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https://issues.apache.org/jira/browse/MAPREDUCE-5221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13677399#comment-13677399
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Mariappan Asokan commented on MAPREDUCE-5221:
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Hi Tsuyoshi,
  See my comments in MAPREDUCE-5294 and MAPREDUCE-5295.  We may not need the 
{{combinerClass}} argument to the constructor of 
{{ShuffleConsumerPlugin.Context}}.  This means the check on whether to use new 
or old API will be in one place namely {{CombinerRunner}}. You can probably 
remove the code that comes up with the value for {{combinerClass}}.

Thanks.

-- Asokan
                
> Reduce side Combiner is not used when using the new API
> -------------------------------------------------------
>
>                 Key: MAPREDUCE-5221
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5221
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>    Affects Versions: 2.0.4-alpha
>            Reporter: Siddharth Seth
>            Assignee: Tsuyoshi OZAWA
>         Attachments: MAPREDUCE-5221.1.patch, MAPREDUCE-5221.2.patch, 
> MAPREDUCE-5221.3.patch
>
>
> If a combiner is specified using o.a.h.mapreduce.Job.setCombinerClass - this 
> will silently ignored on the reduce side since the reduce side usage is only 
> aware of the old api combiner.
> This doesn't fail the job - since the new combiner key does not deprecate the 
> old key.

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