Synchronization is non-trivial, but the question is mostly how fine you need the synchronization to be. E.g., if you are doing a live upgrade using e.g Erlang or Nginx, you can sort-of decide when new processes will be served by the new server, but existing processes and requests may linger with old code until much later.
But there's at least 30 minutes of downtime per week per market (usually per day), and the vast majority of downtimes coincide during the weekend - so this is all moot discussion and needlessly complex solution. If you can afford the downtime, switch midnight GMT between Saturday and Sunday, when all markets are closed.
You're right, yet nearly everybody operates on a daily restart schedule. I guess it's more intuitive, but it's wrong. Still, nobody ever got fired for bouncing systems once a day!
But there's at least 30 minutes of downtime per week per market (usually per day), and the vast majority of downtimes coincide during the weekend - so this is all moot discussion and needlessly complex solution. If you can afford the downtime, switch midnight GMT between Saturday and Sunday, when all markets are closed.