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Sylvain Lebresne commented on CASSANDRA-5062:
---------------------------------------------
If a node receive a PROPOSE for round R there is 2 possible cases:
* either R is a round for which the node has already "learn" the value. In
which case, we will take the "if (C_current.timestamp() >= R)" branch, i.e. the
"repair the proposer that we know is behind"
* or R is a round for which the node hasn't already learn the value. In that
case, we read whatever paxos state for round R. Whether round R has been
"learned" by *other* replica doesn't matter at all. That is, say the last round
the node has "learn" about is R_last, then it's absolutely possible for the R
received in the propose to be R_last+1 or R_last+2 or R_last+whatever. The
point being, every read of the paxos state is parametrized by the round itself.
So a propose for round R_last+2 will look at what the replica has (or doesn't
have) for round R_last+2 only (the state for round R_last+1 is not queried).
Note that node can absolutely have state for more than one round at the same
time, but it will never mix those by construction.
> Support CAS
> -----------
>
> Key: CASSANDRA-5062
> URL: https://issues.apache.org/jira/browse/CASSANDRA-5062
> Project: Cassandra
> Issue Type: New Feature
> Components: API, Core
> Reporter: Jonathan Ellis
> Fix For: 2.0
>
> Attachments: half-baked commit 1.jpg, half-baked commit 2.jpg,
> half-baked commit 3.jpg
>
>
> "Strong" consistency is not enough to prevent race conditions. The classic
> example is user account creation: we want to ensure usernames are unique, so
> we only want to signal account creation success if nobody else has created
> the account yet. But naive read-then-write allows clients to race and both
> think they have a green light to create.
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