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From "Jonathan Ellis (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6023) CAS should distinguish promised and accepted ballots
Date Sat, 14 Sep 2013 21:02:51 GMT


Jonathan Ellis commented on CASSANDRA-6023:

I'm pretty sure we don't need to make everything volatile/concurrent in PrepareCallback, since
the latch and synchronized establish happens-before for the StorageProxy thread.

I'm not crazy about PrepareResponse including two different values for inProgressCommit. 
Can we clean that up somehow?

Might also be worth renaming in_progress_ballot to promised_ballot to be a little more clear
on the distinction vs proposal_ballot.

Rest LGTM.
> CAS should distinguish promised and accepted ballots
> ----------------------------------------------------
>                 Key: CASSANDRA-6023
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Sylvain Lebresne
>            Assignee: Sylvain Lebresne
>             Fix For: 2.0.1
>         Attachments: 0001-Distinguish-between-promised-and-accepted-ballots.txt, 0002-Populate-commitsByReplica-in-PrepareCallback.txt
> Currently, we only keep 1) the most recent promise we've made and 2) the last update
we've accepted. But we don't keep the ballot at which that last update was accepted. And because
a node always promise to newer ballot, this means an already committed update can be replayed
even after another update has been committed. Re-committing a value is fine, but only as long
as we've not start a new round yet.
> Concretely, we can have the following case (with 3 nodes A, B and C) with the current
> * A proposer P1 prepare and propose a value X at ballot t1. It is accepted by all nodes.
> * A proposer P2 propose at t2 (wanting to commit a new value Y). If say A and B receive
the commit of P1 before the propose of P2 but C receives those in the reverse order, we'll
current have the following states:
> {noformat}
> A: in-progress = (t2, _), mrc = (t1, X)
> B: in-progress = (t2, _), mrc = (t1, X)
> C: in-progress = (t2, X), mrc = (t1, X)
> {noformat}
> Because C has received the t1 commit after promising t2, it won't have removed X during
t1 commit (but note that the problem is not during commit, that example still stand if C never
receive any commit message).
> * Now, based on the promise of A and B, P2 will propose Y at t2 (C don't see this propose
in particular, not before he promise on t3 below at least). A and B accepts, P2 will send
a commit for Y.
> * In the meantime a proposer P3 submit a prepare at t3 (for some other irrelevant value)
which reaches C before it receives P2 propose&commit. That prepare reaches A and B too,
but after the P2 commit. At that point the state will be:
> {noformat}
> A: in-progress = (t3, _), mrc = (t2, Y)
> B: in-progress = (t3, _), mrc = (t2, Y)
> C: in-progress = (t3, X), mrc = (t2, Y)
> {noformat}
> In particular, C still has X as update because each time it got a commit, it has promised
to a more recent ballot and thus skipped the delete. The value is still X because it has received
the P2 propose after having promised t3 and has thus refused it.
> * P3 gets back the promise of say C and A. Both response has t3 as in-progress ballot
(and it is more recent than any mrc) but C comes with value X. So P3 will replay X. Assuming
no more contention this replay will succeed and X will be committed at t3.
> At the end of that example, we've comitted X, Y and then X again, even though only P1
has ever proposed X.
> I believe the correct fix is to keep the ballot of when an update is accepted (instead
of using the most recent promised ballot). That way, in the example above, P3 would receive
from C a promise on t3, but would know that X was accepted at t1. And so P3 would be able
to ignore X since the mrc of A will tell him it's an obsolete value.

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