ruby / fingerprint

updated November 30, 2025

I use file-based asset fingerprinting in Ruby web apps to enable aggressive caching with CDNs.

The approach

As part of the deployment build, after building assets with esbuild, a Rake task fingerprints the files:

require "digest"

namespace :assets do
  task :precompile do
    ["public/css/app.css", "public/js/app.js"].each do |old_path|
      hash = Digest::MD5.file(File.expand_path(old_path, __dir__))
      ext = File.extname(old_path)
      base = old_path.chomp(ext)
      new_path = "#{base}-#{hash}#{ext}"
      system "mv #{old_path} #{new_path}"
    end
  end
end

Rails will serve the renamed files:

public/app.css  -> public/app-a1b2c3d4.css
public/app.js   -> public/app-a1b2c3d5.js

Rails configuration

In config/environments/production.rb:

config.public_file_server.enabled = true
config.public_file_server.headers = {
  "Cache-Control" => "public, max-age=31536000, immutable"
}

Since filenames include content hashes, each URL is immutable. Browsers and CDNs can cache aggressively (1 year) without risk of serving stale content.

The immutable directive eliminates revalidation requests even on page reload.

Deployment

Example build command for Render:

npm install && \
npm run build && \
bundle install && \
bundle exec rake db:migrate && \
bundle exec rake assets:precompile

This:

1. Installs JavaScript dependencies
2. Builds and bundles with esbuild
3. Installs Ruby dependencies
4. Migrates the database
5. Fingerprints static assets

Template integration

In config/initializers/assets.rb:

# see rake assets:precompile definition in Rakefile
# and app/views/layouts/application.haml

app_css_path = "/css/app.css"
app_js_path = "/js/app.js"

if ["staging", "production"].include?(ENV.fetch("APP_ENV"))
  path = Dir.glob("#{Rails.root}/public/css/app*.css")&.first
  if path
    app_css_path = path.split("public")[1]
  end

  path = Dir.glob("#{Rails.root}/public/js/app*.js")&.first
  if path
    app_js_path = path.split("public")[1]
  end
end

APP_CSS_PATH = app_css_path.freeze
APP_JS_PATH = app_js_path.freeze

In views such as app/views/layouts/application.haml:

%link{ rel: "stylesheet", href: APP_CSS_PATH }
%script{ src: APP_JS_PATH }

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