Right, so let’s suppose you already segmented the data to two different backing datastores, and your monolith is now connecting to both of them instead of just the one. Now you can do the service migration, at which point you still run into the situation I’m discussing.
Cutovers are hard, to be sure. Ideally they should also be short (the time time a service undergoing mitosis spends talking to the old and new locations should be measured in days or hours or less).
Don't choose general data access patterns for the infrequent occurrence of cutover. Cutover is when you break a few rules and then immediately stop doing so. Build for everyday access patterns instead (which should be through the API of whatever owns the data--SQL is a powerful language and a really shitty API).
