[infinispan-dev] on optimistic locking semnatics

[Sérgio Miguel Fernandes]

Hi,

On 22 Jun 2012, at 15:47, Mircea Markus wrote:

> I've come around a JIRA[1] about optimistic locking and write skew check, and wondering what's the expected behavior in the following scenario:
> tx1.begin()
> read(a); 
> // tx2 modifies 'a' and commits changes
> write(b);
> tx1.commit() - shall I get a write skew (version changed) exception here as a was modified between read and commit?

Yes! Your scenario is an example of a write skew [2].

> In other words, shall I also perform write skew check for the entries that were read during a tx? We currently don't, but I can totally see situations in which this would be needed: if the decision to write to 'b' is made on the value of 'a' and there's an invariant containing both a and b.

I would expect that a configuration named writeSkewCheck, when set to "true", did exactly that kind of check, yes.

I've actually been assuming that this was already implemented, as I've been using writeSkewCheck="true" in Infinispan's configuration to ensure that my applications present a consistent transactional behaviour.

> [1]https://issues.jboss.org/browse/ISPN-1841


[2] Here's some more useful info on this topic:
"A read-only transaction anomaly under snapshot isolation": http://dl.acm.org/citation.cfm?id=1031573
"Making snapshot isolation serializable": http://dl.acm.org/citation.cfm?id=1071615

Cheers,
-- 
Sérgio Miguel Fernandes