Migrating the American Express Payment Network, Twice

Summary

If you tuned in to Monster Scale Summit this year, you may have seen our talk on migrating the American Express Payments Network - not once, but twice — with zero customer-impacting downtime — meaning no transactions were interrupted and no planned maintenance windows were required during either migration. The session focused on how we moved live payments traffic reliably under strict operational constraints. If you missed it, the talk is available to watch on the Monster Scale Summit website. This article expands on the conference talk and dives deeper into the engineering decisions, tradeoffs, and lessons learned across both migrations. Context: The Payments Network The payments network is a mission-critical distributed system responsible for processing critical payments traffic, including live card authorization. It serves as the bridge between American Express merchants, acquirers, and issuers globally. This platform must be continuously available, operate at low latency, and handle large volumes of critical traffic. Migration Constraints In 2018, American Express began a multi-year modernization of our payments network, including migrating from a legacy platform to a new microservices-based architecture. A migration of this scale had to operate within several non-negotiable constraints: The migration had to be performed online, with no planned or unplanned downtime. The new platform had to reimplement existing payment processing logic; regressions in functionality were not acceptable. Latency, throughput, and resiliency characteristics had to remain consistent, and in some cases improve. Payment requests could not be dropped, delayed, or left unanswered. Not only did we need to migrate under these constraints once - we needed to do it twice. Migration #1: From the Legacy Payments Network to the New Platform The first migration involved transitioning live card authorization traffic from the legacy payments network to a new, modernized platform. While the payment...