Cloudfront serving stale CSS for Rails 3.2 app - css

Question

I'm running a Rails 3.2 app on Heroku. For about a week we were serving assets via CloudFront by setting config.action_controller.asset_host to our CloudFront URL in our config/production.rb file. This was working successfully.
This past weekend, however, I noticed that after a deploy to production our website looked very off, and the reason was that it was serving stale CSS. I looked at the css file it was serving (using inspect element in Chrome), and the CSS was an md5-hashed application.css file coming from CloudFront. I removed the asset_host line (so that assets would be served directly from our app) and deployed again (without changing any css), and noticed that the site, which now looked fine, was serving application.css with a different md5 hash.
So it appears that for some reason, CloudFront was serving an old version of application.css, and I'm guessing this is because our application was telling users' browsers to serve an old version of application.css.
To add one more variable: we do cache the home page and part of our our layouts/application.html.erb file (which contains the stylesheet tag), but on each deploy we clear the cache via Rails.cache.clear.
So my best guess is that Rails.cache.clear might not be properly invalidating the cache. We use the dalli memcache client, if that helps.
Any insights or suggestions would be greatly appreciated!
Update:
I tried moving the CSS out of the cached block and re-enabling CloudFront, but the CSS still appears broken. So it doesn't appear to be related to caching the header.
Update 2:
It looks like this is a CloudFront issue, because when I inspect element and change the CSS URL in the to our root domain (instead of the CDN domain), the CSS renders correctly.
Since a md5-hash collision is extremely unlikely, it seems like CloudFront is serving the wrong CSS file when I'm requesting the correct md5-fingerprinted CSS file. Any ideas?

Answer 1

assets are not automatically synced to your CDN when you deploy an app on heroku. In the deployment script on heroku it does do the rake assets:precompile task but it does not then place them into your CDN. You'll have to create some sort of mechanism to do this on your own on deployment of your app.
Somebody else asked a similar question and you might want to have a look at what the suggestions were there: Rails 3 automatic asset deployment to Amazon CloudFront?