Moved a COBOL warehouse and billing system onto a modern platform with strangler-fig seams and parallel runs - no big-bang weekend - while trucks kept rolling.
Warehouse management and billing still ran on COBOL, on hardware held together by a thin support contract, owned by two engineers close to retirement.
Every change sat in a request queue. The batch window set cutoff times customers no longer accepted.
Docs had drifted so far that the only trustworthy spec was the running system.
Two-week assessment: COBOL analysis to recover business rules, dependency maps, and a risk-ranked module list.
Characterization tests first: golden masters that pin current behavior - including the quirks the business still relies on - before any code moves.
Strangler-fig: pull billing out behind a stable interface; run old and new in parallel with nightly reconciliation until the numbers match for a full cycle.
Capture knowledge from the retiring engineers into docs and tests while they can still confirm intent.
Billing runs on the modern platform; the mainframe module is retired. Cutover happened when reconciliation stayed green, not because a calendar said so.
Batch-window limits gone for migrated work; changes ship in sprints instead of quarterly change requests.
Key-person risk turned into documented, tested code the team can own.
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