apache

<VirtualHost *:8000>
  ServerAlias *

  DocumentRoot /var/www/
  <Directory /var/www/>
    Options FollowSymLinks
    AllowOverride None
    Order allow,deny
    Allow from all
  </Directory>

  RewriteEngine On

  RewriteCond %{DOCUMENT_ROOT}/503.html -f
  RewriteCond %{SCRIPT_FILENAME} !503.html
  RewriteRule ^.*$ /503.html [L]
</VirtualHost>

haproxy

backend my-web-app
  server service-unavailable 127.0.0.1:8000 weight 1 backup
  server i-eb1c5083 10.154.148.222:80 weight 1 check
  server i-f41c531e 10.141.158.72:80 weight 1 check

listen my-web-http
  bind :80
  default_backend my-web-app