Suppose your Website is up 24 / 7.
You need to upgrade but have zero downtime.
One way I was thinking you could do is to upgrade one server at a time. So say you had 8 servers, you bring one server offline. You do this by waiting for every single session on it to finish and then when it has no sessions, you bring it offline and upgrade it. Any user that hits this server then will get the upgraded version. You follow this process for all 8 servers.