Watson IoT QoS1/2 Retries

Question

Does anyone know what the Watson IoT broker does if publishes a QoS1 o 2 message and doesn't receive the appropriate acknowledgement from the client? Does it implement a time out (say 20 seconds or so) and then resend the message again? It seems that some brokers do this while others only resend the message on a new connection (if retain is set to 1 of course). The MQTT spec is a little vague on this point.

Answer 1

the message would be considered in-flight for the client that hasn’t acknowledged it, and that message redelivery will only occur when that client disconnects and reconnects (but only if the client was clean session = 0)

For QoS1 and 2: At least once (QoS1)

With quality of service level 1 (QoS1), the message is always delivered at least once. If a failure occurs before an acknowledgment is received by the sender, a message can be delivered multiple times. The message must be stored locally at the sender until the sender receives confirmation that the message was published by the receiver. The message is stored in case the message must be sent again.

Exactly once (QoS2)

The "exactly once" quality of service level 2 (QoS2) is the safest, but slowest mode of transfer. The message is always delivered exactly once and must also be stored locally at the sender, until the sender receives confirmation that the message was published by the receiver. The message is stored in case the message must be sent again. With quality of service level 2, a more sophisticated handshaking and acknowledgment sequence is used than for level 1 to ensure that messages are not duplicated

MQTT keep alive interval

The MQTT keep alive interval, which is measured in seconds, defines the maximum time that can pass without communication between the client and broker. The MQTT client must ensure that, in the absence of any other communication with the broker, a PINGREQ packet is sent. The keep alive interval allows both the client and the broker to detect that the network failed, resulting in a broken connection, without needing to wait for the TCP/IP timeout period to be reached.

If your Watson IoT Platform MQTT clients use shared subscriptions, the keep alive interval value can be set only to between 1 and 3600 seconds. If a value of 0 or a value that is greater than 3600 is requested, the Watson IoT Platform broker sets the keep alive interval to 3600 seconds.

Retained messages

Watson IoT Platform provides limited support for the retained messages feature of MQTT messaging. If the retained message flag is set to true in an MQTT message that is sent from a device, gateway, or application to Watson IoT Platform, the message is handled as an unretained message. Watson IoT Platform organizations are not authorized to publish retained messages. The Watson IoT Platform service overrides the retained message flag when it is set to true and processes the message as if the retained message flag is set to false.