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system-design
real-time
networking

Message Delivery: Guarantees, Ordering & Idempotency

11 questions

At-most-once vs at-least-once vs "exactly-once," why duplicates and gaps happen, and how idempotency, dedup, sequence numbers, and per-key ordering tame them. The duplicate/dedup behavior and HTTP idempotency facts are verified in Node and against RFC 9110.