Re: [Hibernate-devel] caching algorithm

[Gavin_King/Cirrus%<CI...@ci...>]

> some general questions: where does this cache fit in? Is it per
> Datastore, per Session, or something else?

per session factory.

> What's the intended purpose of the cache?

Well, people asked for it......

its intended to increase performance by caching data that changes rarely
(or is never changed by the application) between transactions. Many
applications will have a set of data that it modifies, and another set of
"reference" or "static" data that it can sensibly cache between
transactions. Possibly the static data changes rarely, or it changes on a
predictable cycle (daily, for example). There is no reason to force
hibernate to re-select that data each session. On the other hand, you could
easily mount a convincing case that the application itself should be
responsible for caching this stuff...

There is also data that is mostly read and very rarely updated. Its harder
for the application to manage its own cache in this situation. For
performance reasons, we would like to be able to get some caching going
here.

I am certainly *not* trying to implement caching for the case of data which
is frequently updated. I feel this a harder problem and the caching
algorithm i implemented doesn't improve performance in this case.

> This might sounds like a
> trivial question, but I can't tell from the discussion. A lot of it
> seems to be heading away from just a lightweight persistence layer.

Well, this is the line i have drawn: if implementing caching would require
any significant added complexity to the core persistence engine, or if it
would have a negative performance impact upon the vanilla case of no
caching, I will happily drop the idea. The way i've done it at the moment
satisifies these requirements. This is added to the other requirement that
hibernate never ever uses in-memory locks upon objects.

> Scared, PaulS :|

Yeah, yeah, i know, i know ..... its against the spirit of the project, it
cant scale to a cluster, I can see neither you nor Doug like it and i'm not
that keen on it myself. Its not hurting anything by staying there for the
moment, but I also promise not to document it for the moment. I also
promise we wont head further down this path (i never intended to,
actually).