For a college project i'm thinking of implementing the business layer in Erlang and then accessing it via multiple front-ends using REST. I would like to avail of OTP features like distributed applications, etc.
My question is how do I expose gen_server calls/casts to other applications? Obviously I could make RPC calls via language specific "bridges" like OTP.net or JInterface, but I want a consistent way to access it like REST.
There is valuable approach to design gen_server calls/casts in flavor of REST if possible. You can use messages as
{get, Resource}
{set, Resource, Value} % aka PUT
{delete, Resource}
{add, Resource, Value} % aka POST (possible another names are append, modify or similar)
Then its mapping is easy. You can make some transformation URI->RESOURCE or use identity. For most of your application this should be wort approach and special cases you should handle specially. You can think there will be big margin, where you can't use this approach, but this should be mostly premature optimization.
I did something similar for my job and found it best to use REST to expose the business layer because even Legacy languages such as SoftwareAG's Natural is able to access it. The best mechanism that I have found in Erlang is to use Mochiweb.
You can find more information about using it from the screencast located at Erlang In Practice Screencast. Episode 6 is particularly helpful but all of them are excellent.
A resource to walk you through installation is How To Quickly Set Up Ubuntu 8.04 loaded with Erlang, Mochiweb and Nginx and Migrating a native Erlang interface to RESTful Mochiweb (with a bit of TDD) provides a good start if you don't find the screencasts to your liking.
As already mentioned Yaws or Mochiweb are a great way to go but if you'd like a dead simple way to get your RESTful API done quickly and correctly then use Webmachine. It's a layer on top of Mochiweb that implements proper HTTP behavior based on Alan Dean's amazing HTTP flow diagram and makes it easy to get REST done right.
I'm using it right now to expose a REST API as well as handle a COMET application and it's been pretty easy to do, even for an Erlang newbie such as myself.
Do you really mean a RESTful interface or RPC over HTTP? Building a RESTful interface on top of an existing layer is more work than just exposing existing methods via HTTP.
I'd suggest to use mochiweb or yaws to implement a (generic) rpc layer.
Just an update, Webmachine has moved to bitbucket: new link to Webmachine
The HTTP flow diagram link is dead. The original version and a updated version created in collaboration between Alan Dean and Justin Sheehy ist also hosted in the Webmachine project: link to latest version of the HTTP Diagramm.