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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-6013) CAS may return false but still commit the insert
Date Thu, 12 Sep 2013 15:32:53 GMT
Sylvain Lebresne created CASSANDRA-6013:
-------------------------------------------

             Summary: CAS may return false but still commit the insert
                 Key: CASSANDRA-6013
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6013
             Project: Cassandra
          Issue Type: Bug
            Reporter: Sylvain Lebresne


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 only guarantees that we'll
agree on "a" value (for a given round in our case), but does 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.

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