The send method of a sender has an optional second parameter that controls whether the send call is synchronous or not. A synchronous send call will block until the broker has confirmed receipt of the message. An asynchronous send call will return before the broker confirms receipt of the message, allowing for example further send calls to be made without waiting for a roundtrip to the broker for each message. This is desirable where increased throughput is important.

The sender maintains a list of sent messages whose receipt has yet to be confirmed by the broker. The maximum number of such messages that it will hold is defined by the capacity of the sender, which can be set by the application. If an application tries to send with a sender whose capacity is already fully used up, the send call will block waiting for capacity regardless of the value of the sync flag.

The sender can be queried for the available space (i.e. the unused capacity), and for the current count of unsettled messages (i.e. those held in the replay list pending confirmation by the server). When the unsettled count is zero, all messages on that sender have been successfully sent.

If the connection fails and is transparently reconnected (see Section 1.10.2, “Connection Options” for details on how to control this feature), the unsettled messages for each sender over that connection will be re-transmitted. This provides a transparent level of reliability. This feature can be controlled through the link's reliability as defined in the address (see Table 1.3, “Link Properties”). At present only at-least-once guarantees are offered.