DNS to CDN to Origin

Dan Croak

Content Distribution Networks (CDNs) such as Amazon CloudFront and Fastly have the ability to “pull” content from their origin server during HTTP requests in order to cache them. They can also proxy POST, PUT, PATCH, DELETE, and OPTION HTTP requests, which means they can “front” our web application’s origin like this:

DNS -> CDN -> Origin

Swapping out the concepts for actual services we use, the architecture can look like this:

DNSimple -> CloudFront -> Heroku

Or like this:

DNSimple -> Fastly -> Heroku

Or many other combinations.

Without Origin Pull or an Asset Host

Let’s first examine what it looks like serve static assets (CSS, JavaScript, font, and image files) from a Rails app without a CDN.

We could point our domain name to our Rails app running on Heroku using a CNAME record (apex domains in cloud environments have their own set of eccentricities):

www.thoughtbot.com -> thoughtbot-production.herokuapp.com

We’ll also need to set the following configuration:

# config/environments/{staging,production}.rb
config.serve_static_assets = true

In this setup, we’ll then see something like the following in our logs:

no asset host

That screenshot is from development mode but the same effect will occur in production:

  • all the application’s requests to static assets will go through the Heroku routing mesh,
  • get picked up by one of our web dynos,
  • passed to one of the Unicorn workers on the dyno,
  • then routed by Rails to the asset

This isn’t the best use of our Ruby processes. They should be reserved for handling real logic. Each process should have the fastest possible response time. Overall response time is affected by waiting for other processes to finish their work.

How Can We Solve This

AssetSync is a popular approach that we have used in the past with success. We no longer use it because there’s no need to copy all files to S3 during deploy (rake assets:precompile). Copying files across the network is wasteful and slow, and gets slower as the codebase grows. S3 is also not a CDN, does not have edge servers, and therefore is slower than CDN options.

Asset Hosts that Support “Origin Pull”

A better alternative is to use services that “pull” the assets from the origin (Heroku) “Just In Time” the first time they are needed. Services we’ve used include CloudFront and Fastly. Fastly is our usual default due to its amazingly quick cache invalidation. Both have “origin pull” features that work well with Rails’ asset pipeline.

Because of the asset pipeline, in production, every asset has a hash added to its name. Whenever the file changes, the browser requests the latest version as the hash and therefore the whole filename changes.

The first time a user requests an asset, it will look like this:

GET 123abc.cloudfront.net/application-ql4h2308y.css

A CloudFront cache miss “pulls from the origin” by making another GET request:

GET your-app-production.herokuapp.com/application-ql4h2308y.css

All future GET and HEAD requests to the CloudFront URL within the cache duration will be cached, with no second HTTP request to the origin:

GET 123abc.cloudfront.net/application-ql4h2308y.css

All HTTP requests using verbs other than GET and HEAD proxy through to the origin, which follows the Write-Through Mandatory portion of the HTTP specification.

Making it Work with Rails

We have standard configuration in our Rails apps that make this work:

# Gemfile
gem "coffee-rails"
gem "sass-rails"
gem "uglifier"

group :staging, :production do
  gem "rails_12factor"

# config/environments/{staging,production}.rb:
config.action_controller.asset_host = ENV["ASSET_HOST"] # will look like //123abc.cloudfront.net
config.assets.compile = false
config.assets.digest = true
config.assets.js_compressor = :uglifier
config.assets.version = ENV["ASSETS_VERSION"]
config.static_cache_control = "public, max-age=#{1.year.to_i}"

We don’t have to manually set config.serve_static_assets = true because the rails_12factor gem does it for us, in addition to handling any other current or future Heroku-related settings.

Fastly and other reverse proxy caches respect the Surrogate-Control standard. To get entire HTML pages cached in Fastly, we only need to include the Surrogate-Control header in the response. Fastly will cache the page for the duration we specify, protecting the origin from unnecessary requests and serving the HTML from Fastly’s edge servers.

Caching Entire HTML Pages (Why Use Memcache?)

