Building recurring payment systems involves far more complexity than calling a payment API on a schedule. Seven key backend challenges are covered: reliable payment scheduling (handling time zones, month-end edge cases, leap years), preventing duplicate charges via idempotency keys, handling failed payments with dunning workflows that distinguish temporary vs. permanent failures, maintaining consistent state across distributed services using the Outbox Pattern, correctly processing webhooks that may arrive late, out-of-order, or duplicated, supporting multiple payment models through abstracted billing plan classes, and monitoring payment systems with business-level metrics and alerting. Python code examples illustrate each pattern in a production-like context.
