Application Server deployment

Question

I have been having this heated debate on whether to run multiple applications on the same instance of the application server or on different instances. I am in favor of running them on different applications servers, what I would ask you guys is that if there is an industry standard in this respect and what is the most common configuration used in such a scenario.

Answer 1

OK here I am not talking about the ear files. SInce
I would suppose that one application can have more
than one ear files. In my experience this is not true - an EAR file is the deployment unit for an enterprise application.
WHere I am trying to go is
1) How can I guarantee service levels between
applications? Would you do me a favor and define "service levels" for me?
2) How can I prevent one errant application from
affecting other applications? The app server is supposed to handle this.
3) How can I reconcile differing needs for operating
system patch levels between applications?...and so
on. The app server is supposed to insulate your applications from the OS.
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I think that
1) Errant applications can negatively impact other
running applications. The app server is supposed to rigidly separate applications from each other. This isn't to say that an errant application couldn't negatively impact the app server itself though. The last place I worked had a policy where the infrastructure people would perform something called "Platform Integration" tests (or something like that) where they verified that the new app played well with everyone else. We had something like a dozen fairly heavy applications in production running from a cluster of app servers there with no significant troubles (that I heard about, anyway)
2) There is no way to guarantee performance service
levels for different applications. Oh - do you mean uptime and/or application perfomance? True enough - the platform would be uniform (more or less) between all applications.
3) All applications must work with the same WebLogic
version, JVM version, and associated patch levels. This is true. Though to me having several different servers all at different patch levels and JVM versions sounds like a worse problem.
4) All applications must work with the same
operating-system version and patch level. The app server should insulate the applications from the OS. If you are creating OS dependencies within a J2EE application you have other issues.
5) The potential for upgrade deadlocks between
applications (i.e., an OS or JVM patch that one
application requires ends up breaking another
application). See #3
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6) JVMs tend not to scale well past four processor
configurations, which limits the viability of the
single-JVM model on larger machines.
I wasn't aware of this, this might be an issue.
But despite these points I fail to convince my co
worker as to why applications should run on their own
instances. If you could add to any more pros and
cons.Well, to me the pros are: Uniform platform providing fewer maintenance issues. I don't really see any cons though.