[hibernate-dev] Consistency guarantees of second level cache
Hi,
I've been fixing a lot of consistency issues in Infinispan 2LC lately
and also trying to improve performance. When reasoning about consistency
guarantees I've usually assumed that we don't want to provide stale
entries from the cache after the DB commits - that means, we have to
invalidate them before the DB commit. This is a useful property if there
are some application constraints on the data (e.g. that two entities
have equal attributes). On the other hand, if we want the cache
synchronized with DB only after the commit fully finishes, we could omit
some pre-DB-commit RPCs and improve the performance a bit.
To illustrate the difference, imagine that we wouldn't require such
atomicity of transactions: when we update the two entities in TX1 and
one of them is cached and the other is not, in TX2 we could see updated
value of the non-cached value but we could still hit cache for the other
entity, seeing stale value, since TX1 has committed the DB but did not
finish the commit yet on ORM side:
A = 1, B = 1
TX1: begin
TX1: (from flush) write A -> 2
TX1: (from flush) write B -> 2
TX1: DB (XA resource) commit
TX2: read A -> 2 (handled from DB)
TX2: read B -> 1 (cached entry)
TX1: cache commit (registered as synchronization) -> cache gets updated
to B = 2
TX1 is completed, control flow returns to caller
Naturally, after TX1 returns from transaction commit, no stale values
should be provided.
Since I don't have any deep experience with DBs (I assume that they
behave really in the ACID way). I'd like to ask what are the guarantees
that we want from 2LC, and if there's anything in the session caching
that would loosen this ACIDity. I know we have the nonstrict-read-write
mode (that could implement the less strict way), but I imagine this as
something that breaks the contract a bit more, allowing even larger
performance gains (going the best-effort way without any guarantees).
Thanks for your insight!
Radim
--
Radim Vansa <rvansa(a)redhat.com>
JBoss Performance Team