Confirms (aka Publisher Acknowledgements)

Using standard AMQP, the only way to guarantee that a message isn't lost is by using transactions -- make the channel transactional, publish the message, commit. In this case, transactions are unnecessarily heavyweight and decrease throughput by a factor of 250. To remedy this, a confirmation mechanism was introduced.

To enable confirms, a client sends the confirm.select method. Depending on whether no-wait was set or not, the broker may respond with a confirm.select-ok. Once the confirm.select method is used on a channel, it is said to be in confirm mode. A transactional channel cannot be put into confirm mode and once a channel is in confirm mode, it cannot be made transactional.

Once a channel is in confirm mode, both the broker and the client count messages (counting starts at 1 on the first confirm.select). The broker then confirms messages as it handles them by sending a basic.ack on the same channel. The delivery-tag field contains the sequence number of the confirmed message. The broker may also set the multiple field in basic.ack to indicate that all messages up to and including the one with the sequence number have been handled.

In exceptional cases when the broker is unable to handle messages successfully, instead of a basic.ack, the broker will send a basic.nack. In this context, fields of the basic.nack have the same meaning as the corresponding ones in basic.ack and the requeue field should be ignored. By nack'ing one or more messages, the broker indicates that it was unable to process the messages and refuses responsibility for them; at that point, the client may choose to re-publish the messages.

After a channel is put into confirm mode, all subsequently published messages will be confirmed or nack'd once. No guarantees are made as to how soon a message is confirmed. No message will be both confirmed and nack'd.

An example in Java that publishes a large number of messages to a channel in confirm mode and waits for the acknowledgements can be found here.

When will messages be confirmed?

The broker will confirm messages once:

Notes

The broker loses persistent messages if it crashes before said messages are written to disk. Under certain conditions, this causes the broker to behave in surprising ways.

For instance, consider this scenario:

  1. a client publishes a persistent message to a durable queue
  2. a client consumes the message from the queue (noting that the message is persistent and the queue durable), but doesn't yet ack it,
  3. the broker dies and is restarted, and
  4. the client reconnects and starts consuming messages.
At this point, the client could reasonably assume that the message will be delivered again. This is not the case: the restart has caused the broker to lose the message. In order to guarantee persistence, a client should use confirms. If the publisher's channel had been in confirm mode, the publisher would not have received an ack for the lost message (since the message hadn't been written to disk yet).