Hugo cache busting


Although caching can make page loads notably faster, it comes with a cost. Browsers aren’t always capable of taking note when a cached resource has changed. I’ve noticed recently that Safari utterly refuses to reload .css files even after emptying the browser cache and clearing the web history.


With a lot of help from the a pair of articles written by Ukiah Smith, I’ve developed a workflow for dealing with this problem during the deployment process. He describes two approaches to the problem of static asset caching, one an improvement on the other. I’ve implemented something like what he describes using the git file hash to modify the filename of the css files. When the client browser sees a new filename, it always reloads the resource. So the problem is to figure out how to only change the filename when the contents have changed. Let’s say you tweak a css parameter and want to ensure that client browsers load the correct version. We can use the git file hash, and append it on the filename. Then the only remaining problem is to make sure that the page head template knows how to find the correct version to bake into the pages. Here, our approach is the same as Smith’s.

Where we diverge in the approach described here relates to the fact that I’m using straight CSS and not a pre-processor. The main change is that I’ve moved my custom CSS files to a separate directory css_source at the top level of my site directoy. It’s not parsed in the build process; but it serves as the canonical source of my custom css. During the build process, we run a script that process the contents of the css_source directory, appends the git hash to the filename and creates a JSON manifest that we use in the head template.

The process

First, I just clear the public/css directory of any prior versions of the css files:

import os
import fnmatch

# Clean public/css directory of old CSS files before build process

public_css = "path to my public css directory"
for fname in fnmatch.filter(os.listdir(public_css), '*css'): 

Then, we process the files and build the manifest:

import os
import fnmatch
import subprocess
import json
import sys
import random
import re
from shutil import copyfile

def fileHash(filePath):
    process =['/usr/local/bin/git', 'hash-object',filePath], check=True, stdout=subprocess.PIPE, universal_newlines=True)
    output = process.stdout
    return output
def replaceIdentifier(fn,path):
    result = re.match(r'(.+)_[\d\D]{30}\.css', fn)
        baseName = result.groups()[0]
        result ='(.+)\.css', fn)
        baseName = result.groups()[0]
    identifier = fileHash(os.path.join(path, fn))
    return baseName + "_" + identifier + ".css"

data = {}
path = os.path.abspath(sys.argv[1])
out_path = os.path.abspath(sys.argv[2])
j_data = os.path.abspath(sys.argv[3])
j_data = os.path.join(j_data,"css.json")

# empty the primary css directory
for fname in fnmatch.filter(os.listdir(out_path), '*css'): 
# apply hash to css filename and move to primary css directory
for fname in fnmatch.filter(os.listdir(path), '*css'):
    newFilename = replaceIdentifier(fname,path)
    data[os.path.splitext(fname)[0]] = newFilename
    copyfile(os.path.join(path,fname), os.path.join(out_path,newFilename))
# write the manifest
with open(j_data,'w') as outfile:
    print("Generating css json data file.")
    json.dump(data, outfile, indent = 1)

Now in site/data/tonedeaf/ I have a manifest file css.json that we can use to build the stylesheet links.

   "link-page": "link-page_10eb858a5c0a9e255aef9b40baf623d64ee2c29c\n.css",
   "russian-shortcode": "russian-shortcode_65be890c35a165f7ef13a61b1134c0888a0c384f\n.css",
   "photogallery": "photogallery_4d7b91630e0c31a2764ca52d61f3293438d752d0\n.css",
   "tag_page": "tag_page_121f9dc78b4180d309fbac735da704359c335921\n.css",
   "general": "general_191e79d04191f5dd1a8392b0904ee3c8fc4e9669\n.css",
   "hugo-easy-gallery": "hugo-easy-gallery_d78dfec854168b088b061544f8f01b20a5389efc\n.css"

Using this data in my head template:

<link href="{{ .Site.BaseURL }}css/{{ index .Site.Data.tonedeaf.css "general" }}" rel="stylesheet">
<link href="{{ .Site.BaseURL }}css/{{ index .Site.Data.tonedeaf.css "russian-shortcode" }}" rel="stylesheet">
<link href="{{ .Site.BaseURL }}css/{{ index .Site.Data.tonedeaf.css "photogallery" }}" rel="stylesheet">
<link href="{{ .Site.BaseURL }}css/{{ index .Site.Data.tonedeaf.css "link-page" }}" rel="stylesheet">

Where Smith handles this all in a Makefile, I just use a Keyboard Maestro macro to link all of pieces together. I hope this helps solve caching problems when working with Hugo.