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https://issues.apache.org/jira/browse/CASSANDRA-5062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13592634#comment-13592634
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Cristian Opris edited comment on CASSANDRA-5062 at 3/4/13 9:05 PM:
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Sorry, I've probably edited my comment after your reply.
C_current.timestamp needs to be exactly R-1 if you increment the counter on
sending the proposal.
If it's less, then the acceptor hasn't learned the previously committed value
(R-1) so can't participate in round R, otherwise we're mixing up rounds.
If it's more, then the proposer is behind so you already handle that.
Regarding " If you get quorum for the proposal you can perform the CAS locally
and just
send the new column value with the accept"
By that I meant you can do the validate part of the CAS locally, not actually
write the CAS.
Basically any operation (not just CAS) can be evaluated (in memory) by the
proposal after it gets quorum for the proposal (which guarantees it has the
latest *committed* value) so it obtains the value to send for acceptance. This
is more of an optimization where you exchange and agree on values rather than
operations (state transfer replication). Also solves the problem of where to
validate the CAS.
was (Author: [email protected]):
Sorry, I've probably edited my comment after your reply.
C_current.timestamp needs to be exactly R-1 if you increment the counter on
sending the proposal.
If it's less than the acceptor hasn't learned the previously committed value
(R-1) so can't participate in round R, otherwise we're mixing up rounds.
If it's more, than the proposer is behind so you already handle that.
Regarding " If you get quorum for the proposal you can perform the CAS locally
and just
send the new column value with the accept"
By that I meant you can do the validate part of the CAS locally, not actually
write the CAS.
Basically any operation (not just CAS) can be evaluated (in memory) by the
proposal after it gets quorum for the proposal (which guarantees it has the
latest *committed* value) so it obtains the value to send for acceptance. This
is more of an optimization where you exchange and agree on values rather than
operations (state transfer replication). Also solves the problem of where to
validate the CAS.
> 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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