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https://issues.apache.org/jira/browse/CASSANDRA-12126?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17138578#comment-17138578
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Sylvain Lebresne commented on CASSANDRA-12126:
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Ok, I understand what you are suggesting now and I agree this should work as
well. And it does is more optimal.
I like to think of our algorithm as "pure Paxos instances" separated by the MRC
to tell us when we can forget the previous instance and start a new one.
Committing empty updates as any other updates still fits that mental model,
while your suggestion adds a bit of a special case in that it bends the Paxos
rules slightly, allowing to sometime ignore a previously accepted value in a
promise (when it's empty). Which is not a criticism, just thinking out loud.
It's more performant and this is likely worth the slight special casing since
it's not too hard to reason about its correctness.
I'll sleep on it and modify to your suggestion tomorrow (which is trivial, just
need to massage an appropriate comment to explain it).
> 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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