Use Rails assets from sources in the cloud
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README.md

cloud_assets

cloud_assets enables a Rails app to make transparent use of assets on a remote server in an alternative technology.

Principles and modes of operation

You can avoid incorporating static assets in your Rails app directly, if they exist somewhere else on the internet. With cloud_assets you configure your Rails app as the front end for assets managed in another internet-connected system, e.g. WordPress, Drupal, SharePoint, etc.

Instead of returning a 404 Not Found, cloud_assets will proxy a pooled request to the remote server, cache the content locally, and serve it from Rails.

cloud_assets also provides tools and vocabulary for making Rails views that make use of HTML originating on the remote system, and rewrites URIs in such templates to point either to the local Rails system or a CDN, depending on configuration and utility.

Installation

Add cloud_assets to your Gemfile and bundle install, then define the necessary configuration in an initializer, e.g. config/initializers/cloud_assets.rb.

If you don't define the initializer, all the important values will be read from the environment, which is extra handy if you are running on Heroku.

This initializer will behave the same as no initializer; customize to your taste.

CloudAssets.setup do |config|

  # Required: origin URI for remote assets, e.g. http://yourhost.com
  config.origin = ENV['CLOUD_ASSET_ORIGIN']

  # If needed: HTTP Basic user for remote assets
  config.user = ENV['CLOUD_ASSET_USER']

  # If needed: HTTP Basic user for remote password
  config.password = ENV['CLOUD_ASSET_PASSWORD']

  # Rewrite URIs to use a CDN
  config.cdn = ENV['CLOUD_ASSET_CDN']

  # Activate verbose logging
  config.verbose = false

end

Put a route at the bottom of your routes.rb:

match '*url' => 'cloud_assets#content'

This will allow cloud_assets to handle anything Rails doesn't recognize.

In application_controller.rb:

include CloudAssets

Additional Configuration

If you are serving any non-trivial amount of remotely sourced assets out of your Rails system, you'll want a cache. FIXME: The cache currently uses dalli only. This should be and can be configured more flexibly in the initializer.

To enable it, just define $dalli_cache somewhere in one of your initializers.

Usage

Set a remote layout for an HTML view by defining the URI to the layout on the remote system, e.g.

set_remote_layout '/templates/foo.html'

You can inject additional content into the elements of that template, or override the interior of its elements, using CSS selectors:

inject_into_remote_layout '#notice' => flash[:notice]
override_remote_layout 'body' => yield
replace_remote_layout '#unwanted_stuff' => 'wanted stuff'
remove_remote_layout '.unwanted_classes'

Finally, obtain and show the result (complete with rewrites of CDN URLs and so on) with:

apply_remote_layout

So a complete application layout would look something like this (Haml):

- set_default_remote_layout '/about/'
- inject_into_remote_layout 'head' => (render :partial => 'layouts/headers')
- unless yield.empty?
  - override_remote_layout '#content' => yield
!= apply_remote_layout

This loads the HTML of the remote "about" page, rewrites references to localhost or the CDN as appropriate, injects Rails headers form the headers partial into the HTML head element, and replaces the interior of the element whose id is "content" -- say it's a div -- with the yield of the Rails view that uses this layout.

HTML Fixups

The remote site likely was not designed to have its HTML repurposed and rewritten on other sites. Sites following best practices should not need any manual fixups. However, any number of hacks might produce HTML that cloud_assets does not know how to rewrite. You have an opportunity to fix these by adding some monkey-patches to your initializer:

module CloudAssets
  def self.fixup_html(html)
    html.gsub 'something bad', 'something good'
  end
end

This method is applied immediately before the HTML is delivered to the browser; you can do anything here that you like, but it should be regarded as a hack. If the uncorrectable code is standards-compliant and best practice, it would be good to submit a pull request so cloud_assets can handle it. If not, it would be ideal to fix the remote asset source -- e.g. remove hard-coded references or eliminate unwarranted assumptions.

Remote URL Fixups

You can also use a monkey-patch to rewrite the URLs used to fetch the remote assets, in case they do not align with the URLs of your Rails site. Normally, cloud_assets just passes through the path component of the URL exactly as requested from Rails; this allows you to change that behavior to do anything you want, for example, blacklisting or whitelisting URLs for the remote asset source.

module CloudAssets
  def self.fixup_url(fullpath)
    fullpath.gsub 'wrongness', 'rightness'
  end
end