7.1.4 All instances of your webapp are in the same state.

If you do an update in one place you do it everywhere. It's simple. That way when customer A reports a bug you won't have to say, ``Did we update their server with patch foo?'' Because you know. Either all the apps have it or none of the apps have it. I know, this is an obvious rule. But people violate it all the time because we're human, and thus don't want to do boring work. This is yet another reason you need an automated deployment system.

K. Rhodes 2007-05-18