How do I show a warning when server is over capacity to avoid server goes down?

Twitter sometimes shows an message: Twitter is over capacity

This is to prevent t开发者_如何转开发oo much pressure on the servers. Which avoids that the servers go down.

How do I implement this in my application?

Edit: I am NOT looking for a PHP specific solution.

I thing this can be easily achieved by using a separate software to watch the server status, and on to much pressure, show the specified message. This is very important in a cloud architecture, so you can easily launch new instances. I think Amazon uses CloudWatch for this. Also, you could use apache mod_status to watch the server, also using a separate software.

Hope this helps, Gabriel