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Message QoS

The Quality of Service (QoS) level is an agreement between the sender of a message and the receiver of a message that defines the guarantee of delivery for a specific message. There are 3 QoS levels in MQTT:

  • At most once (0)
  • At least once (1)
  • Exactly once (2).

When you talk about QoS in MQTT, you need to consider the two sides of message delivery:

  • Message delivery form the publishing client to the broker.
  • Message delivery from the broker to the subscribing client.

You can read more about the detail of QoS in MQTT from HiveMQ site.