AssetUrlHelper

This module provides methods for generating asset paths and urls.

image_path("rails.png")
# => "/assets/rails.png"

image_url("rails.png")
# => "http://www.example.com/assets/rails.png"

Using asset hosts

By default, Rails links to these assets on the current host in the public folder, but you can direct Rails to link to assets from a dedicated asset server by setting ActionController::Base.asset_host in the application configuration, typically in config/environments/production.rb. For example, you’d define assets.example.com to be your asset host this way, inside the configure block of your environment-specific configuration files or config/application.rb:

config.action_controller.asset_host = "assets.example.com"

Helpers take that into account:

image_tag("rails.png")
# => <img alt="Rails" src="http://assets.example.com/assets/rails.png" />
stylesheet_link_tag("application")
# => <link href="http://assets.example.com/assets/application.css" media="screen" rel="stylesheet" />

Browsers typically open at most two simultaneous connections to a single host, which means your assets often have to wait for other assets to finish downloading. You can alleviate this by using a %d wildcard in the asset_host. For example, “assets%d.example.com”. If that wildcard is present Rails distributes asset requests among the corresponding four hosts “assets0.example.com”, …, “assets3.example.com”. With this trick browsers will open eight simultaneous connections rather than two.

image_tag("rails.png")
# => <img alt="Rails" src="http://assets0.example.com/assets/rails.png" />
stylesheet_link_tag("application")
# => <link href="http://assets2.example.com/assets/application.css" media="screen" rel="stylesheet" />

To do this, you can either setup four actual hosts, or you can use wildcard DNS to CNAME the wildcard to a single asset host. You can read more about setting up your DNS CNAME records from your ISP.

Note: This is purely a browser performance optimization and is not meant for server load balancing. See http://www.die.net/musings/page_load_time/ for background.

Alternatively, you can exert more control over the asset host by setting asset_host to a proc like this:

ActionController::Base.asset_host = Proc.new { |source|
  "http://assets#{Digest::MD5.hexdigest(source).to_i(16) % 2 + 1}.example.com"
}
image_tag("rails.png")
# => <img alt="Rails" src="http://assets1.example.com/assets/rails.png" />
stylesheet_link_tag("application")
# => <link href="http://assets2.example.com/assets/application.css" media="screen" rel="stylesheet" />

The example above generates “http://assets1.example.com” and “http://assets2.example.com”. This option is useful for example if you need fewer/more than four hosts, custom host names, etc.

As you see the proc takes a source parameter. That’s a string with the absolute path of the asset, for example “/assets/rails.png”.

 ActionController::Base.asset_host = Proc.new { |source|
   if source.ends_with?('.css')
     "http://stylesheets.example.com"
   else
     "http://assets.example.com"
   end
 }
image_tag("rails.png")
# => <img alt="Rails" src="http://assets.example.com/assets/rails.png" />
stylesheet_link_tag("application")
# => <link href="http://stylesheets.example.com/assets/application.css" media="screen" rel="stylesheet" />

Alternatively you may ask for a second parameter request. That one is particularly useful for serving assets from an SSL-protected page. The example proc below disables asset hosting for HTTPS connections, while still sending assets for plain HTTP requests from asset hosts. If you don’t have SSL certificates for each of the asset hosts this technique allows you to avoid warnings in the client about mixed media. Note that the request parameter might not be supplied, e.g. when the assets are precompiled via a Rake task. Make sure to use a Proc instead of a lambda, since a Proc allows missing parameters and sets them to nil.

config.action_controller.asset_host = Proc.new { |source, request|
  if request && request.ssl?
    "#{request.protocol}#{request.host_with_port}"
  else
    "#{request.protocol}assets.example.com"
  end
}

You can also implement a custom asset host object that responds to call and takes either one or two parameters just like the proc.

config.action_controller.asset_host = AssetHostingWithMinimumSsl.new(
  "http://asset%d.example.com", "https://asset1.example.com"
)