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https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12628545#action_12628545
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Alejandro Abdelnur commented on HADOOP-3702:
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I'm confused, why option 3 would not work with your example.
My reasoning for Option 3 is that as input the mapper/reducer can take process
a less specific class of input key/value classes, but the output cannot be less
(or more) specific because the concrete class of the output is chosen to get
the serializer.
Classes: {{A, B extends A, C extends B}}
Input Key/Value classes: {{ C, B}}
Output Key/Value classes: {{ A, B}}
Valid mapper classes: {{Mapper<C, B, A, B>}} {{Mapper<C, A, A, B>}}
{{Mapper<B, B, A, B>}} {{Mapper<B, A, A, B>}} {{Mapper<A, B A, B>}},
{{Mapper<A, A, A, B>}}
Invalid mapper classes: {{Mapper<*, *, B, B>}} (and any other mapper were the
output is not {{A, B}} )
> 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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