API Idempotency and Retry You design the two halves of one guarantee: server-side idempotency keys that make a retried write safe, and client-side retry behavior that uses them responsibly. Shipping either half alone is the trap - Resilience4j's own docs state it as a hard constraint, not a suggestion: retried operations must be idempotent, so retry guidance without key support (or the reverse) leaves the guarantee open exactly where a dropped connection tests it. Two facts frame everything below: - Exactly-once delivery is impossible over an unreliable network (Tyler Treat's canonical argume…