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From "Stefan Podkowinski (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8672) Ambiguous WriteTimeoutException while completing pending CAS commits
Date Wed, 01 Apr 2015 18:27:53 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8672?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14391158#comment-14391158
] 

Stefan Podkowinski commented on CASSANDRA-8672:
-----------------------------------------------

I've added a patch to make it clearer where I actually want to go with this ticket. Should
have done this right at the start instead of posting a wall of text, sorry :/

> Ambiguous WriteTimeoutException while completing pending CAS commits
> --------------------------------------------------------------------
>
>                 Key: CASSANDRA-8672
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8672
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: Stefan Podkowinski
>            Assignee: Tyler Hobbs
>            Priority: Minor
>              Labels: CAS
>             Fix For: 3.0
>
>         Attachments: storyproxybean-2.0-8672.txt, storyproxybean-2.1-8672.txt
>
>
> Any CAS update has a chance to trigger a pending/stalled commit of any previously agreed
on CAS update. After completing the pending commit, the CAS operation will resume to execute
the actual update and also possibly create a new commit. See StorageProxy.cas()
> Theres two possbile execution paths that might end up throwing a WriteTimeoutException:
> cas() -> beginAndRepairPaxos() -> commitPaxos()
> cas() -> commitPaxos()
> Unfortunatelly clients catching a WriteTimeoutException won't be able to tell at which
stage the commit failed. My guess would be that most developers are not aware that the beginAndRepairPaxos()
could also trigger a write and assume that write timeouts would refer to a timeout while writting
the actual CAS update. Its therefor not safe to assume that successive CAS or SERIAL read
operations will cause a (write-)timeouted CAS operation to get eventually applied. Although
some [best-practices advise|http://www.datastax.com/dev/blog/cassandra-error-handling-done-right]
claims otherwise.
> At this point the safest bet is possibly to retry the complete business transaction in
case of an WriteTimeoutException. However, as theres a chance that the timeout occurred while
writing the actual CAS operation, another write could potentially complete it and our CAS
condition will get a different result upon retry.



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