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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-5062) Support CAS
Date Mon, 01 Apr 2013 22:49:16 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-5062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13619268#comment-13619268
] 

Jonathan Ellis commented on CASSANDRA-5062:
-------------------------------------------

bq. During prepare, if a replica "promise", the ballot it sends back in the response is the
one it just got

This is a bug, it's supposed to reply with the ballot of a previously accepted value (so the
coordinator can pick the highest such to re-propose).

bq. Why not just use a System table for the Paxos state?

I was hoping to avoid that, but you're right.

bq.  Should that compare the timestamps of the UUIDs (The clock sequence is not the timestamp).
 Furthermore, wasn't the goal to have a comparator to have it only compare and timestamps
(and thus not break tie on same timestamp)?

Yes, and yes.

bq. wasn't the goal to reuse the ballot timestamp as the timestamp of the columns in the update
we finally commit 

I'm on board with that, but since we're generating ballots server-side that means we need
to swallow CASSANDRA-5293...

bq.  I strongly believe we should return false only if the CAS doesn't apply, but otherwise
we should just restart a new proposal

I agree, but on the other hand "just retry automatically" is something that we avoid elsewhere
for good reason (arbitrarily high latency with no visibility to the client what is going on).
 I also considered introducing a new exception type explaining the state, but throwing an
exception when everything is working fine is also not intuitive.  So I think all the options
kind of suck, but this one sucks least IMO.

Fixes pushed to the same branch.
                
> 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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