[ http://issues.apache.org/jira/browse/HADOOP-115?page=all ]
Owen O'Malley reopened HADOOP-115:
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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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