Re: cluster and cache question

[Edgar Poce]

hi stefan
Stefan Guggisberg wrote:
ItemState instances are cached on multiple layers. Please have a look
at dominique's earlier post which IMO explains jackrabbit's design 
quite nicely:
http://thread.gmane.org/gmane.comp.apache.jackrabbit.devel/1172
thanks for the reference.
the SharedItemStateManager is at the bottom layer and caches 
ItemState instances read from the PM. this memory-sensitive cache, 
apart from helping to reduce PM calls, guarantees that there's only 
*one* ItemState instance for any given id on this layer. this is crucial 
for jackrabbit's support of isolation levels. caching in the PM would 
therefore IMO be redundant if not disadvantageous because it would
use up (memory) resources that could be used more efficiently by 
jackrabbit.
got it.
regarding clustering:
clustering is a very interesting topic and we had quite a few discussions
on how it could be supported in jackrabbit. 
is there an archive of this?
> clustering can IMO not be
entirely delegated to the PM. it has to be tackled on a higher level.
the sticking point is how to synchronize the multiple jackrabbit instances 
in a cluster.

Have you seen the oscache approach for clustering? it seems interesting. 
I'm just thinking out loud, Local and Shared ItemStateManagers could 
favor composition over inheritance. The current ItemStateCache 
implementation is a good fit for the first, and an oscache like approach 
could be a good choice for the latter in a clustering scenario.

Maybe this could be a deployment decision, with a "shared-cache" tag in 
the config file that leaves to the deployer the cache strategy selection 
for shared ItemStates.

A pluggable cache for shared item states would also be useful for 
network deployments with or without clustering. Those who chose a rdbms 
backend would be able to to use a cache that relies also in the local 
filesystem, and not only in mem.

there's no easy answer to that and it would certainly require considerable
work in jackrabbit's core. as our main priority is getting jackrabbit version 
1.0 out clustering support has to wait i'm afraid.
It has sense. It seems many people is waiting for a release in order to 
use it in production.

Sorry for being such a pain, I'm becoming a jackrabbit fun and I'd like 
to use it on any project I get involved.

thanks
edgar