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From "sankalp kohli (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-12126) CAS Reads Inconsistencies
Date Fri, 01 Jul 2016 17:46:11 GMT
sankalp kohli created CASSANDRA-12126:
-----------------------------------------

             Summary: CAS Reads Inconsistencies 
                 Key: CASSANDRA-12126
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12126
             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. 




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