> > * In reference to the Clusterjsp demo we
> showed, one attendee
> > asked what are instances and is there any
> legitimate use for
> > having a cluster on a single machine?
> >
> Having a cluster on a single machine limits the
> ability to horizontally
> scale and creates a single point of failure (that of
> the machine's
> failure).
However, there's another (potential) aspect of clustering on a single machine.
In the past I've benchmarked an (unnamed) app server, and on a 4 core machine, you'd think you would get twice the performance of a 2 core machine.
But in our case we didn't. Could have been some internal scaling issues with the appserver, but we found that running 2 instances on the same machine, with 2 cores each, (we didn't cluster per se, the instances were independent and load balanced), we got better throughput from the machine as a whole and it's 4 cores than if we ran a single instance on all 4 cores.
So, even without virtualization, there can be benefits to running multiple instances on the same machine. Now, as to whether the same effects would happen under Glassfish, I don't know.
Also, while we use the word "cluster", and that implies all of the instances are running the same application, that's not necessarily true either. It's actually a larger management group, and can support multiple applications. So there's another potential argument of having multple instances on the same machine running different applications (whatever application reason -- not all applications are pure JEE and all of its portability and container agnostic glory), but clustered just to simplify central management.
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