At-Least-Once Delivery
A delivery semantic that favors avoiding message loss while allowing the same message to be delivered more than once when failures or acknowledgement uncertainty occur.
When a producer or broker cannot know with certainty whether processing completed, redelivery is the safer choice for avoiding loss; duplicates are the cost. Consumers in at-least-once systems are therefore commonly designed to be idempotent or to use explicit deduplication keys.
“At least once” does not imply that an external database side effect happens exactly once. If message acknowledgement and the transaction boundary belong to different systems, a failure window remains. Safe retry design describes the same uncertainty in request-based systems.