At-Least-Once Delivery

Problem

A broker sends a message to a consumer. The consumer processes it successfully—but crashes before acknowledging it. The broker doesn't know whether processing happened.

Solution — Redelivery

When uncertain, deliver the message again.

Message → Consumer ✓
ACK         ✗ lost
Message → Consumer again

This may produce duplicates.

Core idea: at-least-once delivery prefers possible duplication over silently losing uncertain work.