Slightly hacky rails middleware for serving up precompiled gzipped assets

compressed_static_assets.rb

require 'action_dispatch/middleware/static'

module Middleware
  class FileHandler < ActionDispatch::FileHandler
    def initialize(root, assets_path, cache_control)
      @assets_path = assets_path.chomp('/') + '/'
      super(root, cache_control)
    end

    def match?(path)
      path.start_with?(@assets_path) && super(path)
    end
  end

  class CompressedStaticAssets
    def initialize(app, path, assets_path, cache_control=nil)
      @app = app
      @file_handler = FileHandler.new(path, assets_path, cache_control)
    end

    def call(env)
      if env['REQUEST_METHOD'] == 'GET'
        request = Rack::Request.new(env)
        encoding = Rack::Utils.select_best_encoding(
          %w(gzip identity), request.accept_encoding)

        if encoding == 'gzip'
          pathgz = env['PATH_INFO'] + '.gz'
          if match = @file_handler.match?(pathgz)
            # Get the filehandler to serve up the gzipped file,
            # then strip the .gz suffix
            env["PATH_INFO"] = match
            status, headers, body = @file_handler.call(env)
            path = env["PATH_INFO"] = env["PATH_INFO"].chomp('.gz')

            # Set the Vary HTTP header.
            vary = headers["Vary"].to_s.split(",").map { |v| v.strip }
            unless vary.include?("*") || vary.include?("Accept-Encoding")
              headers["Vary"] = vary.push("Accept-Encoding").join(",")
            end

            headers['Content-Encoding'] = 'gzip'
            headers['Content-Type'] =
              Rack::Mime.mime_type(File.extname(path), 'text/plain')
            headers.delete('Content-Length')

            return [status, headers, body]
          end
        end
      end

      @app.call(env)
    end
  end
end

production.rb

 # Serve pre-gzipped static assets
 middleware.insert_after(
   'Rack::Cache', Middleware::CompressedStaticAssets,
   paths["public"].first, config.assets.prefix, config.static_cache_control)

@bensheldon @marcgg FYI the heroku-deflater gem is slightly different than this gist. The gem will use Rack::Deflater for every response, but tell it to skip binary files via the no-transform Cache-control directive. This gist takes advantage of the higher compression ratio for precompiled assets by serving up the .gz version, but by itself doesn't do anything with other types of requests. I think a combination of the two approaches is the ideal.

@guyboltonking @neersighted @bensheldon @marcgg @runeroniek I have taken the liberty of gemifying this code (with changes to make it play nice with Rack::Deflate). It is available here. It's been working well for me. Let me know if you have any questions.

Another option for serving gzipped assets using Rack::Rewrite https://gist.github.com/eliotsykes/6049536