Update website with 0 Downtime

Hi there,

I'd like to know if there's a tool i can use to help in the deployment of my websites. i know about bitbucket and so forth, but am looking for a way that when i need to update custom themes/plugins, i can do that without any downtime. eg. sometime am on facebook, or even wpmudev website and design structure would change without any form of downtime. what's the secrete lol.

Cheers,

Thomas