Delivery Semantics and the Exactly-Once Myth

1. Three Semantics, and the One You Cannot Buy

Every message system makes one of three promises about delivery, and the names are more confident than the guarantees.

• At-most-once. Each message is delivered zero or one times. Messages can be lost, never duplicated.
• At-least-once. Each message is delivered one or more times. Nothing is lost, duplicates are expected.
• Exactly-once. Each message is delivered precisely one time. This is the one that is usually claimed and rarely delivered.