This post is for myself (so that I won’t forget) and the only other reader of this blog.

Background: you have a shining new web application written with Ruby on Rails 3.0.*, which uses an engine; this engine employs various static assets (CSS stylesheets, images, etc.) stored in the engine’s public directory.

You code your way and during development everything works. Then, a day, you fire up Thin or Mongrel in production mode and… where the heck are the stylesheets?

Static assets in production

Ruby on Rails 3.0.* does not serve static assets (i.e., the contents of public) in production mode. The reasons are that your application server is usually fronted with a web server and this piece of software is better suited than anything else to quickly serve files.

But you insist: you want to see you app, in production mode, on your development machine, running just Thin. Ok, then change the value of config.serve_static_assets in config/environments/production.rb to true or comment out that row.

Restart Thin, reload the app and there they are, your stylesheets, your images, etc. There’s almost everything, but the engine’s assets! And the app is strangely slow…

Turns out that there’s another optimization for production environment at work: X-Sendfile.

Engine’s static assets in production

Even if you’ve just reenabled serving static assets, that setting influences only the contents of your app’s public directory. Engines’ assets are affected by config.action_dispatch.x_sendfile_header: if its value is one out of a short list and not nil, every response containing your assets is replaced with an empty body and a particular header that a well configured web server would intercept and manage for you trasparently.

So, comment out also that row in config/environments/production.rb, restart Thin and, lo!, all your assets are there.

Clean up before deployment

First of all, remember to restore your production.rb file.

Then, double check that the web server used in production can handle X-Sendfile/X-Accel-Redirect: if it can’t, either replace it with another software or comment out again config.action_dispatch.x_sendfile_header in production.rb. It will slow down your app, because engines’ assets will have to be handled by the application server, but they’ll at least be there.