nginx.conf

server {
  error_page 503 @503;
  location = /503.html { }
  location = /down_for_maintenance.html { }

  location @503 {
    # If we are in maintenance mode and returning a 503,
    # show the special maintenance page
    if ( -f $document_root/SHOW_MAINTENANCE_PAGE ) {
      rewrite ^.*$ /down_for_maintenance.html;
    }

    # But if a backend has just returned a 503
    # because it's overloaded, show the 503 page
    if ( !-f $document_root/SHOW_MAINTENANCE_PAGE ) {
      rewrite ^.*$ /503.html;
    }
  }

  location / {
    # Check if the requested file is on the filesystem
    if (!-f $request_filename) {
      set $not_found 1;
    }

    # If SHOW_MAINTENANCE_PAGE exists in webroot,
    # we probably want to return a 503 maintenance page
    if (-f $document_root/SHOW_MAINTENANCE_PAGE) {
      set $maintenance 1;
    }

    # But if it's a static asset, we should skip
    # the 503, provided the file exists
    if ($request_filename ~* (css|jpg|png|gif)$) {
      set $maintenance $not_found;
    }

    if ($maintenance) {
      return 503;
    }
  }