Ilija, probably you are already aware of this, but just in case (perhaps you'll find this interesting):

How To Provide A Web API

Best regards

-Ahum!- Sorry to "bother" you with this, but here's another one:
How to Add an API to your Web Service
I hope this could be useful to you, even though in this particular case I think you are already aware of some aspects cited in the article.

Very best regards