[jira] [Created] (MAPREDUCE-4464) Reduce tasks failing with NullPointerException in ConcurrentHashMap.get()

["Clint Heath (JIRA)" <ji...@apache.org>]

Clint Heath created MAPREDUCE-4464:
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             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 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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