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From "Chris Douglas (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5589) TupleWritable: Lift implicit limit on the number of values that can be stored
Date Wed, 15 Apr 2009 23:47:15 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5589?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12699452#action_12699452
] 

Chris Douglas commented on HADOOP-5589:
---------------------------------------

bq. The patch could be backwards-compatible if the bitset was written to the stream as VLongs
(essentially what had been implemented in HADOOP-5589-2.patch

This seems like a reasonable approach to me. As long as this can read old data, it's OK if
it writes records that cannot be read by the old implementation.

bq. is there still a need to support users who were storing tuples over 64 values but with
incorrect results? 

Such data is already corrupt, so I don't think anything can be done for it.

> TupleWritable: Lift implicit limit on the number of values that can be stored
> -----------------------------------------------------------------------------
>
>                 Key: HADOOP-5589
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5589
>             Project: Hadoop Core
>          Issue Type: Improvement
>          Components: mapred
>    Affects Versions: 0.21.0
>            Reporter: Jingkei Ly
>            Assignee: Jingkei Ly
>         Attachments: HADOOP-5589-1.patch, HADOOP-5589-2.patch, HADOOP-5589-3.patch
>
>
> TupleWritable uses an instance field of the primitive type, long, which I presume is
so that it can quickly determine if a position has been written to in its array of Writables
(by using bit-shifting operations on the long field). The problem with this is that it implies
that there is a maximum limit of 64 values you can store in a TupleWritable.
> An example of a use-case where I think this would be a problem is if you had two MR jobs
with over 64 reduces tasks and you wanted to join the outputs with CompositeInputFormat  -
this will probably cause unexpected results in the current scheme.
> At the very least, the 64-value limit should be documented in TupleWritable.

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