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From "Owen O'Malley (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-723) Race condition exists in the method MapOutputLocation.getFile
Date Thu, 16 Nov 2006 18:29:39 GMT
     [ http://issues.apache.org/jira/browse/HADOOP-723?page=all ]

Owen O'Malley updated HADOOP-723:
---------------------------------

    Attachment: map-out-lost.patch

This patch:
  1. Gives each map copier thread a unique id.
  2. Uses the id to generate unique names for the fetched files.
  3. Adds timeouts for the connect/read in getFile.
  4. Adds checks for interrupted in getFile.
  5. Add exception handlers inside the while loop of the map output copier threads and the
map output lease thread.
  6. Add synchronization around the renames of the temporary files so that there aren't any
race conditions.

We might consider putting this one into a 0.8.1 release.

> Race condition exists in the method MapOutputLocation.getFile
> -------------------------------------------------------------
>
>                 Key: HADOOP-723
>                 URL: http://issues.apache.org/jira/browse/HADOOP-723
>             Project: Hadoop
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.8.0
>            Reporter: Devaraj Das
>         Assigned To: Owen O'Malley
>             Fix For: 0.9.0
>
>         Attachments: map-out-lost.patch
>
>
> There seems to be a race condition in the way the Reduces copy the map output files from
the Maps. If a copier is blocked in the connect method (in the beginning of the method MapOutputLocation.getFile)
to a Jetty on a Map, and the MapCopyLeaseChecker detects that the copier was idle for too
long, it will go ahead and issue a interrupt (read 'kill') to this thread and create a new
Copier thread. However, the copier, currently blocked trying to connect to Jetty on a Map,
doesn't actually get killed until the connect timeout expires and as soon as the connect comes
out (with an IOException), it will delete the map output file which actually could have been
(successfully) created by the new Copier thread. This leads to the Sort phase for that reducer
failing with a FileNotFoundException.
> One simple way to fix this is to not delete the file if the file was not created within
this getFile method.

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