Gem/Plugin for sending system alert message to logged-in users via AJAX?

Anyone know of a gem/plugin that lets you send alerts to currently
logged in users of a Rails app via AJAX?

For example, I want to send an alert like "The system will be going
down in 5 minutes for a quick upgrade. Please save your work now and
log off."

I'm working on an AJAX-heavy Rails app in which the user doesn't
reload the entire page much, so just the standard rails flash messages
and warnings wouldn't be too useful. Perhaps the client side would use
AJAX to poll the server for these global system alerts.

It's no big task to write my own code for this, but this seems like
such a commonly needed feature that I'm wondering if there is already
a gem/plugin for it? Or is there another way that is commonly used to
send a global system message to users?

You would be better off running a push server instead if you will be using alerts on a very regular basis. There’s a few options you can consider:

http://blog.jcoglan.com/2010/02/02/faye-a-comet-client-and-server-for-node-js-and-rack/

http://juggernaut.rubyforge.org/

Best regards

Peter De Berdt