Core banking and APIs: living with the legacy for the next ten years

A banking core cannot be replaced quickly, and in many cases it does not need replacing at all. It can be flanked a piece at a time, moving the functions that lend themselves to it outside and leaving in the core the ones that belong there.

The method is known as the strangler pattern, and the hard part is not technical. It is deciding the scope: which function moves out first, under what contract with the rest of the system, and by what measure the move is declared successful.

Every move needs three things: a stable contract between old and new, a period in which both run on the same data, and a way back that can be triggered in minutes. Rollback is not a failure you planned for — it is the condition that makes the risk acceptable.

Start from the marginal functions and you learn the method with little at stake. Start from the heart of the system and you learn exactly the same things, but in production and with the board in the room.