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https://issues.apache.org/jira/browse/CASSANDRA-12126?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16631226#comment-16631226
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Benedict commented on CASSANDRA-12126:
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bq. We read nothing in node Y, yet node Z read something in the next request.
I think the problem here is that, at the API level, there isn't enough
information to say that X didn't simply 'occur' *after* both Y and Z. That is,
unless the rejection of Y occurs after X's timeout. In this case, it would
seem to be an API-visible error, as at the point of timeout the indeterminacy
should be fixed. Timeouts should not ‘live forever’ as the bogeyman, ready to
mess with history.
I think, though, that the suggested mechanism could result in this.
Take three nodes (RF=3) A, B and C; and any three CAS operations X, Y and Z
such that:
* X and Y can always succeed
* Z can only succeed if X has succeeded
Setup:
# Prepare _and_ Propose X with ballot 1; proposal accepted only by A
#* this will be the last and only node’s proposal acceptance
# Prepare Y with ballot 2; reach B and C before ballot 1, so they do not accept
# Now, lock X and Y in battle, always failing to proceed to the propose step
before the other reaches the prepare step again
# X and Y both timeout having failed to cleanly apply
Part 2:
# Z is now attempted; it prepares to only B and C, seeing no in-progress
proposal
# As a result, it does not see X; it is rejected, so there is no new
proposal/commit
# Read at SERIAL is performed; this time, A is consulted
# Suddenly, a wild X appears. From nowhere.
It does seem, in essence, to be an incidence of the bug (or a very similar one)
described in the ticket.
> CAS Reads Inconsistencies
> --------------------------
>
> Key: CASSANDRA-12126
> URL: https://issues.apache.org/jira/browse/CASSANDRA-12126
> Project: Cassandra
> Issue Type: Bug
> Components: Coordination
> Reporter: sankalp kohli
> Priority: Major
> Labels: LWT
>
> While looking at the CAS code in Cassandra, I found a potential issue with
> CAS Reads. Here is how it can happen with RF=3
> 1) You issue a CAS Write and it fails in the propose phase. A machine replies
> true to a propose and saves the commit in accepted filed. The other two
> machines B and C does not get to the accept phase.
> Current state is that machine A has this commit in paxos table as accepted
> but not committed and B and C does not.
> 2) Issue a CAS Read and it goes to only B and C. You wont be able to read the
> value written in step 1. This step is as if nothing is inflight.
> 3) Issue another CAS Read and it goes to A and B. Now we will discover that
> there is something inflight from A and will propose and commit it with the
> current ballot. Now we can read the value written in step 1 as part of this
> CAS read.
> If we skip step 3 and instead run step 4, we will never learn about value
> written in step 1.
> 4. Issue a CAS Write and it involves only B and C. This will succeed and
> commit a different value than step 1. Step 1 value will never be seen again
> and was never seen before.
> If you read the Lamport “paxos made simple” paper and read section 2.3. It
> talks about this issue which is how learners can find out if majority of the
> acceptors have accepted the proposal.
> In step 3, it is correct that we propose the value again since we dont know
> if it was accepted by majority of acceptors. When we ask majority of
> acceptors, and more than one acceptors but not majority has something in
> flight, we have no way of knowing if it is accepted by majority of acceptors.
> So this behavior is correct.
> However we need to fix step 2, since it caused reads to not be linearizable
> with respect to writes and other reads. In this case, we know that majority
> of acceptors have no inflight commit which means we have majority that
> nothing was accepted by majority. I think we should run a propose step here
> with empty commit and that will cause write written in step 1 to not be
> visible ever after.
> With this fix, we will either see data written in step 1 on next serial read
> or will never see it which is what we want.
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