Can someone please tell me what an
'application server' is/does ?
It's a term that has been nobbled by the marketroids, so the practical
meaning is "Almost anything (but we sell one, and would you please buy
it)". If you see the term used twice, it's likely to have three
different meanings.
The core of it is "a server that runs applications" where an
"application" is a piece of "business logic" implemented as code, with
the absolute minimum of extra effort that isn't the business logic
itself. The "server" represents all the code necessary that isn't
custom written for the application, so as to allow the application to
run in a useful manner. So for a typical case, you implement the
application as an EJB and the App Server allows this to operate as a web
service, either responding to SOAP or even a more traditional HTTP/HTML
world.
It's generally a web thing. They existed before, but weren't called
application servers. They also rely on some sort of component-based
software, so they're generally based around EJB these days but could
equally use COM or even Corba. There's no requirement for an App Server
to use Java, but it's a long search to find one that doesn't.
As to what it _does_, then that can be pretty minimal or very complex.
As a minimum they do interface marshalling between the SOAP and the
program API (for your local choices of either interface). At a complex
level they manage object lifetimes, correct versioning, access control,
load balancing across machine locations etc.
