Sports betting exchanges have been doing that for a very long time. There is never a good time to take the system down for maintenance - event settlements happen every few minutes, and live games with in-play betting are going on somewhere in the world at any given time.

Makes things damn hard indeed, because you have to truly learn asynchronicity, CQRS and complex live migrations. (Incidentally, engineers who have worked on such systems tend to be over-represented in extreme HA businesses.)