[ 
https://issues.apache.org/jira/browse/CASSANDRA-12126?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17117597#comment-17117597
 ] 

Benedict Elliott Smith commented on CASSANDRA-12126:
----------------------------------------------------

I've pushed various test cases 
[here|https://github.com/belliottsmith/cassandra/tree/12126-tests-3.0] - most 
of them marked {{@Ignore}} because they are known to fail, and won't be 
resolved immediately.

> 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. 



--
This message was sent by Atlassian Jira
(v8.3.4#803005)

---------------------------------------------------------------------
To unsubscribe, e-mail: commits-unsubscr...@cassandra.apache.org
For additional commands, e-mail: commits-h...@cassandra.apache.org

Reply via email to