seq", LongWritable(2), key, new Text("Chau"), "seq_b");
 mos.write(key, new Text("value"), generateFileName(key, new Text("value")));
 ...
 }

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

 }
 

When used in conjuction with org.apache.hadoop.mapreduce.lib.output.LazyOutputFormat, MultipleOutputs can mimic the behaviour of MultipleTextOutputFormat and MultipleSequenceFileOutputFormat from the old Hadoop API - ie, output can be written from the Reducer to more than one location.

Use MultipleOutputs.write(KEYOUT key, VALUEOUT value, String baseOutputPath) to write key and value to a path specified by baseOutputPath, with no need to specify a named output. Warning: when the baseOutputPath passed to MultipleOutputs.write is a path that resolves outside of the final job output directory, the directory is created immediately and then persists through subsequent task retries, breaking the concept of output committing:

 private MultipleOutputs<Text, Text> out;
 
 public void setup(Context context) {
   out = new MultipleOutputs<Text, Text>(context);
   ...
 }
 
 public void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
 for (Text t : values) {
   out.write(key, t, generateFileName(<parameter list...>));
   }
 }
 
 protected void cleanup(Context context) throws IOException, InterruptedException {
   out.close();
 }
 

Use your own code in generateFileName() to create a custom path to your results. '/' characters in baseOutputPath will be translated into directory levels in your file system. Also, append your custom-generated path with "part" or similar, otherwise your output will be -00000, -00001 etc. No call to context.write() is necessary. See example generateFileName() code below.

 private String generateFileName(Text k) {
   // expect Text k in format "Surname|Forename"
   String[] kStr = k.toString().split("\\|");
   
   String sName = kStr[0];
   String fName = kStr[1];

   // example for k = Smith|John
   // output written to /user/hadoop/path/to/output/Smith/John-r-00000 (etc)
   return sName + "/" + fName;
 }
 

Using MultipleOutputs in this way will still create zero-sized default output, eg part-00000. To prevent this use LazyOutputFormat.setOutputFormatClass(job, TextOutputFormat.class); instead of job.setOutputFormatClass(TextOutputFormat.class); in your Hadoop job configuration.

Constructor Summary

Constructors

Constructor and Description
MultipleOutputs(TaskInputOutputContext<?,?,KEYOUT,VALUEOUT> context) Creates and initializes multiple outputs support, it should be instantiated in the Mapper/Reducer setup method.

Method Summary

Methods

Modifier and Type	Method and Description
static void	addNamedOutput(Job job, String namedOutput, Class<? extends OutputFormat> outputFormatClass, Class<?> keyClass, Class<?> valueClass) Adds a named output for the job.
void	close() Closes all the opened outputs.
static boolean	getCountersEnabled(JobContext job) Returns if the counters for the named outputs are enabled or not.
static void	setCountersEnabled(Job job, boolean enabled) Enables or disables counters for the named outputs.
void	write(KEYOUT key, VALUEOUT value, String baseOutputPath) Write key value to an output file name.
<K,V> void	write(String namedOutput, K key, V value) Write key and value to the namedOutput.
<K,V> void	write(String namedOutput, K key, V value, String baseOutputPath) Write key and value to baseOutputPath using the namedOutput.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

MultipleOutputs

public MultipleOutputs(TaskInputOutputContext<?,?,KEYOUT,VALUEOUT> context)

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

Parameters:

context - the TaskInputOutputContext object

Method Detail

addNamedOutput

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

Adds a named output for the job.

Parameters:

job - job 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(Job job,
                      boolean enabled)

Enables or disables counters for the named outputs. The counters group is the MultipleOutputs class name. The names of the counters are the same as the named outputs. These counters count the number records written to each output name. By default these counters are disabled.

Parameters:

job - job to enable counters

enabled - indicates if the counters will be enabled or not.

getCountersEnabled

public static boolean getCountersEnabled(JobContext job)

Returns if the counters for the named outputs are enabled or not. By default these counters are disabled.

Parameters:

job - the job

Returns:

TRUE if the counters are enabled, FALSE if they are disabled.

write

public <K,V> void write(String namedOutput,
               K key,
               V value)
           throws IOException,
                  InterruptedException

Write key and value to the namedOutput. Output path is a unique file generated for the namedOutput. For example, {namedOutput}-(m|r)-{part-number}

Parameters:

namedOutput - the named output name

key - the key

value - the value

Throws:

IOException

InterruptedException

write

public <K,V> void write(String namedOutput,
               K key,
               V value,
               String baseOutputPath)
           throws IOException,
                  InterruptedException

Write key and value to baseOutputPath using the namedOutput.

Parameters:

namedOutput - the named output name

key - the key

value - the value

baseOutputPath - base-output path to write the record to. Note: Framework will generate unique filename for the baseOutputPath Warning: when the baseOutputPath is a path that resolves outside of the final job output directory, the directory is created immediately and then persists through subsequent task retries, breaking the concept of output committing.

Throws:

IOException

InterruptedException

write

public void write(KEYOUT key,
         VALUEOUT value,
         String baseOutputPath)
           throws IOException,
                  InterruptedException

Write key value to an output file name. Gets the record writer from job's output format. Job's output format should be a FileOutputFormat.

Parameters:

key - the key

value - the value

baseOutputPath - base-output path to write the record to. Note: Framework will generate unique filename for the baseOutputPath Warning: when the baseOutputPath is a path that resolves outside of the final job output directory, the directory is created immediately and then persists through subsequent task retries, breaking the concept of output committing.

Throws:

IOException

InterruptedException

close

public void close()
           throws IOException,
                  InterruptedException

Closes all the opened outputs. This should be called from cleanup method of map/reduce task. If overridden subclasses must invoke super.close() at the end of their close()

Throws:

IOException

InterruptedException