Hmm, good point. Potentially also Session.refresh(...), although I'm not
sure if the implementation of that method skips the 2LC and goes right
to the db.
Paul Ferraro wrote:
After thinking this through, the only scenario I can think of where the
2LC would be subject to a repeated read is after a session cache
eviction (i.e. via Session.clear() or Session.evict(...)). Without
REPEATABLE_READ isolation on the 2LC, any subsequent request withing the
same transaction for an evicted entity could return an updated value, if
the cache was updated by a concurrent request.
Paul
On Thu, 2008-07-17 at 12:59 -0500, Brian Stansberry wrote:
Can anyone see a reason to use REPEATABLE_READ as the JBoss Cache
isolation level in the 2nd level cache use case? I'm not seeing one, and
it certainly hurts performance by forcing cache writes to block waiting
for an earlier tx that did a read to commit.
There are 4 types of data cached:
1) Entities
If an entity is read from the 2LC, for the life of the tx it will be
cached in the Session, so AIUI there should be no second read during the
tx. So no benefit to RR.
2) Collections
Same as entities.
3) Queries
If an application executes a query twice in the same tx, I wouldn't
think they'd expect the same result. In any case, if an update to the
query cache is blocking waiting for a tx that previously read the query
result to release, the existence of the update that means the
underlying entities and their timestamps have changed. So a repeated
read of the cached query will just result in it being discarded as out
of date anyway.
4) Timestamps
Here you don't want an RR semantic. You always want to get the most
up-to-date data.
Anyone see any holes in my thinking?
--
Brian Stansberry
Lead, JBoss AS Clustering
JBoss, a division of Red Hat
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Brian Stansberry
Lead, AS Clustering
JBoss, a division of Red Hat
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