Migrating the American Express Payment Network, Twice

American Express migrated its mission-critical payments network twice—from a legacy platform to a new microservices-based architecture—with zero customer-impacting downtime. The migration operated under strict constraints including no dropped or delayed payment requests, no latency regressions, and continuous availability of live card authorization traffic. The system relies on a Global Transaction Router (GTR) managing long-lived TCP connections carrying ISO8583 messages between acquirers, issuers, and the payments processing platform.