ruby script/plugin install http://rails.sanityinc.com/plugins/maintenance_server/

After installation, a new script/maintenance_server command is available, which runs a small (Mongrel) web server instance listening on the specified port.

The web server will serve files under RAILS_ROOT/public as usual,while unconditionally returning a "503 Unavailable" server response for any other request, and rendering a nice apologetic HTML page at RAILS_ROOT/public/maintenance.html (provided).


Example
=======

The installation will create a new 'script/maintenance_server' script, and a reasonable default 'unavailable' page at 'public/maintenance.html'.

Start the maintenance_server on the port of your choice, as follows:

 ruby script/maintenance_server PORT

(You may want to use software such as daemontools or runit to ensure that the server is automatically started with your system and restarted if it fails or is killed.)

Given mongrel_rails back-ends on ports 8000, 8001 etc., and maintenance_server running on port 8020, sample configuration for Pound might look something like this:

  ListenHTTP
    Address 0.0.0.0
    Port    80
    Service
      BackEnd
        Address 127.0.0.1
        Port    8000
      End
      BackEnd
        Address 127.0.0.1
        Port    8001
      End
      ...  # other back-ends here
      Emergency
        Address 127.0.0.1
        Port    8020
      End
    End
  End

In the same scenario, one might start the excellent Pen load-balancer with the following command-line:

  pen -f -r -b 5 -e localhost:8020 \
    localhost:80 \
    localhost:8000 localhost:8001 localhost:8002 \
    ...

There is currently no advanced documentation for this plugin.