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https://issues.apache.org/jira/browse/CASSANDRA-5062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13588390#comment-13588390
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Sergio Bossa commented on CASSANDRA-5062:
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Thanks for clarifying, [~slebresne].
{quote}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).{quote}
The Zab deviation from standard 2PC here is that the coordinator doesn't need
to wait for the ack from replicas on commit phase.
If a replica fails during prepare phase, it will just be out of quorum.
If a replica fails after prepare but before completing the commit, it will
recover later from the leader: so in your example, when 2 and 3 come up, they
will join the leader which may hint them the correct values.
If the third replica died in your example was actually the coordinator, a new
coordinator will be elected among the ones that have seen either the last
commit or the latest *proposed* commit, which will become committed.
So there's no lost-ack problem as there's actually no ack at all in the commit
phase: it will be "eventually" committed or recovered.
By the way, I'm not saying this is better than Paxos for sure: I just *think*
this is easier and more practical (which yes doesn't mean can be implemented
easily on top of Cassandra).
> 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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