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https://issues.apache.org/jira/browse/PIG-5290?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16129631#comment-16129631
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Erik Krogen edited comment on PIG-5290 at 8/16/17 11:48 PM:
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Attaching initial patch implementing the ideas I discussed in the JIRA. Uploads
at a random file name, then moves it to the original filename. Uses listStatus
to be able to share identical JAR files with nonidentical names. Makes an
attempt at cleaning up hanging temp files if an issue occurs while uploading /
renaming.
I did not add any unit tests as it was not very clear to me how to do so / what
the policy on that would be. Let me know if I should add some and if so, where
they would go. I did manually test the patch on our Hadoop 2.6 cluster and it
worked as expected, populating the cache when empty and reusing files when
present (even at different names).
[~rohini], please let me know if you have a chance to review. Thanks!
was (Author: xkrogen):
Attaching initial patch implementing the ideas I discussed in the JIRA. Uploads
at a random file name, then moves it to the original filename. Uses listStatus
to be able to share identical JAR files with nonidentical names. Makes an
attempt at cleaning up hanging temp files if an issue occurs while uploading /
renaming.
I did not add any tests as it was not very clear to me how to do so / what the
policy on that would be. Let me know if I should add some and if so, where they
would go.
[~rohini], please let me know if you have a chance to review. Thanks!
> User Cache upload contention can cause job failures
> ---------------------------------------------------
>
> Key: PIG-5290
> URL: https://issues.apache.org/jira/browse/PIG-5290
> Project: Pig
> Issue Type: Bug
> Affects Versions: 0.13.0
> Reporter: Erik Krogen
> Assignee: Erik Krogen
> Attachments: PIG-5290.patch
>
>
> We recently enabled the User Cache (PIG-2672) feature and found that
> occasionally jobs would fail because of contention when uploading JARs into
> the cache. Although the cache is designed to be fail-safe, i.e. to fall back
> to normal behavior if anything goes wrong by catching all {{IOException}},
> the portion of code which closes the output stream _is not_ wrapped within a
> {{try}} statement and thus an exception during the closing of that stream
> causes the entire job to fail. If multiple jobs are attempting to upload the
> same JAR failure simultaneously, the contention can cause this close
> statement to fail.
> The current strategy also has two other flaws. First, consider the scenario
> where job A begins uploading jar X. Job B also needs jar X, sees that the
> file exists, and launches its tasks. Yet, job A has not yet finished
> uploading jar X (perhaps it is large). So, the tasks are localizing a
> half-completed version of jar X. Second, the original design allowed for the
> same JAR (identical contents) to be shared between jobs even if a different
> name was used. In PIG-3815, however, this ability was removed, and now JARs
> are only shared if they have the same name.
> I propose we solve all of these issues simultaneously by returning to the
> listStatus based behavior (used prior to PIG-3815), but filter out entries
> ending in {{.tmp}}. When uploading, upload to {{randomNumber.tmp}}, then once
> the file is completed, do a rename to the original name of the JAR file. This
> ensures that incomplete files are never in a location that would be accessed
> by other jobs, and the only write operation accessing a shared path is a
> single rename operation.
> An alternative design is to use a single canonicalized name for all JAR files
> (they will still be unique since they are inside of directories based on
> their SHA1). Upload to a tmp file as previously described, then rename to the
> canonical name. This removes the need to do a listStatus call; however it
> will result in classpaths that are human unreadable since the name of the JAR
> file has been lost. I think it's worth it from a debugging standpoint to go
> with the first design.
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