Manik Surtani wrote:
AFAIR the last time we discussed this (last summer in Austin with Steve
and Gavin) we came to the conclusion that R_C was optimal for the 2LC
use case.
Yep. I was looking for reasons to flat out say "never use R_R, always
R_C" in docs and such. Seems it's a bit more subtle than that.
I did neglect to change the default in the AS 5 / Hib 3.3 config files,
which will shortly be corrected.
On 17 Jul 2008, at 22:59, Brian Stansberry wrote:
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.
You'd need to check with Steve on this, but to the best of my knowledge,
once a session has started, it copies stuff to a "first-level cache"
which is a Map associated with the session. A Session.clear()/evict()
would only flush the 2LC, the 1LC would still be intact to provide R_R
to the caller. Although it does sound a bit odd that a clear() or
evict() won't affect 1LC and go straight to 2LC, so I could be wrong. :-)
Session.evict() and clear() clear the 1LC.[1] The 2LC is managed via
the SessionFactory API.
In my now-recollected usage of Session.evict() at my previous employer,
we used it to free memory during transactions that processed a lot of
data, exactly what [1] describes.
So, so far it seems R_R in the cache config only makes sense if:
1) You are using Session.clear() evict() or refresh().
2) AND your app needs R_R semantics for the entities/collections
affected by those calls.
AIUI, the whole point of using refresh() is because you don't want R_R
semantics [2], so clear() and evict() are the key points.
[1]
http://www.hibernate.org/hib_docs/v3/reference/en/html/performance.html#performance-sessioncache
[2]
http://www.hibernate.org/hib_docs/v3/reference/en/html/objectstate.html#objectstate-loading
Cheers,
Manik
--
Manik Surtani
Lead, JBoss Cache
[EMAIL PROTECTED]
--
Brian Stansberry
Lead, AS Clustering
JBoss, a division of Red Hat
[EMAIL PROTECTED]
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