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Categories of Message Delivery Reliability

at-most-once delivery means that for each message handed to the mechanism, that message is delivered zero or one times; in more casual terms it means that messages may be lost.
at-least-once delivery means that for each message handed to the mechanism potentially multiple attempts are made at delivering it, such that at least one succeeds; again, in more casual terms this means that messages may be duplicated but not lost.
exactly-once delivery means that for each message handed to the mechanism exactly one delivery is made to the recipient; the message can neither be lost nor duplicated.

The first one is the cheapest—highest performance, least implementation overhead—because it can be done in a fire-and-forget fashion without keeping state at the sending end or in the transport mechanism. The second one requires retries to counter transport losses, which means keeping state at the sending end and having an acknowledgement mechanism at the receiving end. The third is most expensive—and has consequently worst performance—because in addition to the second it requires state to be kept at the receiving end in order to filter out duplicate deliveries.

Direct quote from Akka, Message Delivery Reliability. Other introductions: Kafka Message Delivery Semantics or Distributed systems – theory.

Why at-least once semantics is popular

  • At-most-once semantics can only be used if it is acceptable to lose messages. To guarantee data delivery, exactly or at-least once semantics must be used.
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  1. Sep 16, 2014

    From https://twitter.com/trondhjort/status/511784945921310720:
    Some interpret Udi Dahan's blog post about Life without distributed transactions to indicate that NServiceBus can provide exactly once semantics without the use of expensive mechanisms like distributed transactions. If I interpret MS DTC correctly, this IS in fact distributed transactions. Whether it performs better than other implementations which support exactly once semantics, that is another question.