Skip to main content

Automation FTW: Bust Your Cache

04-07-14 Patrick Simpson

Manually changing filenames of cached assets to bust cache is a nightmare. Patrick will tell you how to automate cache busting and make you want to give him a high five.

Have you ever made a change to a CSS or JavaScript file and noticed that you’re not seeing updates in the browser? Well, you might just have a cache problem. A quick way to bust cache is to simply add a query string to the filename. However, query strings are not the best.

The best way to bust your cache is to use a hash (based on the file’s contents) as part of the filename itself to create a file “fingerprint”. Changing the filename of your cached asset (CSS, JS, favicon) files appears like a new (uncached) file. The server then makes a new request to a fresh (updated) copy of your asset file, which is great: users will get the latest updated assets without having to clear out their cache.

Fingerprinting files is great, but doing that manually is a nightmare.

Automation FTW

At Sparkbox, we use Grunt (task automater) to build amazing websites. To automate cache busting, you can combine two Grunt tasks, Bushcaster, and string-replace for a solution to the cache problem: Bushcaster-string-replace.

Let’s go over the important stuff in the


      grunt.cacheMap = []

You’ll need this to store the pattern/replacement object collected in the bushcaster “onComplete” call.

In the Bushcaster options, you’ll probably want the noProcess flag.

  noProcess: true

I learned this the hard way. Without this flag, Grunt will modify references in the files, which screws up binary files, like images, favicons, etc. It’s best just not to process any of the files for now.

String Replace

The string-replace was the best Grunt task that I could find that would accept a pattern/replacement object built in the onComplete step previously.

So all you’ll need to do is specify the replacements as that object previously collected in grunt.cacheMap (call it whatever you want).

        replacements: grunt.cacheMap


This repo has a build directory, which will place all the .html files inside. It’s important to know that that might not be the case for the project you’re working on. Be careful NOT to overwrite your actual source files.

              'build/': '*.html' # dest: source

Wrapping Up

When you have cache busting in place, you can enjoy setting a long expires to your asset files. We know that fingerprinted assets will always be different, you can then adjust your .htaccess file to allow for longer caching times on assets you are busting. Here is an example.

Our build process (using Assemble and Grunt) needed a solution to the cache problem. Using these two Grunt libraries, you have an asset fingerprinting, cache-busting tool for your static site. It’s important to add, Ruby on Rails already has cache-busting functionality with the asset pipeline.

So there you have it! This is our solution for now for our static sites. Now that you know, give it a shot. Enjoy busting all the caches!

Related Content

User-Centered Thinking: 7 Things to Consider and a Free Guide

Want the benefits of UX but not sure where to start? Grab our guide to evaluate your needs, earn buy-in, and get hiring tips.

More Details

See Everything In

Want to talk about how we can work together?

Katie can help

A portrait of Vice President of Business Development, Katie Jennings.

Katie Jennings

Vice President of Business Development