Structuring Rails applications: the Modular Monorepo Monolith

Root Insurance runs their application as a monolithic Rails application – but they've modularized it inside its repository. Here is their approach in summary:

Strategy

• Keep all code in a single repository (monorepo)
• Have a Rails Engine for each logical component instead of writing a single big Rails Application
• Build database-independent components as gems
• Thus: gems/ and engines/ directories instead of app/
• Define a dependency graph of components. It should have few edges.
• Gems and Engines can be extracted easier once necessary.

Advantages over a classic Rails monolith

• By (automatically) only requiring expected/allowed dependencies in tests, components cannot accidentally use (i.e. depend on) code they're not allowed to
• Onboarding is easier because of the clear separation and dependencies. No need to know the whole application.
• Faster test runs, because only dependent components need to run.
• A growing team can still keep up the development pace because developers concentrate on specific components. Merge requests target a specific component.

Advantages over a distributed application setup (multiple repositories, multiple applications)

• Everything is still deployed at once. No need to orchestrate branch merges and deploys across N repos.

Other resources

• https://www.airpair.com/ruby-on-rails/posts/ruby-on-rails-the-modular-way
• https://engineering.gusto.com/building-toward-a-modular-monolith
• https://railsconf.com/2020/video/vladimir-dementyev-between-monoliths-and-microservices