Remotely flush your page and fragment caches using rake and capistrano

Anton Jenkins | February 26, 2009

The key to this technique is making your cache sweepers easy to call from a rake task. For this website I’ve created a simple site sweeper which flushes the entire page cache :

# app/sweepers/site_sweeper.rb

class SiteSweeper < ActionController::Caching::Sweeper
  observe Page, Post, Comment, Tag, Tagging
  
  def after_save(site)
    self.class::sweep
  end
  
  def after_destroy(site)
    self.class::sweep
  end
  
  def self.sweep
    cache_dir = ActionController::Base.page_cache_directory
    unless cache_dir == RAILS_ROOT+"/public"
      FileUtils.rm_r(Dir.glob(cache_dir+"/*")) rescue Errno::ENOENT
      RAILS_DEFAULT_LOGGER.info("Cache directory '#{cache_dir}' fully swept.")
    end
  end
end

By declaring the sweep method as a class method we make it easy to call from rake to force a sweep.

To do so we need to create a new rake file :

# lib/tasks/pixellated.rake

namespace :pixellated do
  namespace :cache do
    desc "Expire the flickr_sidebar fragment cache"
    task :expire_flickr => :environment do
      ActionController::Base.new.expire_fragment('flickr_sidebar')
      RAILS_DEFAULT_LOGGER.info("Flickr sidebar cache swept")
    end

    desc "Expire page cache"
    task :expire_pages => :environment do
      SiteSweeper.sweep
    end

    desc "Expire everything"
    task :expire_all => [:expire_flickr, :expire_pages] do
      RAILS_DEFAULT_LOGGER.info("All caches swept")
    end
  end
end

The tasks in here allow me to flush the page cache as well as flush the flickr_sidebar fragment which you can see to the right of each page. It takes around 8 seconds to talk to flickr and refresh this page element so it’s a perfect candidate for fragment caching.

We can now use rake to flush the caches from the command line :

123	rake pixellated:cache:expire_flickrrake pixellated:cache:expire_pages rake pixellated:cache:expire_all

But what about on the production server?

Well that’s your local machine sorted, but we need a way to flush the caches on the live server. Capistrano to the rescue again!

Because we can perform cache flushes using rake we just need a way to invoke the rake tasks on the server. Of course we will need to do a deploy to get our newly created rake tasks on to the server first (very important and easy to forget!) :

1	cap deploy

Now we can add the following capistrano tasks to depoy.rb to invoke these rake tasks remotely :

# config/deploy.rb

namespace :pixellated do
  namespace :cache do
    set :remote_rake_cmd, "/usr/local/bin/rake"

    desc "Expire everything"
    task :expire_all do
      run("export RAILS_ENV=production; cd #{deploy_to}/current; #{remote_rake_cmd} pixellated:cache:expire_all")
    end

    desc "Expire page cache"
    task :expire_pages do
      run("export RAILS_ENV=production; cd #{deploy_to}/current; #{remote_rake_cmd} pixellated:cache:expire_pages")
    end

    desc "Expire the flickr_sidebar fragment cache"
    task :expire_flickr do
      run("export RAILS_ENV=production; cd #{deploy_to}/current; #{remote_rake_cmd} pixellated:cache:expire_flickr")
    end
  end
end