Scaling Complex Rails Applications

After ten years of development Rails is a mature framework, and a recurring theme at conferences is how to build and maintain Rails applications that are increasingly complex. Rails achieved early success as an opinionated framework that emphasizes MVC and convention over configuration, but as developers have tackled ever more ambitious projects there has been divergence as to how to structure them.

What is generally agreed is that business logic should reside in models, away from controllers, views and API implementations. This has led to ‘fat’ models and discussion of strategies to manage them such as this blog post from CodeClimate titled ‘7 Patterns to Refactor Fat ActiveRecord Models‘. An important point often missed when first learning Rails is that app/models is not intended exclusively for classes persisted with ActiveRecord as it may include other domain objects. In addition to domain objects other popular patterns used to structure code include form objects, presenters, decorators and service objects. Presenters and decorators intermediate between controllers and views, whereas service objects intermediate between controllers and models.

Presenters and decorators are probably less controversial in the Rails community and have done much to replace view helpers. Service objects, however, have been disparaged by DHH in favor of concerns, but many developers with a traditional OOP background are unconvinced by his approach. One example project that uses service objects is GitHub itself. More recently Collective Idea have published an ‘interactor’ gem that elaborates this approach.

These strategies all result in more classes, and probably the first step in managing a codebase with many classes is to introduce namespaces. The Rails class loader automatically searches sub-folders matching namespaces so source files can be arranged similarly to the namespace hierarchy. Given most developers nowadays navigate code using global search or ‘jump anywhere’ features in their text editor the neater project directory layout is of limited benefit, but namespacing can reduce name conflicts with libraries or between different areas of an application.

The next step beyond namespacing, suggested by Stephan Hagemann is to divide a project into internal engines or gems, an approach labelled Component Based Rails Architecture. An example project is provided at https://github.com/shageman/the_next_big_thing Each component has its own test suite so the scope of regression testing of any feature change is more limited. If a component proves reusable it can be split out from the original project altogether.

Beyond a component based architecture the final step to scaling a complex application is to break it apart into separate services interacting with an internal API, an approach referred to as Service Oriented Architecture (SOA). I will defer comment on the benefits and challenges of such an approach to a later post.

See also

• Railsconf ’14 presentation ‘Concerns, Decorators, Presenters, Service Objects, Helpers, Help Me Decide!’
• Railscast on Form objects
• Railscast on Presenters
• Railscast on Service Objects