RESTful app design means that the same action can generate both HTML and XML. One problem I’ve encountered is that XML in the form of an RSS or ATOM feed could be page cached, but the HTML version should not be (since it might show flash messages or be customized for authenticated users).

http://peepcode.com/products # => Generates customized HTML
http://peepcode.com/products.rss # => Changes infrequently and could be cached

The Solution

Edge Rails has a nice feature that could possibly be used to cache some formats but not others. In the meantime, here’s an alternate strategy that works for simple sites that need to cache only a few feeds:

• Write a rake task to hit the webserver and save the relevant content to disk. When requested, the webserver will serve it normally and Rails won’t be involved. A cron task or a Capistrano after_deploy task could do the caching.
• Use a normal sweeper to expire the content when items are changed inside the Rails app.

The Code

A rake task:

desc "Cache specific feeds manually. To be called from cron." 
task :cache_feeds do
  require 'open-uri'
  %w(
    products.rss
    products.atom
  ).each do |filename|

    unless File.exist?("public/#{filename}")
      File.open("public/#{filename}", "w") do |f|
        f.write open("http://peepcode.com/#{filename}").read
      end
    end

  end
end

A cron:

@hourly cd my_rails_app/current && rake cache_feeds RAILS_ENV=production

Here’s a sweeper that will delete the cached feeds when content is created or saved.

class ProductSweeper < ActionController::Caching::Sweeper

  observe Product

  def after_save(record)
    expire_page hash_for_formatted_products_path(:format => :rss)
    expire_page hash_for_formatted_products_path(:format => :atom)
  end

end

To activate, put the following line in the products controller:

class ProductsController < ApplicationController

  cache_sweeper :product_sweeper, :only => [:create, :update]

HTML content will still be generated dynamically by Rails, but the cached RSS and ATOM versions will be served from disk, reducing load on the server.

See Also

• Sweeper generator plugin
• Sweeper API Docs