We migrated a 22-year-old mainframe with the lights on

Vantatek's system of record was COBOL, undocumented, and processing four hundred thousand orders a day. Two previous rewrite attempts had been abandoned mid-flight — which is the normal outcome, because a rewrite asks an organisation to hold its breath for two years.

We didn't rewrite. We put an event gateway in front of the mainframe and moved one bounded context at a time behind it. Every context dual-wrote to both systems until the new path had proven itself against real production traffic — not staging traffic, which is a fiction.

The rollback flag stayed live for the entire fourteen months. We used it four times. That number is the whole argument: a migration where rollback is theoretical is a migration where nobody is willing to move fast.

Zero downtime, zero lost orders, 3.4× throughput. The unsexy conclusion is that big-bang rewrites don't fail on technology. They fail because they ask for a year of faith, and organisations don't have a year of faith to give.