Two previous rewrites had failed. We never attempted a cutover — we strangled the mainframe context by context, shipping value every fortnight for eighteen months.
Annual run cost of the scheduling estate, year over year.
With no cutover weekend and no feature freeze at any point.
Each extracted, tested and documented before migration.
Handled without a single customer-visible scheduling incident.
Lumen's scheduling mainframe handled 3.2 million shipments a month and encoded two decades of undocumented business rules. Two big-bang rewrites had been cancelled at a combined cost of $14M. The COBOL team was three people, two within four years of retirement.
Before writing anything we extracted business rules from the COBOL into an executable specification — 1,840 rules, each with a test, each reviewed by an operations lead.
All traffic moved behind a routing facade on day one. From then on, migrating a context meant flipping a route, and rolling back meant flipping it back.
Nine bounded contexts migrated in dependency order, each fully in production before the next began. Rating went first because it was the most isolated; scheduling went last.
Each context ran in parallel with the mainframe for at least three weeks, with automated output comparison, before the mainframe path was disabled.
“The first thing they delivered was a list of every business rule buried in our COBOL. Our own operations directors learned things about the company from that document.”
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