Yes, both versions must be running at some point. The load balancer starts accepting connections on Server2 and stops accepting new connections on Server1. The

Yes, both versions must be running at some point. The load balancer starts accepting connections on Server2 and stops accepting new connections on Server1. Then, Server1 disconnects when all of its connections are closed. It could be different Servers or multiple Workers on one server. During that window, as the other comments said, migrations have to be backwards compatible.