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https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12619840#action_12619840
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Enis Soztutar commented on HADOOP-3702:
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bq. On #1, Yes but this introduces a backwards incompatibility as there is code
out there (outside of core) that uses the write(OutputStream) with
DataOuputStream instances and the change is breaking such usages (as the
previous patch found).
Yes, at some level it will introduce backwards incompatibility. But thinking in
abstract terms, having {{func(Interface1 i1)}}, and introducing
{{func(Interface2 i2)}} is not a direct incompatibility. Only those calls where
the object both implements Interface1 and Interface2 would be affected(in this
case DataOutputStream), and there is a clear workaround for this. I think
introducing a Serialization for Configuration is far more worse. I suggest we
keep write(OutputStream), introduce write(DataOutput), fix all the cases where
DataOutputStream is passed (including contrib), and mark this change as
incompatible with clear documentation in release note.
bq. On #2, I'm missing something here, under what circumstances would it make
sense to use the Chain* classes with generics that would be checked at compile
time? If it is just a way of avoiding the @SuppressWarnings annotations I'd
prefer the anotations as, IMO, they are meant for this cases.
@SuppressWarnings annotations are just "hacks" for the compiler to stop
complaining. There are valid reasons for the compiler to issue warnings, and
instead of fixing them, we say the compiler to ignore these, which is not
desired.
The motivation to use generics is the same as the one for the use of generics
in Mapper, Reducer, etc. I guess with some little extra effort, we could make
this change, no?
> 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
> Attachments: 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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