To build a Rails application that will be an API server, you can start with a more limited subset of Rails in Rails 5.
To generate a new Rails API app:
rails new my_api --api
--api does is to remove functionality that is not needed when building an API. This includes sessions, cookies, assets, and anything that makes Rails work on a browser.
It will also configure the generators so that they don't generate views, helpers, and assets when generating a new resource.
When you compare the
ApplicationController on a web app versus an API app, you will see that the web version extends from
ActionController::Base, whereas the API version extends from
ActionController::API, which includes a much smaller subset of functionality.