While setting the asset host is a great start, a DNS to CDN to Origin architecture also lets us cache entire HTML pages. Here’s an example of caching entire HTML pages in Rails with High Voltage:

class PagesController < HighVoltage::PagesController
  before_filter :set_cache_headers


  def set_cache_headers
    response.headers["Surrogate-Control"] = "max-age=#{1.day.to_i}"

This will allow us to cache entire HTML pages in the CDN without using a Memcache add-on, which still goes through the Heroku router, then our app’s web processes, then Memcache. This architecture entirely protects the Rails app from HTTP requests that don’t require Ruby logic specific to our domain.

Rack Middleware

If we want to cache entire HTML pages site-wide, we might want to use Rack middleware. Here’s our typical config.ru for a Middleman app:

$:.unshift File.dirname(__FILE__)

require "rack/contrib/try_static"
require "lib/rack_surrogate_control"

ONE_WEEK = 604_800

use Rack::Deflater
use Rack::SurrogateControl
use Rack::TryStatic,
  root: "tmp",
  urls: %w[/],
  try: %w[.html index.html /index.html],
  header_rules: [
      %w(css js png jpg woff),
      { "Cache-Control" => "public, max-age=#{ONE_WEEK}" }
      %w(html), { "Cache-Control" => "public, max-age=#{FIVE_MINUTES}" }

run lambda { |env|
      "Content-Type"  => "text/html",
      "Cache-Control" => "public, max-age=#{FIVE_MINUTES}"
    File.open("tmp/404.html", File::RDONLY)

We build the Middleman app at rake assets:precompile time during deploy to Heroku, as described in Styling a Middleman Blog with Bourbon, Neat, and Bitters. In production, we serve the app using Rack, so we are able to insert middleware to handle the Surrogate-Control header:

module Rack
  class SurrogateControl
    # Cache content in a reverse proxy cache (such as Fastly) for a year.
    # Use Surrogate-Control in response header so cache can be busted after
    # each deploy.
    ONE_YEAR = 31557600

    def initialize(app)
      @app = app

    def call(env)
      status, headers, body = @app.call(env)
      headers["Surrogate-Control"] = "max-age=#{ONE_YEAR}"
      [status, headers, body]

CloudFront Setup

If we want to use CloudFront, we use the following settings:

  • “Download” CloudFront distribution
  • “Origin Domain Name” as www.thoughtbot.com (our app’s URL)
  • “Origin Protocol Policy” to “Match Viewer”
  • “Object Caching” to “Use Origin Cache Headers”
  • “Forward Query Strings” to “No (Improves Caching)”
  • “Distribution State” to “Enabled”

As a side benefit, in combination with CloudFront logging, we could replay HTTP requests on the Rails app if we had downtime at the origin for any reason, such as a Heroku platform issue.

Fastly Setup

If we use Fastly instead of CloudFront, there’s no “Origin Pull” configuration we need to do. It will work “out of the box” with our Rails configuration settings.

We often have a rake task in our Ruby apps fronted by Fastly like this:

# Rakefile
task :purge do
  api_key = ENV["FASTLY_KEY"]
  site_key = ENV["FASTLY_SITE_KEY"]
  `curl -X POST -H 'Fastly-Key: #{api_key}' https://api.fastly.com/service/#{site_key}/purge_all`
  puts 'Cache purged'

That turns our deployment process into:

git push production
heroku run rake purge --remote production

For more advanced caching and cache invalidation at an object level, see the fastly-rails gem.

Back to the Future

Fastly is really “Varnish as a Service”. Early in its history, Heroku used to include Varnish (an awesome open source reverse proxy) as a standard part of its “Bamboo” stack. When they decoupled the reverse proxy in their “Cedar” stack, we gained the flexibility of using different reverse proxy caches and CDNs fronting Heroku.

Love is Real

We have been using this stack in production for thoughtbot.com (a Rails app), robots.thoughtbot.com (a Middleman app), playbook.thoughtbot.com (a Rack app), and many other apps for almost a year. It’s a stack in real use and is strong enough to consider as a good default architecture.

Give it a try on your next app!