Caching URLs with Query Strings

Posted by Mike Brittain on June 11, 2009
WWW / 1 Comment

A common method for breaking a cached object, or for specifying a version of an object, is to put a version number into a query string or some other part of the file name.  I’ve been wary of using query strings in URLs of static assets that I cache, either in browser cache or on a CDN.  I hadn’t seen much hard evidence, but generally felt query strings provide another area of uncertainty in a network path that you already have little control over.

Today, I just found Google’s recommendation for leveraging proxy caches, which states, “to enable proxy caching for these resources, remove query strings from references to static resources, and instead encode the parameters into the file names themselves.”

What is a good alternative?  One method I like is to use a version number that is part of the URL path:


The “12345” is prepended in your page templates using a variable.  The value can be taken from anywhere you want… it could be part of a timestamp, a revision number in your source control, or maybe even manually changed in a constants file anytime your are updating files in your “css” directory.  If you’re using Apache, this portion of the path can be easily stripped off the request using mod_rewrite.

  RewriteEngine On
  RewriteRule ^/\d{5,}(/.+)$ $1

With this method, you’re not literally using directories with version number in them.  Rather, you just use your regular old /htdocs/css directory. The same can be applied to JavaScript, images, or other directories with static assets.

What you gain is the ability to break your cached versions of files when necessary, but to also set long-term cache times on these assets (while they’re not changing) and expect cache servers between you and your users to properly store these objects.

More to come…

I’ll be speaking at Velocity in two weeks about how to optimize caching with your CDN.  I have a lot of content that I’d like to present, but only 20 minutes to do it.  Over the coming weeks I’ll be writing a number of posts covering topics that don’t make it into the presentation.

Tags: , , ,