The plugins in the application are mostly distributed as internal gems, but then there has to be some parts in the core application (controllers, models, etc.) that call these plugin classes explicitly. To me this is not a typical plugin architecture, just some way to organize the code in gems and define a unified interface for its configuration. Besides, it doesn't follow the Open-Closed Principle, because, as far as I can tell, the system has to know about the plugins in order to use them, so the only way to actually write a plugin is modifying the code in the core system to make it aware of the new options/features.