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I'm currently working on this process, replacing 40+ year Cobol systems with modern services. It's in private business, so we probably have more flexibility than in government, but a lot of the same principles apply.

What we don't do is look at the old code, document how it works, and then reproduce that. Prior to this legacy system being built the entire company worked with paper processes and documentation, so it was a paradigm shift for how the business worked. That system is slow to update, so how they work is heavily influenced by the business' thinking 40 years ago. Our replatforming project is seen as essential for the business' continual survival, so we're allowed to question processes, simplify where we can, and work as equals with the business in defining new processes. There are definitely hold-outs and resistance from some quarters, but once you launch some successes people start converting and accepting the process.

interesting. I guess extremely old systems are a different beast than just old ones.

it's also probably very different whether the system has been continuously adjusted and fixed for 40 years, or if it's just stayed there.

If a system has been continuously adjusted over time (like ours) we still work from first principles. Even though we have institutional knowledge in the form of people who have worked here for 40 years, it’s often impossible to know the reasoning behind why a change was made. Most ageing code bases contain redundant logic due to some situation in the past which no longer occurs, or a constraint imposed by a dependency which will now be removed.

For example, what was once a file based batch process meant that other processes had to wait and split their processes accordingly. Replace that with an event based system and everything can run contiously and a lot of the restrictions disappear.

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