How Idempotency and Retry Logic Keep Payment Systems From Double-Charging Users

Senior backend engineer Alexander Frolov has detailed the idempotency and retry architecture he built for ROSSTRAFFY's payment core, which processes over 20,000 transactions per day at 99.99% uptime. The core challenge in payment systems is that network failures can cause clients to retry requests the server already processed, risking duplicate charges if operations are not designed to be idempotent. Frolov's approach enforces idempotency at three layers — API, handler, and storage — treating each payment as a state machine where every transition is safe to apply more than once. The system uses exponential backoff, jitter, dead-letter queues, and load balancing across five or more acquirers to achieve 99.8% acceptance reliability. The architecture delivered measurable results, including a roughly 10% improvement in payment conversion, a 25% reduction in erroneous transactions, and 15% faster median processing times, all validated through A/B testing on live traffic.