[andoriyu]

> I dunno here, is Rails a server application library because I can progressively integrate the different components of its total API, e.g. first use ActiveRecord, then adopt rails-api for the frontend, then adopt ActiveView and Turbolinks for the end-user frontend? Or is there different idea of framework at work here?

I think you misunderstood parent comment. Progressively integrate here means: you have a web page (facebook.com), you want to replace a part of it with react (chat function), you rewrite just that portion of the webpage in react.

You can't get a Sinatra application and replace just one route with rails. Well, you can use ActiveController, which is just an opinionated wrapper around Rack with a lot of sugar, but that wouldn't be rails and you won't get any rails benefits of doing that.