MessageAck Event

Fired when an incoming or outgoing message has completed all acknowledgment steps.

Syntax

Sub mqttcontrol_MessageAck(PacketId As Integer, Direction As Integer, Index As Integer, ResponseCode As Integer)

Remarks

The MessageAck event fires once an incoming or outgoing message with a QoS of 1 or 2 has successfully completed all acknowledgment steps as required by its QoS level. (Note that this event does not fire for messages with a QoS of 0 since they do not require acknowledgment.)

• PacketId: The Id of the original PUBLISH packet for the message.
• Direction: Shows whether the client (0) or the server (1) is sending the data.
• Index: The index at which the message resides in either the IncomingMessage* properties or OutgoingMessage* properties.
• ResponseCode: In MQTT 5, all response packets contain Reason Codes. This argument contains any code encountered in message acknowledgment packets (PUBACK, PUBREC, PUBREL, PUBCOMP).

Possible MQTT-specific values for ResponseCode are:

Inbound Message Processing

Incoming messages with a QoS of 1 follow these steps:

1. The message is added to IncomingMessage* when the control receives the PUBLISH packet.
2. The control sends a PUBACK (publish acknowledgment) packet in response.
3. The MessageAck event is fired.
4. The message is removed from IncomingMessage*.
5. The MessageIn event is fired.

Incoming messages with a QoS of 2 follow these steps:

1. The message is added to IncomingMessage* when the control receives the PUBLISH packet.
2. The control sends a PUBREC (publish received) packet in response.
3. The control waits to receive a PUBREL (publish release) packet.
4. The control sends a PUBCOMP (publish complete) packet in response.
5. The MessageAck event is fired.
6. The message is removed from IncomingMessage*.
7. The MessageIn event is fired.

Outbound Message Processing

Outgoing messages with a QoS of 1 follow these steps:

1. The control sends the PUBLISH packet, then adds the message to OutgoingMessage*.
2. The control waits to receive a PUBACK (publish acknowledgment) packet.
3. The MessageAck event is fired.
4. The message is removed from OutgoingMessage*.
5. The MessageOut event is fired.

Outgoing messages with a QoS of 2 follow these steps:

1. The control sends the PUBLISH packet, then adds the message to OutgoingMessage*.
2. The control waits to receive a PUBREC (publish received) packet.
3. The control sends a PUBREL (publish release) packet in response.
4. The control waits to receive a PUBCOMP (publish complete) packet.
5. The MessageAck event is fired.
6. The message is removed from OutgoingMessage*.
7. The MessageOut event is fired.

In MQTT 3.1.1, the RepublishInterval configuration setting, if set to a non-zero value (default), controls how long the control will wait to receive a PUBACK (for QoS 1) or PUBREC (for QoS 2) before automatically republishing an outgoing message. In MQTT 5, messages are only republished if the client is disconnected before receiving a PUBACK or PUBREC.