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Date: Tue, 21 Oct 2008 15:28:44 -0700 (PDT)
From: "Robert Chansler (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Updated: (HADOOP-3149) supporting multiple outputs for M/R
 jobs
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     [ https://issues.apache.org/jira/browse/HADOOP-3149?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Robert Chansler updated HADOOP-3149:
------------------------------------

    Release Note: Introduced MultipleOutputs class so Map/Reduce jobs can write data to different output files. Each output can use a different OutputFormat. Outpufiles are created within the job output directory. FileOutputFormat.getPathForCustomFile() creates a filename under the outputdir that is named with the task ID and task type (i.e. myfile-r-00001).  (was: MultipleOutputs class helps Map/Reduce jobs to write data to different output files using a collector like API. Each output can use a different OutputFormat. Outpufiles are created within the job output directory.

FileOutputFormat provides a new static method, getPathForCustomFile, that creates a custom filename under the outputdir that is namespace with the task ID and task type (i.e. myfile-r-00001).)

> supporting multiple outputs for M/R jobs
> ----------------------------------------
>
>                 Key: HADOOP-3149
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3149
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: mapred
>         Environment: all
>            Reporter: Alejandro Abdelnur
>            Assignee: Alejandro Abdelnur
>             Fix For: 0.19.0
>
>         Attachments: patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt, patch3149.txt
>
>
> The outputcollector supports writing data to a single output, the 'part' files in the output path.
> We found quite common that our M/R jobs have to write data to different output. For example when classifying data as NEW, UPDATE, DELETE, NO-CHANGE to later do different processing on it.
> Handling the initialization of additional outputs from within the M/R code complicates the code and is counter intuitive with the notion of job configuration.
> It would be desirable to:
> # Configure the additional outputs in the jobconf, potentially specifying different outputformats, key and value classes for each one.
> # Write to the additional outputs in a similar way as data is written to the outputcollector.
> # Support the speculative execution semantics for the output files, only visible in the final output for promoted tasks.
> To support multiple outputs the following classes would be added to mapred/lib:
> * {{MOJobConf}} : extends {{JobConf}} adding methods to define named outputs (name, outputformat, key class, value class)
> * {{MOOutputCollector}} : extends {{OutputCollector}} adding a {{collect(String outputName, WritableComparable key, Writable value)}} method.
> * {{MOMapper}} and {{MOReducer}}: implement {{Mapper}} and {{Reducer}} adding a new {{configure}}, {{map}} and {{reduce}} signature that take the corresponding {{MO}} classes and performs the proper initialization.
> The data flow behavior would be: key/values written to the default (unnamed) output (using the original OutputCollector {{collect}} signature) take part of the shuffle/sort/reduce processing phases. key/values written to a named output from within a map don't.
> The named output files would be named using the task type and task ID to avoid collision among tasks (i.e. 'new-m-00002' and 'new-r-00001').
> Together with the setInputPathFilter feature introduced by HADOOP-2055 it would become very easy to chain jobs working on particular named outputs within a single directory.
> We are using heavily this pattern and it greatly simplified our M/R code as well as chaining different M/R. 
> We wanted to contribute this back to Hadoop as we think is a generic feature many could benefit from.

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