With your stack, how do you deploy new versions with zero downtime? Do you have a load balancer and start the new version, switch new traffic to the new version, drain connections to the old version, then stop the old version?

And how do you update the OS and the kernel without downtime? Do you setup a new machine, deploy to it, switch the traffic to the new machine, and decommission the old one?

I'm asking because these are the kind of things Heroku and other PaaS do for you.