rack-pagespeed on Heroku

TL;DR Use memcached, not disk storage

A few weeks ago I heard about rack-pagespeed and decided to use it on this blog. A week later I was randomly checking out the site and found it completely unstyled. The CSS file that rack-pagespeed had generated was throwing a 404. I scratched my head a bit, asked on GitHub and realized that I’d stupidly been using the tmp directory for storage. It’s called a temp directory for a reason, and Heroku was flushing it out periodically, hence why it looked fine after a deploy but broke a little while later.

The solution is to use a persistent cache like memcached. Depending on the size of your site and the amount of CSS/JS you have, this might not really be worth it, but assuming it is, here’s how you do it.

First install memcached locally using Homebrew¹:

$ brew install memcached

Then just specify the store as memcached. Heroku can sometimes be finicky about relative paths, so I like to be really explicit and set the root of the app to the current directory, then build off that. settings is just a class method on the rack app, so you can access it from outside the app²:

# minimalog.rb
class Minimalog < Sinatra::Base
  set :root, File.dirname(__FILE__)

  ...
end

# config.ru
use Rack::PageSpeed, :public => "#{Minimalog.settings.root}/public" do
  store :memcached
  combine_css
end

Redeploy to Heroku and everything should be good.