----- Original Message -----
From: Robert Claeson <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Friday, February 11, 2000 4:26 PM
Subject: Singleton EJB's?
> I've yet to come up with a good solution on this one myself, so I'm turning
> it over to you now: what would be the best approach to implement the
> equivialent of a singleton EJB - i.e. one that contains shared data for all
> sessions WITHOUT going through a database. I'm looking for something like a
> singleton stateful session bean. A typical example would be a community web
> site with two or more web servers working against an application server,
> where it would be beneficial to keep shared data in memory on the app
> server, i.e. things like the current number of users and the like.
Distributed shared memory is a *really* hard problem. The singleton pattern
works fine for small numbers of users but *never* scales. This is implicit in
the notion of a singleton (single point of contention, after all). There are
some companies who claim to perform magic and transparently replicate
read/write singleton state across multiple machines. I haven't tried their
stuff, so I can't say if they have succeeded. However, I'm skeptical of any
solutions here because I know what these solutions would require (unless
someone has figured out far more clever ways than I'm aware of - always a
possibility).
This issue is much like the problem of Gravity.
You can build a terrific bridge if you don't have to deal with gravity. It
makes the design so much cleaner, easier to maintain and understand.
However, it doesn't work in real life, and unfortunately that is where it has
to work.
Dealing with distributed state from the beginning of the application will
prevent the phenomenon where your demo's work great but do not scale beyond 10
users.
This is why building scalable applications is hard work. If it was as easy as
just making an Entity bean, everyone would have done it already. Databases are
there for really, really, really good reasons.
Hal
__
What are we going to do tonight, Brain?
Same thing we do every night, Pinky. Try to take over the world.
http://www.hellblazer.com
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