I have multiple instances of NHibernate running on the same server. This seems somewhat wasteful. I was wondering if anyone had a solution where they ran one shared instance of NHibernate (similar to Java EJBs in an app server) shared by the various app instances.

You should share the SessionFactory (by making it singleton).

Hey thanks for the reply Pixel. I did make the SessionFactory a singlton. Problem is that my system is spread out over several program instances, hardware drivers, web-app, HL7 interface etc. All seperate executables.

I was hoping there was some way to share the NHibernate instance (and caches) between multiple apps on the same server similar to how a Java application server can share EJBs over multiple apps within the app server.

what you needed is an entprise server like com+ or ejb , its really a huge project, and i think you can wrap NHibernate with com+ to profer the function you need

I would consider running it within a windows service and using .net remoting to communicate with it. A lot of people use this technique for custom session state servers etc.

Sorry to drop so late into this thread, but kerry, could you explain a bit better this Windows Service/Remoting approach.

Sounds very interesting...

Sorry for the late reply, just saw your post.

Basically you write a standard windows service using VS.NET that runs a single thread. You have one function call that is exposed via remoting that returns your ISession object from your factory.

Your client application then runs and when it needs an ISession makes a remote call to the windows service process to get it.

This is a pretty heavy handed approach however, it has greater overheads in both processing time (marshalling costs etc) and ongoing system maintainability (another component, another point of failure).

A singleton design at least only initialises once per process, and you don't get ongoing processing overheads.