Squid and Rails caching

At Vquence our Rails setup looks something like this.

------------     ---------     ------------ 
| Internet |---->| Squid |---->| Mongrels | 
------------     ---------     ------------ 

(Who needs Inkscape when you have ASCII art)

This infrastructure is hosted in the US and up until recently squid hadn’t been doing much of anything except really sitting there.

Now a few months ago when we signed a contract with an Australian customer we decided we needed to place a squid cache in Australia which would actually cache content. For two reasons, firstly the US is a long way away and the 300ms latency is really noticeable and secondly because some of our pages involving graphs have long statistical calculations which can take minutes to render. (OK its really because no one has had a chance to optimise them yet but lets pretend that’s not the case). So we changed the above setup for the Australian customers to look like the following.

------------     ------------     ------------     ------------
| Internet |---->| Squid AU |---->| Squid US |---->| Mongrels |
------------     ------------     ------------     ------------

We hand out urls like http://www.client.b2b.vquence.com/widget to Australian customers and the rails backend is smart enough to make sure all the URLs look similar (I’ll blog about how I did that another time).

Without much time to look into thing properly I did some really nasty things on the AU squid cache to make sure it cached the pages.

refresh_pattern /client/graph  1440 0% 1440 ignore-no-cache ignore-reload
refresh_pattern /client/static 1440 0% 1440 ignore-no-cache ignore-reload
refresh_pattern /client/video  1440 0% 1440 ignore-no-cache ignore-reload

This is evil, breaks a whole heap of RFCs but it did the trick and got us out of a bind quickly.

A few weeks ago I moved the production site to Rails 2.0, I noticed around this time that the caching had stopped working. The client was no longer using our services as their campaign had finished so it wasn’t an urgent concern.

It seems that Rails 2.0 goes one step further to ensure that caches don’t cache content and instead of just sending

Cache-Control: no-cache

it now sends

Cache-Control: private, max-age=0, must-revalidate

I tried adding ignore-private, since if you’re breaking some aspects of the RFC you may as well break a couple more, but squid still refused to cache the content. After struggling with this for a bit I decided that the universe was trying to tell me I should actually do things properly.

So with squid set back to its defaults I went exploring how to accomplish this. Google wasn’t all that helpful at first since most Rails caching articles talk about caching to static files as most sites don’t implement reverse proxying for caching. It turns out however its fairly simple. In the appropriate actions in your controllers simply do the following.

class VideoController < ApplicationController
  def vquence
    # Lots of code here

    expires_in 8.hours, :private => false
    render :template => "videos/vquence"
  end
end

This will send the following header and cache the page for 8 hours.

Cache-Control: max-age=28800

Now everything is much faster!!