Applications that receive messages should acknowledge their receipt by calling the session's acknowledge method. As in the case of sending messages, acknowledged transfer of messages to receivers provides at-least-once reliability, which means that the loss of the connection or a client crash does not result in lost messages; durable messages are not lost even if the broker is restarted. Some cases may not require this however and the reliability can be controlled through a link property in the address options (see Table 1.3, “Link Properties”).

The acknowledge call acknowledges all messages received on the session (i.e. all message that have been returned from a fetch call on a receiver created on that session).

The acknowledge call also support an optional parameter controlling whether the call is synchronous or not. A synchronous acknowledge will block until the server has confirmed that it has received the acknowledgement. In the asynchronous case, when the call returns there is not yet any guarantee that the server has received and processed the acknowledgement. The session may be queried for the number of unsettled acknowledgements; when that count is zero all acknowledgements made for received messages have been successful.