Retry and Idempotency Are Not the Same — Confusing Them Costs Money

A common misconception among engineers is treating retry logic and idempotency as interchangeable concepts, but they are distinct and complementary properties. Retry logic resides on the client side and governs when to resend a failed request, while idempotency is a server-side guarantee that processing the same request multiple times produces the same outcome as processing it once. Without both working together via a shared idempotency key, payment systems risk charging customers multiple times during network timeouts or crashes. Proper implementation also requires a distributed lock and durable storage to prevent race conditions and data loss across server restarts. The transactional outbox pattern is recommended to atomically record both the payment result and the idempotency key, eliminating a critical failure window.