I have noticed the following:
Full Engine
With a full engine, the parent application inherits the routes from the engine. It is not necessary to specify anything in parent_app/config/routes.rb. Specifying the gem in Gemfile is enough for the parent app to inherit the models, routes etc. The engine routes are specified as:
# my_engine/config/routes.rb
Rails.application.routes.draw do
# whatever
end
No namespacing of models, controllers, etc. These are immediately
accessible to the parent application.
Mountable Engine
The engine’s namespace is isolated by default:
# my_engine/lib/my_engine/engine.rb
module MyEngine
class Engine < Rails::Engine
isolate_namespace MyEngine
end
end
With a mountable engine, the routes are namespaced and the parent app can bundle this functionality under a single route:
# my_engine/config/routes.rb
MyEngine::Engine.routes.draw do
#whatever
end
# parent_app/config/routes.rb
ParentApp::Application.routes.draw do
mount MyEngine::Engine => "/engine", :as => "namespaced"
end
Models, controllers, etc are isolated from the parent application – although helpers can be shared easily.
These are the main differences I have spotted. Perhaps there are others? I have asked over here, but have yet to receive a response.
My impression is that since a full engine does not isolate itself from the parent application, it is best used as a standalone application adjacent to the parent app. I believe name clashes could occur.
A mountable engine could be used in situations where you want to avoid name conflicts and bundle the engine under one specific route in the parent application. For example, I am working on building my first engine designed for customer service. The parent application could bundle it’s functionality under a single route such as:
mount Cornerstone::Engine => "/cornerstone", :as => "help"
If I’m way off in my assumptions, someone please let me know and I’ll fix this response. I have made a small article about the subject here Cheers!