hardScenarioCase StudyMidSeniorLead
You are mapping data from a legacy system into a replacement and the two define a customer differently. How do you work that out?
4 min readUpdated 2026-08-15
Answer
Short answer
Agree the business meaning and the grain of each entity before touching field names, profile the real data to find the overloaded and sentinel values, then write a mapping where every target field has a source, a rule, a default and a decision for records that cannot be mapped.
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Likely follow-ups
- The legacy system has 40,000 customer records and 6,000 look like duplicates. Who decides which survives, and how do you frame that decision?
- How would you handle a mandatory field in the target that simply has no source anywhere?
- What reconciliation would you give the finance team on the morning after go-live?
- The two systems will run in parallel for three months. What does that do to your mapping?
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