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From "Clint Heath (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MAPREDUCE-4464) Reduce tasks failing with NullPointerException in ConcurrentHashMap.get()
Date Thu, 19 Jul 2012 18:16:34 GMT
Clint Heath created MAPREDUCE-4464:

             Summary: 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

If DNS does not resolve hostnames properly, reduce tasks can fail with a very misleading exception.

as per my peer Ahmed's diagnosis:

In ReduceTask, it seems that event.getTaskTrackerHttp() returns a malformed URI, and so host
String host = u.getHost();
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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