22. The modernization tax

In a legacy modernization, a team replaces an old technology stack with a modern one, and keeps the behavior the business has built up over years. The people who could once explain the legacy system are often retiring, scarce or already gone. The legacy code is the only authoritative record of what the system actually does. The cost of working from that record by hand is the Modernization Translation Tax, and the tax has four components. The reverse-engineering tax is the work of finding out, from the legacy code, what the system does, which needs senior engineers. The lost-context tax covers business decisions the code does not show, such as deliberate edge cases and old workarounds. The validation-vacuum tax arises because nothing can test the old behavior automatically, so every migrated module needs a hand-built check that it behaves as before. The knowledge-disappearance tax arrives after the project, when the people who built up the understanding leave with the engagement. The first three components make each other worse. Without an executable contract that proves the new system behaves like the old one, nobody can show the migration is complete. A modernization that cannot be proven complete stalls before it can deploy.

On the site: The Modernization Version of the Problem · The Four Components · Why the Tax Compounds