There can't be anything atomic in a distributed system. You can't even atomically hot upgrade it on a single VM anyway -- you instead load the new version of the module and let dispatcher know to route new calls into it, the same as you would do with a load balancer and a bunch of load bearing docker hosts, just inside your app.
erlang has a code_change function in the otp that allows the gen_server to update its current state and start using new code. No connections need be broken with clients, no long running processes need be stopped. Just updated in place.
It's a routing change in a sense that gen_server is routing function calls to the new module definition. I know about gen_server and code_change, the point was that conceptually the same mechanism, just on a different level of abstraction.
