Rails 3.1: Engine vs. Mountable App

Question

Can someone help me understand the differences between a Rails Engine and a Mountable app? In Rails 3.1, you can create either one with the \"rails new plugin _

Answer 1

i was wondering the same and, hence, ended up here. it seems to me that the earlier answers basically cover the question, but i thought the following might help as well:

# generate plugins (NOTE: using same name each time to minimize differences)
# -----------------------------------------------------------------------------

$ rails plugin new test-plugin -T
$ mv test-plugin{,.01}

$ rails plugin new test-plugin -T --mountable
$ mv test-plugin{,.02}

$ rails plugin new test-plugin -T --full
$ mv test-plugin{,.03}

$ rails plugin new test-plugin -T --full --mountable
$ mv test-plugin{,.04}




# compare "stock" (01) with "mountable" (02)
# -----------------------------------------------------------------------------

$ diff -r test-plugin.01 test-plugin.02

Only in test-plugin.02: app
Only in test-plugin.02: config
Only in test-plugin.02/lib/test-plugin: engine.rb
diff -r test-plugin.01/lib/test-plugin.rb test-plugin.02/lib/test-plugin.rb
0a1,2
> require "test-plugin/engine"
> 
Only in test-plugin.02: script
diff -r test-plugin.01/test-plugin.gemspec test-plugin.02/test-plugin.gemspec
18a19
>   # s.add_dependency "jquery-rails"




# compare "stock" (01) with "full" (03)
# -----------------------------------------------------------------------------

$ diff -r test-plugin.01 test-plugin.03
Only in test-plugin.03: app
Only in test-plugin.03: config
Only in test-plugin.03/lib/test-plugin: engine.rb
diff -r test-plugin.01/lib/test-plugin.rb test-plugin.03/lib/test-plugin.rb
0a1,2
> require "test-plugin/engine"
> 
Only in test-plugin.03: script
diff -r test-plugin.01/test-plugin.gemspec test-plugin.03/test-plugin.gemspec
18a19
>   # s.add_dependency "jquery-rails"




# compare "mountable" (02) with "full" (03)
# -----------------------------------------------------------------------------

$ diff -r test-plugin.02 test-plugin.03

Only in test-plugin.03/app/assets/javascripts/test-plugin: .gitkeep
Only in test-plugin.02/app/assets/javascripts/test-plugin: application.js
Only in test-plugin.03/app/assets/stylesheets/test-plugin: .gitkeep
Only in test-plugin.02/app/assets/stylesheets/test-plugin: application.css
Only in test-plugin.03/app/controllers: .gitkeep
Only in test-plugin.02/app/controllers: test-plugin
Only in test-plugin.03/app/helpers: .gitkeep
Only in test-plugin.02/app/helpers: test-plugin
Only in test-plugin.03/app/mailers: .gitkeep
Only in test-plugin.03/app/models: .gitkeep
Only in test-plugin.03/app/views: .gitkeep
Only in test-plugin.02/app/views: layouts
diff -r test-plugin.02/config/routes.rb test-plugin.03/config/routes.rb
1c1
< TestPlugin::Engine.routes.draw do
---
> Rails.application.routes.draw do
diff -r test-plugin.02/lib/test-plugin/engine.rb test-plugin.03/lib/test-plugin/engine.rb
3d2
<     isolate_namespace TestPlugin




# compare "mountable" (02) with "full & mountable" (04)
# -----------------------------------------------------------------------------

$ diff -r test-plugin.02 test-plugin.04

<no difference>




# compare "full" (03) with "full & mountable" (04)
# -----------------------------------------------------------------------------

