[ 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 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. -- This message is automatically generated by JIRA. If you think it was sent incorrectly, please contact your JIRA administrators: https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa For more information on JIRA, see: http://www.atlassian.com/software/jira