Assaf Arkin wrote:
> Paul Hodgetts wrote:
>
> > Even though this may be vendor specific, how does this all work
> > in a clustered environment? Are the object caches in each
> > container kept consistent across multiple container instances?
> > If so, how is it done - what are the limitations?
>
> They should through shared locking and other mechanisms of consistency
> checks. There are no limitations, but you pay a penalty in performance.
> It's not going to be slower than using a database, but it won't be as
> fast as Hashtable either.
The specific scenario I'm thinking of is this:
1. Container A instantiates an entity bean with a PK of 26. This
entity is not in Container A's cache (in its VM), so it reads
it from the database and caches it. The entity is used, and is
eventually passivated.
2. Container B instantiates the same entity bean (PK 26). It's
not in its cache, so it reads it from the database. After
updating some fields in the entity, the entity is stored back
to the database.
3. Container A gets another request to use the entity (PK 26).
It finds the entity in its cache, and uses the cached entity.
But this copy is no longer up to date, because it doesn't
reflect the changes made through container B.
Obviously, something needed to happen after step 2 to cause the
entity in container A's cache to be invalidated so that it will
refresh it from the database on the next access. If the cluster
is the only thing that will be accessing the entity, then these
notifications can be generated from container to container,
perhaps using JMS. If arbitrary apps can update the database,
then the notification would have to come from the database,
perhaps via triggers or some such.
Is there any CMP product on the market that handles this scenario
(assuming that the cluster can own the database for now)?
Thanks,
-Paul Hodgetts
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