I mean they can all be. It's more where is it running.. Ruby has gem and rails has engine support, you could build most of the code in gems and mount them as

I mean they can all be. It's more where is it running.. Ruby has gem and rails has engine support, you could build most of the code in gems and mount them as engines so their url's and paths would be isolated. Or you could just be super specific about how you build things from day 1. A recent app I worked on had no logic in models or controllers, but everything was in name-spaced active_jobs. Super easy to find what you're looking for and DRY was not something this team loved. Simple and contained was way more important.