What at most once means
At most once delivery promises that a message is processed either zero times or one time, but never more. The system accepts the risk of loss to eliminate the risk of duplicates.
How it is achieved
The sender fires a message and does not retry on uncertainty.
- It sends once and forgets, or sends and gives up after the first attempt.
- If the acknowledgement is lost, it assumes the message is gone rather than resending.
Because there is no retry, a copy can never be delivered twice. The tradeoff is plain: when an ack is lost but the message actually arrived, the sender wrongly treats it as lost, yet the receiver processed it.
When it fits
At most once suits work where a missing item is cheap but a duplicate is harmful or confusing:
- Best effort metrics where one dropped sample does not matter.
- High volume telemetry where exact counts are unnecessary.
Key idea
At most once trades the possibility of loss for a firm guarantee of no duplicates by never retrying an uncertain send.