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https://issues.apache.org/jira/browse/CASSANDRA-12126?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17138464#comment-17138464
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Benedict Elliott Smith edited comment on CASSANDRA-12126 at 6/17/20, 2:12 PM:
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The problem here stems only from the overload of
{{mostRecentInProgressCommitWithUpdate}}, which (seems to) assume that an empty
update is for a higher promise (since the meaning is overloaded in the response
message) rather than an "incomplete" proposal. If the empty proposal were to be
correctly merged with {{mostRecentInProgressCommitWithUpdate}}, it would
override the early non-empty incomplete proposal.
Which is a long-winded way of saying that I am fairly confident there's no need
to update the paxos state table with the "committed" status of this empty
proposal so long as it remains in the table _as an accepted proposal_, and so
long as this accepted proposal continues to override earlier in progress
proposals.
was (Author: benedict):
The problem here stems only from the overload of
{{mostRecentInProgressCommitWithUpdate}}, which (seems to) assume that an empty
update is for a higher promise (since the meaning is overloaded in the response
message) rather than an "incomplete" proposal. If the empty proposal were to be
correctly merged with {{mostRecentInProgressCommitWithUpdate}}, it would
override the early non-empty incomplete proposal.
Which its a long-winded way of saying there's no need to update the paxos state
table with the "committed" status of this empty proposal so long as it remains
in the table _as an accepted proposal_ and so long as this accepted proposal
continues to override earlier in progress proposals.
> 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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