EJB's - when to use Remote and/or local interfaces?

Question

I'm very new to Java EE and I'm trying to understand the concept of Local interfaces and Remote interfaces. I've been told that one of the big advantages of Java EE is that it is easy to scale (which I believe means you can deploy different components on different servers). Is that where Remote and Local interfaces come in? Are you supposed to use Remote interfaces if you expect your application to have different components on different servers? And use Local interfaces if your application is only going to reside on one server?

If my assumptions above are correct, how would you go about choosing whether to use Local or Remote interfaces for a new application, where your unsure of what the volume of traffic would be? Start off by using Local interfaces, and gradually upgrade to Remote interfaces where applicable?

Thanks for any clarification and suggestions.

Answer 1

While I agree with most of what is written above, I would like to refine the "how to start" ideas a bit.

My suggestion to you is to never ever program directly to EJB interfaces within your code. Always use a regular, business-oriented interface, program to it (meaning, have your code call methods on the business-oriented interface) and provide the EJB "glue" code as a pluggable implementation. Your program should be focused on business logic, and not on implementation details such as EJB.

That way, you can easily switch between remote and local implementations - and if you use an IoC container such as Spring, you can do it by means of configuration only.

A special note about switching from local to remote: note that there are a few semantic differences between the two. For example, calling an EJB method via its "remote interface" results in arguments being passed by-value, while calling through the "local interface" results in arguments being passed by-reference. This is a major difference; so if you "start with local", make sure that you design your system in a way that it takes "remote" semantics into consideration as well.

If your design relies on EJB methods changing passed-in objects, then it would be tricky for you to "switch to remote" later; perhaps even impossible.

Good luck.