"Rails is half of your application"

Only a week ago I was still standing strong behind the "Rails is not your application" statement. It felt natural to me and I haven't bothered to think deeper about it. Since then I discussed this topic with several people and I read Domain Logic in Rails by Reed Law which mentions DHH's and Uncle Bob's differing approaches to building applications. A torrent of thoughts hit me then and I realized the truth lays somewhere in between. I started analyzing the subject deeper and this article is my attempt to explain what your application actually is and where it stands in relation to "Rails application" label.

Let's first find out what different types of code we put in the application. What does a Rails app directory contain? When you run rails new you get several directories created for you - your Rails app structure. Three most interesting directories are app , lib and config . You put your controllers to app/controller , views to app/views . You configure your routes in config/routes.rb . The business logic goes to app/models , app/services and to some other directories under app or lib . Many people still put it into controllers though.

We can group entities contained in the mentioned directories in two groups: things that can be used outside of Rails application, and things that cannot. Your domain logic, services and models fall into the first group. Controllers, views, helpers and routes fall into the second group. And no, putting entities from the second group into a Rails engine (and making a gem out of it) is not the way of re-use I'm talking about - this engine can't be used in let's say an iOS app, right? Sounds obvious, but bear with me.

What this means is that you can reuse business logic (backend) but you can't reuse the UI (frontend). You don't actually want to reuse the frontend. How likely it is that you will need your routes, controllers and views in another app? Unlikely. You just build another UI, be it a mobile, html5/js, or a desktop app. So Rails is your frontend. In fact it's a business logic delivery mechanism. You have chosen Rails to be on the frontline, to serve the most user facing duty of your business.

Let's say you actually want to build another application to use the existing business logic. We know it's possible, so you extract the logic into a gem or a separate ruby application that is an API (Sinatra or rails-api). When extracting your business logic into an API you still need to access it - thin adapter(s) used from the controllers can do the job. Now your original app becomes only the UI. A "UI application", "frontend application" or just "one of the applications using your API/gem". As a matter of fact the code that is now left in app/* means nothing outside of Rails app. Think about it. Also, what stands after run in your config.ru file? Rails::Application . It's your own flavor of it (you inherit from it) but it's still a Rails::Application . When you start a server Rails boots and your code in app/* and config/* just fills some slots in the Rails app. Not the other way around! Your code doesn't use Rails - Rails uses your code. Can we say then that Rails is your application? It may be more fair to say that Rails is your application runtime. It runs your application and provides many facilities to it. Anyway, if you don't like that idea then there are some nice new Ruby web frameworks emerging (Lotus, Pakyow).

We can look at it from a slightly different perspective. In reality most Rails apps serve both purposes. They provide the UI and domain logic at the same time, keeping both under the same source code tree. The number of Rails apps that don't implement any business logic themselves is pretty damn low. Regardless of where the business logic technically sits, we have the Rails dependent part (UI) and the Rails independent part (logic). Can we say that Rails dependent part, together with Rails itself is one half of your application and the logic part the second half? I think we can. Some developers believe that only the part of the code that is below the controllers (services and models) is "the application". Is it really the case? What's worth your "application" without the UI? What do you think?

In the next post I'm going to find out where the exact line between those parts goes (hint hint: controller) and how we can pull this line to our side to maximize benefits (where should the authorization go? where should the loading/building of models happen? Why ActiveSupport can invisibly make your domain logic Rails dependent?)