title: Rails mountable engine: how should apps set configuration variables?
body: <p>I have a mountable engine called <code>Blog</code> that apps can use.</p>
  
  <p>What's the best way to allow apps that use the engine to set a configuration variable like <code>site_name</code> (so that the engine can display it in the design)?</p>
  
  <p><strong>Update:</strong></p>
  
  <p>I've seen some gems create a 'config/initializers/gem_name.rb' file. Is there any specification on how to:</p>
  
  <ol>
  <li>Create a file like that on the engine's side</li>
  <li>Copy it into the app's side</li>
  <li>How to access those set variables on the engine's side?</li>
  </ol>
  
  <p>I tried creating <code>Blog.site_name = "My Site"</code> in the app's <code>config/initializers/blog.rb</code> file but get an Undefined method error.</p>