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Modular Monolith

Context and Problem Statement​

Large applications need clear boundaries to avoid turning into a ball of mud. However, Ruby does not provide a good solution to enforcing boundaries between code.

Resources:

Proposed solution​

Rebuild the application in a modular way:

  1. Easier to understand, less cognitive load: It’s easier to see where the bounded contexts are drawn.
  2. Easier and faster to test: You only need to test the engines affected and the ones up your dependency chain.
  3. Less coupling, easier to make changes: If you have your boundaries correctly defined it will be easier to swap out components.
  4. Less merge conflicts: Because a change is more isolated to a specific domain the changes will be confined to that module only.

Considered Options​

There are two different problems to solve with complementary tools:

  1. Modules loading: how these modules are loaded by the Rails application
  2. Modules boundaries: how to set modules dependencies, strict boundaries and a clear public/private APIs

Modules loading​

  1. Split app into multiple directories added to Rails autoload path
  2. Create Rails Engines

Modules boundaries​

  1. Multiple gems. (example)
  2. Packwerk (example)
  3. Packs Packwerk + extensions

Decision Outcome​

Chosen option: Rails Engine + Packs rails

Module loading​

Adding folders to the autoload path is simpler but Rails Engines are the native solution to create isolated modules, additionally it provides the following features:

  • module gemspec (isolate gems)
  • module isolation: isolate controllers, models, etc

Modules boundaries​

Gems are the native Ruby tool to package libraries but they require extra tooling and deployment. This would force us to generate a monorepo/multi-repo for all of them and updating our CI tools, worsening development experience. Packwerk is a mature tool from Shopify, also creators of the Zeitwerk Ruby class loader built for monolithic applications. It can be used with any Ruby project and can be easily integrated to a Rails project. Packs extend Zeitwerk and defines a convention for modules and solves the autoloding through Engines and RSpec integration. Additionally it has the following checks:

  • A privacy checker that ensures other packages are using your package's public API
  • A visibility checker that allows packages to be private except to an explicit group of other packages.
  • An experimental architecture checker that allows packages to specify their "layer" and requires that each layer only communicate with layers below it.