cassandra-commits mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Sylvain Lebresne (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-12126) CAS Reads Inconsistencies
Date Tue, 05 Jul 2016 09:34:11 GMT


Sylvain Lebresne commented on CASSANDRA-12126:

I "think" you are right that this can happen, and that committing an empty commit on SERIAL
reads "should" fix it. Paxos is however subtle enough that I would feel more confident with
this if we had a reproduction test first, if only so we can validate whatever patch we come
up with. [~jkni] I believe you've spend some time on jespen-like tests for paxos, do you think
this is something we could use to try to reproduce something like that relatively consistently?

> CAS Reads Inconsistencies 
> --------------------------
>                 Key: CASSANDRA-12126
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: sankalp kohli
> 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

View raw message