What does the At-Most-Once message processing mode mean and how does it differ from At-Least-Once?

The At-Most-Once message processing mode guarantees that each message will be processed no more than once, meaning message loss is possible, but duplicates are excluded. This is achieved by not acknowledging the message again, and it may be lost in case of failure.

In contrast, the At-Least-Once mode guarantees that each message will be processed at least once, meaning duplicates are possible, but message loss is excluded. For this, messages may be redelivered until an acknowledgment is received.

Example:

• At-Most-Once: if the message is not delivered due to a failure, it is simply lost.
• At-Least-Once: if acknowledgment is not received, the message will be resent, which may lead to duplicate processing.