QoS 0
also: QoS 0, at-most-once delivery, fire-and-forget
QoS 0 is the lowest quality-of-service level in MQTT messaging that offers 'at most once' delivery—messages are sent once without confirmation, so delivery is not guaranteed.
QoS 0 (Quality of Service level 0) is a message delivery guarantee in the MQTT protocol commonly used in IoT and Linux-based messaging systems. At this level, a publisher sends a message once to a broker and expects no acknowledgment; if the broker is offline or unreachable, the message is lost.
This level is fastest and uses minimal bandwidth, making it suitable for non-critical data where occasional message loss is acceptable—such as continuous sensor readings where losing one sample is inconsequential.
Example: A temperature sensor publishing to an MQTT broker with QoS 0 sends readings once; if the broker briefly disconnects, that reading is discarded rather than queued for later delivery.
MQTT also offers QoS 1 (at least once, with acknowledgment) and QoS 2 (exactly once, with two-phase handshake) for more reliable scenarios.