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

["Stefan Podkowinski (JIRA)" <ji...@apache.org>]

Stefan Podkowinski created CASSANDRA-8672:
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             Summary: 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
            Priority: Minor


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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