RabbitMQ
Acknowledgements & Delivery Guarantees
Use consumer acks correctly so messages are never silently lost on failure.
RabbitMQ uses consumer acknowledgements to know when a message has been successfully handled. With manual acks (the safe default for important work), the broker keeps a delivered message in an "unacknowledged" state until the consumer sends a basic.ack. If the consumer crashes or its connection drops before acking, the broker requeues the message for another consumer — giving at-least-once delivery.
A courier requiring a signature on delivery. Until you sign (ack), the parcel is still "out for delivery" and will be re-attempted if the courier loses it. Sign too early at the door before checking the contents (auto-ack) and a damaged parcel is your problem.
Key Concepts
1
The dangerous alternative is auto-ack (acknowledge on delivery), which is effectively at-most-once: if the consumer dies mid-processing, the message is already considered delivered and is lost. A consumer can also basic.nack or basic.reject a message, optionally with requeue=false to send it to a dead-letter exchange instead of looping forever.
2
Because redelivery is possible, consumers should be idempotent — the same message may be processed more than once.