Like B is slightly out of date ( replication wise ) the service modify something, then A comes with change that modify the same data that you just wrote.

How do you ensure that B is up to date without stopping write to A ( no downtime ).

4.0 Freeze old system

4.1 Cut over application traffic to the new system.

4.2 merge any diff that happened between snapshot 1. and cutover 4.1

4.3 go live

to me, the above reduces the pressure on downtime because the merge is significantly smaller between freeze and go live, than trying to go live with entire environment. If timed well, the diff could be minuscule.

What they are describing is basically, live mirror the resource. Okay, that is fancy nice. Love to be able to do that. Some of us have a mildly chewed bubble gum, a foot of duct tape, and a shoestring.