Replacing the Gearbox (while Driving Down the Motorway)

Upgrading systems might not be the most sexy of topics, but you can't put it off. And if you do, perhaps due to business pressures, then unfortunately the job will only get bigger when eventually you do get round to it. It's either that, or scrap the system.This is where we found ourselves, a small team of 2.5 FTE, looking after the core business system (orders, invoices and such like) for our company, with a multi-billion EUR turnover. And yet, being such a small team, we needed to be able to keep delivering on features for the business... the car had to keep driving down the motorway.Moreover, this wasn't just a little upgrade; we basically needed to rework the app to run on a new framework, in essence retrofitting Spring Boot underneath. (The Apache Causeway framework does the heavy lifting, sitting in between our domain app and Spring Boot).So this is a talk about how we managed to do it, by developing a number of techniques that we thought might be useful to pass along.