Inside a Core Banking Migration

The bank had run the same core since 1997. Nobody on the team had been there when it was installed. The documentation was a folder of scanned printouts.

The strangler, patiently applied

They moved product by product, running the old and new cores in parallel and reconciling nightly for eighteen months. It was slow, expensive, and the only approach that has ever worked.

Big-bang core migrations have a success rate. It is not zero. It is close enough that no board should accept it.