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  1. Cassandra
  2. CASSANDRA-6013

CAS may return false but still commit the insert



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      If a Paxos proposer proposes some value/update and that propose fail, there is no guarantee on whether this value will be accepted or not ultimately. Paxos guarantees that we'll agree on "a" value (for a given round in our case), but does not guarantee that the proposer of the agreed upon value will know it. In particular, if for a given proposal at least one accepter has accepted it but not a quorum does, then that value might (but that's not guaranteed either) be replayed (and committed) by another proposer.

      Currently, if a proposer A proposes some update U but it is rejected, A will sleep a bit and retry U. But if U was accepted by at least one acceptor, some other proposer B might replay U, succeed and commit it. If A does its retry after that happens, he will prepare, check the condition, and probably find that the conditions don't apply anymore since U has been committed already. It will thus return false, even though U has been in fact committed.

      Unfortunately I'm not sure there is an easy way for a proposer whose propose fails to know if the update will prevail or not eventually. Which mean the only acceptable solution I can see would be to return to the user "I don't know" (through some exception for instance). Which is annoying because having a proposal rejected won't be an extremely rare occurrence, even with relatively light contention, and returning "I don't know" often is a bit unfriendly.


        1. 6013-v4.patch
          6 kB
          Sylvain Lebresne
        2. 6013-v3.txt
          6 kB
          Jonathan Ellis
        3. 6013-v2.txt
          6 kB
          Sylvain Lebresne
        4. 6013.txt
          3 kB
          Jonathan Ellis



            jbellis Jonathan Ellis
            slebresne Sylvain Lebresne
            Jonathan Ellis
            Sylvain Lebresne
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