Don’t rely on atomic commits when you don’t have atomic deploys. If X and Y are each running on multiple machines, you must plan for new X interacting with old

Don’t rely on atomic commits when you don’t have atomic deploys. If X and Y are each running on multiple machines, you must plan for new X interacting with old Y and vice versa, because that is going to happen at least while you are rolling the change out or back. This even goes for a Javascript app talking to backend hosts.