Hi Dominik,In terms of writing an API Backend there are a few options I know of which have recently surfaced:
- Rails API - https://github.com/spastorino/rails-api - there is also a screen cast on this here: http://railscasts.com/episodes/348-the-rails-api-gem
- Grape - https://github.com/intridea/grape
- RocketPants - https://github.com/filtersquad/rocket_pants
- Custom responders - Tim wrote a blog post on what he did for DecafSucks here: http://icelab.com.au/articles/decaf-sucks-launch-countdown-finishing-the-api/
There were other things mentioned but I'll let the other guys chime in.