Webpacker and sprockets manifests

[benwalsh]

I experimented with the Webpacker configuration and add_local_files but our circumstances required a bit of tweaking. In particular, we have multiple web servers and want to avoid multiple asset compilation jobs, and we need to work with an existing CDN configuration that looks for everything under the pathname '/assets'.

This Capistrano task assumes there is a single server with the assets role and one or more with the web role. After asset compilation, this task downloads the Sprockets manifest .sprockets-<fingerprint>.json and the Webpacker manifest manifest.json, and uploads them to all the web servers.

# config/deploy.rb

set :manifest_dir_sprockets, -> { File.join(current_path, 'public', 'assets') }
set :manifest_dir_webpacker, -> { File.join(current_path, 'public', 'packs') }
set :manifest_location_sprockets, -> { File.join(fetch(:manifest_dir_sprockets), '.sprockets-*.json') }
set :manifest_location_webpacker, -> { File.join(fetch(:manifest_dir_webpacker), 'manifest.json') }

after :compile_assets, :copy_manifests

desc 'Download manifest files from asset compilation server and upload them to other web servers'
  task :copy_manifests do
    on roles(:assets) do
      within(shared_path) do
        %i[sprockets webpacker].each do |type|
          execute(:rm, '-f', fetch(:"manifest_path_#{type}"))
          set(:"manifest_path_#{type}", capture(:ls, '-t', fetch(:"manifest_location_#{type}"), '|', 'head', '-n1'))
          set(:"manifest_filename_#{type}", fetch(:"manifest_path_#{type}").split('/').last)
          download!(fetch(:"manifest_path_#{type}"), "tmp/#{fetch(:"manifest_filename_#{type}")}")
        end
      end
      within(current_path) do
        rake('webpacker:upload')
      end
    end
    on roles(:all) do
      within(shared_path) do
        %i[sprockets webpacker].each do |type|
          execute(:rm, '-rf', File.join(fetch(:"manifest_dir_#{type}"), '*'))
          upload!("tmp/#{fetch(:"manifest_filename_#{type}")}", fetch(:"manifest_path_#{type}"))
        end
      end
    end
  end

The Rake task we invoke here is used to upload the transpiled asset packs to our desired location on S3. We also need to modify the manifest itself so that it prepends the assets/ string to the filename.

# lib/tasks/webpacker.rake

namespace :webpacker do
  desc 'Upload webpacker files to S3 asset host'
  task :upload do
    s3 = Aws::S3::Client.new
    file = Rails.root.join('public', 'packs', 'manifest.json')
    payload = JSON.parse(File.read(file))
    payload.each do |key, filename|
      filename.gsub!(%r{\A\/}, '')
      target = "assets/#{filename}"
      payload[key] = target
      next if s3.list_objects(bucket: S3_ASSETS_BUCKET, prefix: target).contents.any?
      body = File.read(Rails.root.join('public', filename.gsub('_/', '../')))
      s3.put_object(acl: 'public-read', body: body, bucket: S3_ASSETS_BUCKET, key: target)
    end
    File.open(file, 'w') { |f| f.puts(payload.to_json) }
  end
end

It's a bit brittle and I'd like it to be more configurable and transparent, but it does what's required.