Using Gulp asset versioning with Hugo data files

I recently finished converting four of the websites I run from Wordpress to Hugo (my original blog, a Chinese learning blog, a Chinese grammar site and this blog), and hosting them with S3 and CloudFront.

It’s been a great change to make and has improved everything from my workflow to loading times and hosting costs. I’d recommend anyone running blogs and other text-focused sites to switch away from Wordpress and use a static site generator instead. Hugo would be my recommendation because of its efficiency and flexibility, but there are many options.

One small issue I faced was asset versioning. As the sites are stored in S3 and served from CloudFront with cache headers set to public, it often takes over 24 hours for changes to propagate. For typo fixes and content updates this is fine, but it causes bigger problems when assets like CSS files get out of sync with markup and make the site look bad.

Gulp plays a big part in my workflow (e.g. see this gulpfile), and there’s a Gulp module designed to solve asset versioning problems: gulp-rev. That plugin uses a hashing function to add a unique prefix to asset filenames. My gulp CSS task looks like this:

gulp.task('style', function() {
  return gulp.src('./themes/chineseboost/scss/cba.scss')
      'advanced': true,
      'aggressiveMerging': true,
      'keepBreaks': true

Adding the unique prefix is only part of the solution. You’ve also got to get the uniquely named files into your markup. Hugo’s data files provide the perfect way to do this. The feature lets you read data from files and use it in templates as the site is being generated.

As shown above, the gulp-rev plugin writes a map of the original filename to the versioned one as a JSON file. The contents will look like this:

  "cba.css": "cba-36335a46.css"

You simply need to get that written to the data directory in your Hugo project and Hugo will make it available on .Site.Data. You can then write it into your templates like this:

<link rel="stylesheet"
  href='/css/{{ index .Site.Data.rev_manifest "cba.css" }}'

This way, it’s not possible for markup to come out of sync with styling as each version of the CSS file has a specific name in whatever cache it is stored in. It also has the advantage of ensuring that new pages will get the latest version of the asset. With this set up, it’s handled for you by Gulp and Hugo so you don’t have to think about it.

Tech mentioned