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From Stefan Guggisberg <stefan.guggisb...@gmail.com>
Subject Re: cluster and cache question
Date Sat, 02 Apr 2005 17:21:22 GMT
hi edgar

On Apr 1, 2005 6:24 PM, Edgar Poce <edgarpoce@gmail.com> wrote:
> Hi
> 
> I'm trying to use jackrabbit in a cluster scenario. Since jdbc calls are
> rather expensive caching results is a common practice, but as you all
> pointed PMs are not designed to be smart. So, do you think caching
> should be in the PM?, it should be somewhere else?, it shouldn't be at
> all? any more option?

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

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.

regarding clustering:
clustering is a very interesting topic and we had quite a few discussions
on how it could be supported in jackrabbit. 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.

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.

cheers
stefan

> 
> I'm thinking of a cluster scenario with
> jdbc PM + distributed cache + jackrabbit
> instead of
> any PM + jackrabbit + rmi server + rmi client
> 
> I prefer the first config because there are products that build the
> cache on each server and don't pass the cached objects around the cluster.
> 
>   It seems I often feel tempted to bad practices :(, so I'd like to know
> your opinion before taking a decision.
> 
> thanks in advance
> edgar
>

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