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    <p>I have noticed the following: </p> <p><strong>Full Engine</strong></p> <p>With a full engine, the parent application inherits the routes from the engine. It is not necessary to specify anything in <code>parent_app/config/routes.rb</code>. Specifying the gem in Gemfile is enough for the parent app to inherit the models, routes etc. The engine routes are specified as: </p> <pre><code># my_engine/config/routes.rb Rails.application.routes.draw do # whatever end </code></pre> <p>No namespacing of models, controllers, etc. These are immediately accessible to the parent application. </p> <p><strong>Mountable Engine</strong></p> <p>The engine's namespace is isolated by default:</p> <pre><code># my_engine/lib/my_engine/engine.rb module MyEngine class Engine &lt; Rails::Engine isolate_namespace MyEngine end end </code></pre> <p>With a mountable engine, the routes are namespaced and the parent app can bundle this functionality under a single route: </p> <pre><code># 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 =&gt; "/engine", :as =&gt; "namespaced" end </code></pre> <p>Models, controllers, etc are isolated from the parent application - although helpers can be shared easily. </p> <p>These are the main differences I have spotted. Perhaps there are others? I have asked over <a href="http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/6a4691f62d0f1420">here</a>, but have yet to receive a response.</p> <p>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.</p> <p>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:</p> <pre><code>mount Cornerstone::Engine =&gt; "/cornerstone", :as =&gt; "help" </code></pre> <p>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 <a href="http://www.astjohn.ca/2011/08/06/rails-31-engines-mountable-or-full-part-1/">here</a> Cheers!</p>
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    1. PORails 3.1: Engine vs. Mountable App
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    1. COCan a mountable engine ever be routed/mounted at the root of the parent app?
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    2. CO@JustinM you could try `mount MyEngine::Engine => "/"`. It works for resources, maybe that's also the case for engines.
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    3. CO@astjohn Great sum-up of your blogs. But wouldn't it be the other way round? Would a Full Engine be "incomplete" and need the parent app to work, whereas the Mountable Engine can work stand-alone, since it is "isolated" from the parent app?
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