QoS 1
also: MQTT QoS 1, At Least Once
QoS 1 is a Quality of Service level in MQTT messaging that guarantees a message is delivered at least once to the subscriber, with acknowledgment-based confirmation between broker and client.
QoS 1 (Quality of Service level 1) is a delivery guarantee in the MQTT (Message Queuing Telemetry Transport) protocol. At this level, the broker ensures a message reaches the subscriber at least once by requiring the subscriber to send an acknowledgment (PUBACK) back to the broker when the message is received.
If the broker doesn't receive an acknowledgment within a timeout period, it will retransmit the message. This means a subscriber might receive the same message multiple times, so applications using QoS 1 should be designed to handle potential duplicates.
Example: A smart home system publishing temperature sensor readings at QoS 1 ensures the subscriber receives every update, but may occasionally process the same reading twice if network issues cause retransmission.
QoS 1 is a middle ground between QoS 0 (fire-and-forget, no guarantee) and QoS 2 (exactly-once delivery, higher overhead).