[sobani]

That reminds me of the "Strangler Fig" pattern where you replace a service by first sending the requests to both the old and new implementation so you can compare their outputs. Then only when you're confident the new service functions as expected do you actually retire the old service.

[CuriouslyC]

The key part of the strangler fig is the facade and gradual migration of capabilities rather than trying to do a rewrite and swap (which never ends well).