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https://issues.apache.org/jira/browse/CASSANDRA-12126?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17138370#comment-17138370
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Sylvain Lebresne commented on CASSANDRA-12126:
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I noticed that the previous version of the patches wasn't working in all cases
due to an existing quirk of the CAS implementation.
Namely, accepted updates that were empty were not replayed by
{{beginAndRepairPaxos}}. Which is a problem for the new empty commits made
during serial reads/non-applying CAS. I added tests to show that if the commit
messages for those empty commits were lost/delayed, we could still have
linearizability violations.
Now, the logic of not replaying empty updates looks wrong to me. There
shouldn't be anything special about an empty update, and if one is explicitely
accepted by a quorum of nodes, we shouldn't ignore it, or that's a break of the
Paxos algorithm (as kind of can be demonstrated by the tests I added).
To be clear, that logic was added *by me* in CASSANDRA-6012 and that was the
sole purpose of that ticket. Except that I can't make sense of my reasoning
back then, and since I didn't included a test to demonstrate the problem I was
solving back then (which was wrong, mea culpa), I have to assume that I was
just confused (maybe I mixed in my head promised ballots and accepted ones?).
Anyway, I think the fix here is simply to remove that bad logic, which fixes
the issue, and I included an additional commit for that.
> CAS Reads Inconsistencies
> --------------------------
>
> Key: CASSANDRA-12126
> URL: https://issues.apache.org/jira/browse/CASSANDRA-12126
> Project: Cassandra
> Issue Type: Bug
> Components: Feature/Lightweight Transactions, Legacy/Coordination
> Reporter: Sankalp Kohli
> Assignee: Sylvain Lebresne
> Priority: Normal
> Labels: LWT, pull-request-available
> Time Spent: 10m
> Remaining Estimate: 0h
>
> 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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