[scanr]

Thanks for responding! And I'll update my response to the backwards compatibility to say that I see the value in having a closed set of clients that you need to test for backwards compatibility.

That said, these are some of the things that I've done to help with backwards compatibility through only the serverside API:

* Track production requests and use them as test cases

* Build a large suite of test cases against the API

* Build the API in a statically typed language (yeah, I know, contentious, I love Rails but there's something about an externally facing API that makes me want to use a statically typed language).

* And then the ultimate -- build the API as an app that talks back to the business logic... essentially, it's your 'client library' but deployed on the server between the actual code and the client. Then, never change the part of it that faces the client, only the mappings on the the real business logic.