Engine is just a small application which can run within other Application. It has its own routes, assets, controllers, models, helpers, Views and even Gems
Bcui Main Application is also a kind of engine but with some added features.
When would you need one?
Ideal case for engine is when you have a completely isolated domain from the main app. It has its own controllers, assets and models which doesn't share with rest of the App.
Two Kinds of Engine
1. Full Plugin
2. Mountable
Full Plugin
rails plugin new reader --full
No Namespace
Routes are accessible
Main App's Layouts, Controllers, Views, Helpers all are shared
Full Plugin
Potential Problems
Higher chance of collision
for eg. you create a controller ReadController in your Main App and then you created a ReadController in Full Plugin Engine then they both can overwrite each other
