Task at hand: Migrating Live Healthcare Services Without Dropping a Single Request When you're processing healthcare revenue cycle transactions worth millions of dollars daily, downtime isn't just inconvenient—it's financially catastrophic and potentially impacts patient care. This is the story of how we migrated 15+ microservices from AWS ECS to EKS across 6 engineering teams with zero downtime, zero rollbacks, and zero production incidents. The stakes: AR Finance and Posting Modernisation services handling real-time remittance processing for U.S. healthcare providers. The constraint: Absolute zero tolerance for downtime or data loss. The scope: Domain-wide cutover coordinating Rules Core, Payment Processing, Reconciliation, Analytics, Data Pipeline, and Platform teams. Why We Migrated: ECS Limitations at Scale Our ECS-based architecture was showing cracks: 1. Autoscaling Lag During Traffic Spikes ECS service autoscaling based on CloudWatch metrics had a 3-5 minute delay.…