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https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12628622#action_12628622
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Alejandro Abdelnur commented on HADOOP-3702:
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With *option 3* the _//should fail_ line fails.
Your comment _However the output classes can indeed be more specific than
declared. An example of this is a mapper which does not change the input key,
and directly outputs it._ is correct, nasty but correct.
This bring then another question, currently when adding a mapper to the chain
the {{addMapper}} method checks that the input key/value classes match (being
equal, a ==) the output key/value classes of the previous mapper. Should this
be changed to an {{isAssignable()}}? Or is it too much?
> add support for chaining Maps in a single Map and after a Reduce [M*/RM*]
> -------------------------------------------------------------------------
>
> Key: HADOOP-3702
> URL: https://issues.apache.org/jira/browse/HADOOP-3702
> Project: Hadoop Core
> Issue Type: New Feature
> Components: mapred
> Environment: all
> Reporter: Alejandro Abdelnur
> Assignee: Alejandro Abdelnur
> Fix For: 0.19.0
>
> Attachments: Hadoop-3702.patch, patch3702.txt, patch3702.txt,
> patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt,
> patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt, patch3702.txt
>
>
> On the same input, we usually need to run multiple Maps one after the other
> without no Reduce. We also have to run multiple Maps after the Reduce.
> If all pre-Reduce Maps are chained together and run as a single Map a
> significant amount of Disk I/O will be avoided.
> Similarly all post-Reduce Maps can be chained together and run in the Reduce
> phase after the Reduce.
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