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From "Alejandro Abdelnur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3702) add support for chaining Maps in a single Map and after a Reduce [M*/RM*]
Date Fri, 05 Sep 2008 04:29:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12628545#action_12628545
] 

Alejandro Abdelnur commented on HADOOP-3702:
--------------------------------------------

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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