At TaskRabbit, we have gone through a few iterations on how we make our app(s). In the beginning, there was the monolithic Rails app in the standard way with 100+ models and their many corresponding controllers and views. Then we moved to several apps with their own logic and often using the big one via API. Our newest project is a single “app” made up of several Rails engines. We have found that this strikes a great balance between the (initial) straightforwardness of the single Rails app and the modularity of the more service-oriented architecture.