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Clustering: Why 2 instances on 1 machine?

From: <glassfish_at_javadesktop.org>
Date: Tue, 23 Sep 2008 06:23:49 PDT

Hi,
I've read [url=http://developers.sun.com/appserver/reference/techart/glassfishcluster/]this[/url] paper about clustering in GlassFish and in Figure 4 you can see, that there are on each machine two instances. Which sense does this make? Is it recommended to have more than one instance on each (physical) machine? Increases this the availability of my cluster? Or should I have just one instance per machine?

My other question is about memory replication. Suggest a stateful session bean, Instance1 replicate it to Instance2, Instance2 to Instance3 and so on.
Does that mean, if a client sends a request for the first time to Instance1, so that a new stateful session bean is created, that this SFSB is send (and backuped) on Instance2, after that the same SFSB is send (and backuped) to Instance3 and so on or does it just mean, that this SFSB is just duplicated on Instance2, so that when Instance1 and Instance2 crash at the same time, this SFSB would be lost?

Thank's for your anwsers.
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