This post is part of my collaborative research with Shinsei Bank on highly-evolvable enterprise architectures. It is licensed under the Creative Commons Attribution-ShareAlike 3.0 license. I am indebted to Jay Dvivedi and his team at Shinsei Bank for sharing with me the ideas developed here. All errors are my own.
The systems in a large bank handle huge volumes of transactions collectively worth many millions or billions of dollars, so errors or sabotage can cause serious damage in short order. For this reason, Jay often compares running bank systems to flying jumbo jets. To make matters worse, the jets must be maintained and modified in flight. Jay’s approach, which he demonstrated to great effect when he successfully migrated Shinsei from mainframes to inexpensive servers, focuses on building up new systems one component at a time in parallel with existing systems and achieving functional parity. Once parity has been reached, the old systems can be shut down.