It would depend on the program. If you've got a load balancer in front, or they're receiving requests through a database or message queue, then there won't be a

It would depend on the program. If you've got a load balancer in front, or they're receiving requests through a database or message queue, then there won't be an outage. Arguably anything where uptime is important should be setup this way. Upgrades can fail, and that will cause downtime anyway.