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From "Karthik Kambatla (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-4464) Reduce tasks failing with NullPointerException in ConcurrentHashMap.get()
Date Thu, 19 Jul 2012 20:25:35 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-4464?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13418642#comment-13418642

Karthik Kambatla commented on MAPREDUCE-4464:

Clint, thanks a lot for looking into this issue. 

*Minor comment: Would it be better to throw an IOException wrapped with your message, so that
we can avoid the subsequent NullPointerException?
> Reduce tasks failing with NullPointerException in ConcurrentHashMap.get()
> -------------------------------------------------------------------------
>                 Key: MAPREDUCE-4464
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-4464
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: task
>    Affects Versions: 1.0.0
>            Reporter: Clint Heath
>            Priority: Minor
>         Attachments: MAPREDUCE-4464.patch
>   Original Estimate: 1h
>  Remaining Estimate: 1h
> If DNS does not resolve hostnames properly, reduce tasks can fail with a very misleading
> as per my peer Ahmed's diagnosis:
> In ReduceTask, it seems that event.getTaskTrackerHttp() returns a malformed URI, and
so host from:
> {code}
> String host = u.getHost();
> {code}
> is evaluated to null and the NullPointerException is thrown afterwards in the ConcurrentHashMap.
> I have written a patch to check for a null hostname condition when getHost is called
in the getMapCompletionEvents method and print an intelligible warning message rather than
suppressing it until later when it becomes confusing and misleading.

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