Testing Strategy for a Modular Rails Application

This is an adapted excerpt from Chapter 13 of Modular Rails: Architecture for the Long Game , my book on building maintainable Ruby on Rails applications using Rails Engines. Your test suite takes 40 minutes. Mine takes 4. That is not because I write fewer tests. It is because when I change billing code, I only run billing tests. When I change notification code, I only run notification tests. The…