[jira] [Updated] (CASSANDRA-8672) Ambiguous WriteTimeoutException while completing pending CAS commits

["Philip Thompson (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-8672?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Philip Thompson updated CASSANDRA-8672:
---------------------------------------
    Fix Version/s: 3.0
         Assignee: Tyler Hobbs

> 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
>
>
> 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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