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Date: Mon, 3 Apr 2006 17:14:56 +0100 (BST)
From: "Owen O'Malley (JIRA)" <jira@apache.org>
To: hadoop-dev@lucene.apache.org
Subject: [jira] Commented: (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.
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    [ http://issues.apache.org/jira/browse/HADOOP-115?page=comments#action_12372967 ] 

Owen O'Malley commented on HADOOP-115:
--------------------------------------

I should have addressed the combiner before. *smile* Of course the combiner input and output has to match the map output types. So, it looks like:

map: k1,v1 -> seq(k2,v2)
combine: k2,seq(v2) -> seq(k2,v2)
reduce: k2, seq(v2) -> seq(k3,v3)

So the only extra code is to set/get the types for k2/v2 (or equivalent k3/v3), although I would recommend adding a type check in the reduce collector. It is completely upward compatible.

As for user confusion, I've already had to explain this restriction (k2==k3 and v2==v3) far more times than I'd like.

On a side note, we could hack around the problem by defining an OutputFormat that uses SequenceFileWriter, but doesn't open the file until the first key/value pair is written and takes the types from the first instances. But that breaks when someone puts the type check into the reduce collector.

> 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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