Idempotency, Retries & Delivery Guarantees

Networks fail mid-request, so real systems retry — and retries are where correctness quietly breaks. Build intuition for at-least-once vs. at-most-once vs. 'exactly-once' (and why the last is a myth), idempotency keys and server-side dedup, exponential backoff with jitter, safe vs. unsafe retries, the Two Generals' Problem, and poison messages — including how to make a payment survive a duplicate without double-charging.