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From "Owen O'Malley (JIRA)" <j...@apache.org>
Subject [jira] Reopened: (HADOOP-115) Hadoop should allow the user to use SequentialFileOutputformat as the output format and to choose key/value classes that are different from those for map output.
Date Sat, 01 Apr 2006 08:21:26 GMT
     [ http://issues.apache.org/jira/browse/HADOOP-115?page=all ]
     
Owen O'Malley reopened HADOOP-115:
----------------------------------


Let's reopen this. I've had discussions with Runping today, and it seems to me that:

1. It is basically free with respect to the framework.
2. It allows more applications to be written using the framework rather than working around
the framework.
3. It is less clear that we should allow the user to change the key type in the reduce, but
since the current API does allow them to change the value (if not the type), I think we should
be consistent and allow a type change too.

I propose:

1. Add {set,get}MapOutput{Key,Value}Class functions in JobConf.
2. The default values for getMapOutput{Key,Value}Class are the values from getOutput{Key,Value}Class.
3. Always check the types in the output collector rather that the OutputFormat, so that even
text output files are check for type correctness.

We should include a javadoc comment for setMapOutputKeyClass will warn that changing the key
in the reduce will mean that your output is NOT sorted.

> Hadoop should allow the user to use SequentialFileOutputformat as the output format and
to choose  key/value classes that are different from those for map output.
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>          Key: HADOOP-115
>          URL: http://issues.apache.org/jira/browse/HADOOP-115
>      Project: Hadoop
>         Type: Improvement
>   Components: mapred
>     Reporter: Runping Qi

>
> When map tasks write intermediate data out, they always use SequencialFile RecordWriter
with key/value classes from the job object.
> When the reducers write the final results out, its output format is obtained from the
job object. By default, it is TextOutputFormat, and no conflicts.
> However, if one wants to use SequencialFileFormat for the final results, then the key/value
classes are also obtained from the job object, the same as the map tasks' output. Now we have
a problem. It is impossible for the map outputs and reducer outputs use different key/value
classes, if one wants the reducers generate outputs in SequentialFileFormat.
> A simple fix would be to add another two attributes to JobConf class: mapOutputLeyClass
and mapOutputValueClass. That allows the user to have different key/value classes for the
intermediate and final outputs.

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