Lesson 18 / 32

Delivery Guarantees

Q: Most real-world 'exactly-once' systems are actually built from which combination?

At-least-once delivery + idempotent consumers. Delivering at least once and then deduping on the consumer side is the practical way most systems achieve exactly-once effect.

At-least-once, at-most-once, and the myth of exactly-once.

Three guarantees

At-most-once: send and forget, message may be lost but never duplicated. At-least-once: retry until acknowledged, message may be duplicated but never lost. Exactly-once: each message has effect exactly one time — the hardest and most expensive to guarantee end-to-end.

Registered mail vs a shout

At-most-once is shouting across a noisy room — fast, might not land. At-least-once is registered mail with return receipts — you keep resending until confirmed, so the recipient might get two copies. Exactly-once needs the recipient to also dedupe by a tracking number, effectively at-least-once delivery plus idempotent processing.

Quick check: Most real-world 'exactly-once' systems are actually built from which combination?

At-most-once delivery + retries
At-least-once delivery + idempotent consumers
A single global lock

Answer

At-least-once delivery + idempotent consumers — Delivering at least once and then deduping on the consumer side is the practical way most systems achieve exactly-once effect.