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Date: Fri, 13 Jan 2006 10:52:54 +0000
From: Jules Gosnell <jules@coredevelopers.net>
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Subject: Re: Replication using totem protocol
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lichtner@bway.net wrote:

>>lichtner@bway.net wrote:
>>    
>>
>>>Well, you guys let me know if I can help you in any way.
>>>      
>>>
>>Keep on talking ;-)
>>    
>>
>
>Okay. I will ask you a question then. What are you doing as far caching
>entity beans?
>  
>
In terms or replication or some form of distributed invalidation, I'm 
not aware that this has been discussed yet.

This is another one for the forthcoming doc - briefly :

If you cluster an entity bean on two nodes naively, you lose many of the 
benefits of caching. This is because neither node, at the beginning of a 
transaction, knows whether the other node has changed the beans contents 
since it was last loaded into cache, so the cache must be assumed 
invalid. Thus, you find yourself going to the db much more frequently 
than you would like, and the number of trips increases linearly with the 
number of clients - i.e. you are no longer scalable.

If you can arrange for the cache on one node to notify the cache on 
other nodes, whenever an entity is changed, then the other caches can 
optimise their actions, so that rather assuming that all beans are 
invalid, they can pinpoint the ones that actually are invalid and only 
reload those.

You could go one step further and send, not an invalidation, but a 
replication message. This would contain the Entity's new value and head 
off any reloading from the DB at all....

All of this needs to be properly integrated with e.g. transactions, 
locking etc...

Perhaps Totem might be useful here ?


Jules


-- 
"Open Source is a self-assembling organism. You dangle a piece of
string into a super-saturated solution and a whole operating-system
crystallises out around it."

/**********************************
 * Jules Gosnell
 * Partner
 * Core Developers Network (Europe)
 *
 *    www.coredevelopers.net
 *
 * Open Source Training & Support.
 **********************************/