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https://issues.apache.org/jira/browse/CASSANDRA-5062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13589676#comment-13589676
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Jonathan Ellis commented on CASSANDRA-5062:
-------------------------------------------
I'm working on a sketch of this approach.
{code}
. public static boolean cas(ByteBuffer key, ColumnFamily expected,
ColumnFamily updates)
{
// read existing row
// compare current value with expected
if (expected does not match current)
return false;
// apply the update!
}
{code}
I'm adding an optional "[time]UUID paxosBallot" to RowMutation for the proposal
IDs. The updates to RMVH and callbacks should be pretty straightforward. I
think Sylvain talked me into using a system CF instead of a flat file to hold
uncommitted proposals.
The "read" here must be paxos-aware to prevent our lost-ack problem: it must
return both committed value and the highest accepted proposal. Making
read/resolve paxos-aware is going to be the messy part.
Since we need it for CAS anyway, I think we should also expose this read path
as {{ConsistencyLevel.SERIAL}} or similar.
Starting with Thrift because "boolean cas(...)" is straightforward. What
syntax do we use to expose this to CQL? A WHERE clause to UPDATE means
something different in SQL and would IMO be too confusing.
> 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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