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← all concepts

At-least-once delivery.

also: at-least-once · at least once

Messages are retried until acknowledged, so none are lost, but some may arrive more than once.

the two-minute refresher

The sender waits for an ACK and retries if none comes.
If the message arrived but the ACK was lost, the retry is a duplicate.
The industry default (webhooks, queues): pair it with idempotent consumers.

Where the course teaches it

W00 · Part 6System Design Terminology & Theorylearn it →W02 · Day 4Webhook Delivery Systemrevisit →W03 · Day 1Queue vs Streamrevisit →

Related concepts

At-most-once delivery
Exactly-once processing
Idempotency