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  1. Cassandra
  2. CASSANDRA-8672

Ambiguous WriteTimeoutException while completing pending CAS commits



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      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 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.


        1. storyproxybean-2.0-8672.txt
          2 kB
          Stefan Podkowinski
        2. storyproxybean-2.1-8672.txt
          2 kB
          Stefan Podkowinski



            spod Stefan Podkowinski
            spod Stefan Podkowinski
            Stefan Podkowinski
            Sylvain Lebresne
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