Chapter 8. Reliability and Quality of Service

An acquiring message consumer (also known as a competing message consumer) is a message consumer who removes messages from a queue, and makes them unavailable to other consumers. When an acquiring message consumer fetches a message from the broker over a reliable link, the message is set to acquired. In the acquired state the message is not visible to other consumers. It is to all intents and purposes acquired by the consumer, but the broker maintains its copy in acquired state until the consumer acknowledges acquisition. At that point the broker considers the message reliably delivered, and will delete its copy.

When a message is sent to the broker over a reliable link, the sender maintains its local copy until the broker acknowledges receipt. At that time the sender deletes the local copy. When sending synchronously this causes the application to block until this exchange has taken place. When sending asynchronously these unacknowledged sent messages are stored in the sender replay buffer.

All links to queues are reliable by default. It is not necessary to explicitly request a reliable link when connecting to a queue.

To request an unreliable link, specify link: {'reliability': 'unreliable'} in the address for the receiver or sender. For example: