MQTT Quality of Service [MQTT-4.3]
QoS 1 - At least once delivery This quality of service ensures that the message arrives at the receiver at least once.
QoS 0 - At most once delivery The message is delivered according to the capabilities of the underlying network. No response is sent by the receiver and no retry is performed by the sender. The message arrives at the receiver either once or not at all.
QoS 2 - Exactly once delivery
This is the highest quality of service, for use when neither loss nor duplication of messages are acceptable. There is an increased overhead associated with this quality of service.
Note that, while this client supports QoS 2, the AWS IoT Core service does not support QoS 2 at time of writing (May 2020).
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