At Least Once

At Least Once is an MDP. With this pattern the sender would send the message, and have a Retry Mechanism in place in case sending the message fails. Though there is a mechanism that ensures the message is being sent, there is no mechanism in place to prevent the message from being processed more than once.

One example would be where one sends an HTTP request to a service that implements a business process: some steps could succeed, but if one of the steps causes a timeout exception, the sender will resend the message and the message could be processed more than once.

One could use this pattern in scenarios where it doesn’t matter if a message is processed more than once (because the operation is Idempotent), or where the receiver implements Duplicate Detection (thus achieving Exactly Once in a different way). Again, a Metric could be a good example: if we store twice that the disk was used for 65% at a certain point in time, that doesn’t really change the meaning of the data.