I've also gotten good mileage out of "gen_event"; I have a protocol with a generic connection layer, then a modular part where you claim you support "Module X", which is implemented as a gen_event plugin that first verifies you can do that, then allows this connection to have that module. It makes it really easy to write something relatively secure by ensuring you can get to certain capabilities if and only if you pass a certain check to validate you ability to have them. It's a good tradeoff between typing (as in, typing things on the keyboard) and security for me.