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From "Tom White (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-326) The lowest level map-reduce APIs should be byte oriented
Date Fri, 12 Feb 2010 23:47:28 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-326?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12833248#action_12833248
] 

Tom White commented on MAPREDUCE-326:
-------------------------------------

Thanks for looking at the proposal, Owen.

> I have reservations about doubling the width of the *public* API by having both a raw
and object level APIs. That will make a much much harder problem to guarantee compatibility
and still enable us to make improvements to the sort and shuffle.

This is not a risk since the API is marked "unstable" so we retain the freedom to change it
in any way we like. You can think of this change as a refactor to make the MapReduce shuffle
more accessible to framework developers, which will make (e.g.) MAPREDUCE-1183, MAPREDUCE-1220
more straightforward.

>It is a software engineering truism that if you can accomplish something in library code,
it is much better to do so rather than the framework.

I totally agree. I'm proposing that the new (context objects) MapReduce API is implemented
in library code. The idea here is to reduce the amount of kernel code, which would be a good
thing.

> So instead of making a new lower level API, I'd propose layering your API on top of the
object API.

I think this is the wrong way round. The low-level API has fewer features than the object
API - for example it doesn't have the concept of record reader, since this is done in higher-level
framework code - whereas the low-level API is designed to support the old and new Java APIs,
and would be able to directly support Streaming, Pipes, etc.


> The lowest level map-reduce APIs should be byte oriented
> --------------------------------------------------------
>
>                 Key: MAPREDUCE-326
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-326
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>            Reporter: eric baldeschwieler
>         Attachments: MAPREDUCE-326-api.patch, MAPREDUCE-326.pdf
>
>
> As discussed here:
> https://issues.apache.org/jira/browse/HADOOP-1986#action_12551237
> The templates, serializers and other complexities that allow map-reduce to use arbitrary
types complicate the design and lead to lots of object creates and other overhead that a byte
oriented design would not suffer.  I believe the lowest level implementation of hadoop map-reduce
should have byte string oriented APIs (for keys and values).  This API would be more performant,
simpler and more easily cross language.
> The existing API could be maintained as a thin layer on top of the leaner API.

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