How do you replace a system that cannot be switched off?
Answer
Short answer
Put a routing facade in front of the old system, migrate one business capability at a time, and treat dual-write as a reconciliation problem rather than two inserts: name a writer of record at every moment, shadow-compare before flipping reads, and sequence slices so value lands early.
A strangler fig migration is useful when legacy migration has to happen while the old system still earns money. Good strangler fig work moves one capability at a time, routes traffic deliberately, and proves rollback before expanding the slice. Weak legacy migration treats the strangler fig as a branding exercise while the shared data model stays untouched. Strong legacy migration uses the strangler fig pattern to reduce risk, not to avoid making hard ownership decisions.
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Likely follow-ups
- Shadow comparison shows a small but persistent mismatch rate. How do you decide whether that blocks the read flip?
- Writes into the old system arrive from three batch jobs whose owners have left. What do you do?
- How do you migrate history when the two data models disagree about what a customer is?
- Funding is halved twelve months in. What do you cut, and what must survive?
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