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https://issues.apache.org/jira/browse/HADOOP-4668?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12649547#action_12649547
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Chris Douglas commented on HADOOP-4668:
---------------------------------------

+1

> Improve JavaDoc on JobConf.setCombinerClass to better document restrictions 
> on combiners
> ----------------------------------------------------------------------------------------
>
>                 Key: HADOOP-4668
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4668
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: documentation
>            Reporter: Owen O'Malley
>            Assignee: Owen O'Malley
>         Attachments: combiner-doc.patch, combiner-doc.patch
>
>
> It is often not clear to developers that Combiners may be called 0, 1, or 
> many times and that combiners input and output types must be the same and the 
> combiner must not have side effects. Since there isn't a Combiner class, the 
> best place to put this documentation is on JobConf.setCombinerClass.

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