Glossary
At-Least-Once Delivery
At-least-once delivery guarantees a message will be delivered one or more times, but never zero — favoring the risk of a duplicate over the risk of losing the message entirely, since in most systems a duplicate is easier to handle safely than a silent loss. Achieving a stronger, exactly-once guarantee is possible but expensive enough in most distributed systems that at-least-once, paired with idempotency on the receiving end, is the more common practical choice.
This shifts the responsibility for correctness onto the receiver: a system built on at-least-once delivery has to treat every incoming message as a possible duplicate and process it accordingly, rather than assuming delivery-layer guarantees alone are sufficient to prevent a message's effect from happening more than once.