Situation: Our core payments processing system was running on a legacy monolith with monolithic database locks that prevented us from supporting new international currencies, requiring a complete migration to a distributed microservice.
Task: I served as lead architect responsible for executing the migration across millions of active transactions with zero downtime, zero data loss, and no interruption to ongoing feature releases.
Action: I designed a multi-phase migration using the Strangler Fig pattern. First, we implemented an abstraction proxy in front of the payment endpoints that routed requests while verifying contracts. Second, we built the new microservice and initiated dual-writing: all transactions executed on the legacy system while asynchronously replaying against the new service to validate output equivalence and latency. We automated data reconciliation scripts that ran continuously to catch any discrepancies. Once we achieved 99.999% parity over two weeks, we gradually shifted read and write traffic using feature flags in 5%, 25%, 50%, and 100% increments.
Result: The migration completed successfully over three months with zero seconds of unplanned downtime and zero financial discrepancies. The new service enabled our international expansion on schedule, reducing transaction latency by 45%.