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https://issues.apache.org/jira/browse/CASSANDRA-5062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13588344#comment-13588344
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Sylvain Lebresne commented on CASSANDRA-5062:
---------------------------------------------
bq. once the coordinator gets the acks from the prepare phase, it can commit
without having to wait for all acks
What Jonathan means by lost ack is: what happens if when the coordinator sends
the commits to replicas, but only a minority of replicas get that commit (say 1
of 3 replica got it (and persist it), the two other dies between the prepare
and commit phase). And later on, the 2 replica get back up while the 3rd one
now dies, and we do a new CAS (that would have a majority and so should work).
In that case, probably the coordinator should hint something when he don't get
the commit-ack from the 2 replicas that died. It could either try to revert
from the 1 replica that did received the commit, but said replica can now be
dead (playing devil's advocate here), so it might be a hint really. Or we rely
on the commit being hinted to the 2 died replica when they come back up. But in
that any case, replicas would need to check and wait for those hints when they
come back up before joining the ring, and that's a problem (any other node
could have hint, we don't want to wait for everyone).
To be clear, I'm not saying Zab doesn't work or anything like that, just
explaining the lost-ack problem we have identified with a naive 2PC
implementation.
> 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
>
>
> "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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