$ diff -r test-plugin.03 test-plugin.04

Only in test-plugin.03/app/assets/javascripts/test-plugin: .gitkeep
Only in test-plugin.04/app/assets/javascripts/test-plugin: application.js
Only in test-plugin.03/app/assets/stylesheets/test-plugin: .gitkeep
Only in test-plugin.04/app/assets/stylesheets/test-plugin: application.css
Only in test-plugin.03/app/controllers: .gitkeep
Only in test-plugin.04/app/controllers: test-plugin
Only in test-plugin.03/app/helpers: .gitkeep
Only in test-plugin.04/app/helpers: test-plugin
Only in test-plugin.03/app/mailers: .gitkeep
Only in test-plugin.03/app/models: .gitkeep
Only in test-plugin.03/app/views: .gitkeep
Only in test-plugin.04/app/views: layouts
diff -r test-plugin.03/config/routes.rb test-plugin.04/config/routes.rb
1c1
< Rails.application.routes.draw do
---
> TestPlugin::Engine.routes.draw do
diff -r test-plugin.03/lib/test-plugin/engine.rb test-plugin.04/lib/test-plugin/engine.rb
2a3
>     isolate_namespace TestPlugin

of particular interest (to me) is the fact that there is no difference between

rails plugin new test-plugin -T --mountable

and

rails plugin new test-plugin -T --full --mountable

Answer 2

Both options will generate an engine. The difference is that --mountable will create the engine in an isolated namespace, whereas --full will create an engine that shares the namespace of the main app.

The differences will be manifested in 3 ways:

1) The engine class file will call isolate_namespace:

lib/my_full_engine/engine.rb:

module MyFullEngine
  class Engine < Rails::Engine
  end
end

lib/my_mountable_engine/engine.rb:

module MyMountableEngine
  class Engine < Rails::Engine
    isolate_namespace MyMountableEngine # --mountable option inserted this line
  end
end

2) The engine's config/routes.rb file will be namespaced:

Full engine:

Rails.application.routes.draw do
end

Mounted engine:

MyMountableEngine::Engine.routes.draw do
end

3) The file structure for controllers, helpers, views, and assets will be namespaced:

create app/controllers/my_mountable_engine/application_controller.rb
create app/helpers/my_mountable_engine/application_helper.rb
create app/mailers create app/models
create app/views/layouts/my_mountable_engine/application.html.erb
create app/assets/images/my_mountable_engine
create app/assets/stylesheets/my_mountable_engine/application.css
create app/assets/javascripts/my_mountable_engine/application.js
create config/routes.rb create lib/my_mountable_engine.rb
create lib/tasks/my_mountable_engine.rake
create lib/my_mountable_engine/version.rb
create lib/my_mountable_engine/engine.rb

---

Explanation

The use case for the --full option seems to be very limited. Personally I can't think of any good reason why you'd want to separate your code into an engine without isolating the namespace as well- It would essentially just give you two tightly coupled applications sharing identical file structures and all the conflicts and code leakage that entails.

Every piece of documentation I've seen demonstrates the --mountable option, and indeed the current edge guide strongly encourages you to include isolate namespace- which is the same as saying use --mountable over --full.

Finally there's terminology confusion: Unfortunately rails plugin -h shows the following descriptions:

[--full] # Generate a rails engine with bundled Rails application for testing
[--mountable] # Generate mountable isolated application

This gives the impression that you use --full to create an "engine" and --mountable to create something else called a "mountable application", when in fact they're both engines - one namespaced and one not. That's bound to lead to confusion as users looking to create an engine will likely assume that --full is the more relevant option.

Conclusion

• rails plugin new something --full = Engine in your app's namespace. (Why would you?)
• rails plugin new something --mountable = Engine with it's own namespace. (Awesome)

---

References

• http://edgeguides.rubyonrails.org/engines.html
• http://api.rubyonrails.org/classes/Rails/Engine.html
• http://railscasts.com/episodes/277-mountable-engines
• https://github.com/rails/rails/pull/6499

Answer 3

The difference, I believe, is that a mountable app's are isolated from the host app, so they can't share classes - models, helper etc. This is because a Mountable app is a Rack endpoint (i.e a Rack app in its own right).

Disclaimer: I have, like most, only just started toying with Rails 3.1.

Answer 4

My understanding of the difference is that engines are like plugins, and add functionality to existing applications. While mountable apps are essentially an application, and can stand alone.

So if you want to be able to run it by itself or within another application you would make a mountable app. If you intend for it to be an addition to existing applications, but not run by itself you would make it an engine.

Answer 5

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!