I will add an example. My goal is to leave the framework as lean as possible and add functionality by using various extensions. I can see the security functionality added as a µ Extension enabling a set of filters and specific web pages; ala Devise. The Extensions allow you to create a completely independent "mini-site" and inject its functions into the main app. With extensions you can add your own filters, routes, controllers and specialized dynamic content/web-UI. Please have a look at: https://github.com/florinpatrascu/micro-extensions/tree/mast..., as an example implementation of the aspects mentioned above. I see something like Devise being built this way.