Reconstruct the behavior from three sources: the data it produces, the people who use it, and the code or configuration if anyone can read it. Work backward from the outputs, because the outputs are the requirements that actually matter. Expect undocumented rules, log every one you find, and plan a period of parallel running to catch the ones you missed.
Why interviewers ask this
Legacy replacement is a large share of analyst work, and this checks whether you have a method for archaeology. Interviewers want multiple evidence sources triangulated, output driven reconstruction, and above all humility: an assumption that hidden rules exist and a plan such as parallel running or reconciliation to catch them. Confidence that you would fully document it up front is a warning sign.
How to structure your answer
- Triangulate from data, users, and code or configuration.
- Work backward from outputs, since those are the real requirements.
- Interview the long serving users about the strange cases.
- Assume hidden rules exist and log every one you uncover.
- Plan parallel running or reconciliation to catch what you missed.
Example answer
I work backward from the outputs, because whatever it produces is what the business actually depends on, and the internal logic is only interesting where it changes an output. So I get samples of everything it emits, reports, files, interfaces, and I trace back what has to be true for each one. In parallel I talk to whoever has been using it longest and ask specifically about the weird cases, because that is where the hidden rules live. If anyone can still read the code or the configuration, that is the third source and it settles disputes. On one replacement we found a rounding rule buried in a stored procedure that nobody alive had known about, and it affected about a fifth of transactions by a small amount that finance had been reconciling manually for years. The thing I insist on is parallel running, because you never find every rule, and reconciliation against the old system is how you find the ones you missed before customers do.
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See how it worksFollow-up questions to expect
- How do you decide which legacy behavior to replicate and which to drop?
- How long would you run in parallel, and what do you compare?
- What do you do when the source code is genuinely unreadable?
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