Before rewriting the old system, discover what it knows.
Undocumented behavior is still behavior, especially to customers and operations.
Interpretation
A rewrite starts with a cleaner model and therefore risks excluding messy truths the old system absorbed. Observe real inputs, outputs, exceptions, and workflows before replacing it. The strangest rules often encode a contract that disappeared from documentation but not from reality.
Practice
- Capture production behavior with characterization tests.
- Interview support and operations before defining parity.
- Compare old and new outputs on real-shaped data.
Anti-pattern
Defining rewrite requirements solely from the current UI and a stale wiki.