1-frontend_server.conf

upstream haproxy {
  server 127.0.0.1:85;
}

server {
  server_name example.com;

  # ... other stuff (locations, etc.)

  location / {
    proxy_pass http://haproxy;
  }

  location ^~ /assets/ {
    # Originally I had a rewrite... but I'd rather not redirect the client.
    # rewrite ^ $scheme://<%= @asset_server_name %>$request_uri permanent;
    proxy_set_header Host assets.example.com;
    proxy_pass http://asset_server;
  }
  
  location ~* "^/foo.js$" {
    proxy_set_header Host assets.example.com;
    proxy_pass http://asset_server;
  }
}

upstream asset_server {
  server xxx.xxx.xxx.xxx:8000;
}

server {
  server_name assets.example.com;
  # Is this location block necessary if there's nothing else in this sever block?
  location / {
    proxy_pass http://asset_server;
  }
}

2-asset_server.conf

server {
  listen      8000;
  server_name assets.example.com;
  root        /home/foo/app/public;

  # Allow all assets to be cached by CloudFront and browsers.
  add_header Cache-Control public;

  # Default expires of 1 hour.
  expires 1h;

  # Only set far-future expires for assets with hashes. Note: since max_expire
  # contains curly brackets in the regexp, we must use quotes.
  location ~* "-[0-9a-f]{32}\.(ico|css|js|gif|jpe?g|png|eot|ttf|otf|woff|svg)(\.gz)?$" {
    expires max;
  }

  # Legacy support for /foo.js: it should be in /assets: /assets/foo.js:
  location ~* "^/foo.js$" {
    # Originally I had a rewrite... but I'd rather not redirect the client.
    # rewrite ^ $scheme://assets.example.com/assets/foo.js permanent;
    root /home/foo/app/public/assets;
  }
}