A persistent message is a message that must not be lost even if the broker fails. To make a message persistent, set the delivery mode to PERSISTENT. For instance, in C++, the following code makes a message persistent:

message.getDeliveryProperties().setDeliveryMode(PERSISTENT);

If a persistent message is delivered to a durable queue, it is written to disk when it is placed on the queue.

When a message producer sends a persistent message to an exchange, the broker routes it to any durable queues, and waits for the message to be written to the persistent store, before acknowledging delivery to the message producer. At this point, the durable queue has assumed responsibility for the message, and can ensure that it is not lost even if the broker fails. If a queue is not durable, messages on the queue are not written to disk. If a message is not marked as persistent, it is not written to disk even if it is on a durable queue.

When a message consumer reads a message from a queue, it is not removed from the queue until the consumer acknowledges the message (this is true whether or not the message is persistent or the queue is durable). By acknowledging a message, the consumer takes responsibility for the message, and the queue is no longer responsible for it.