Fire and forget - sir, yes, sir!

QoS 0 is the minimal QoS level, often referred to as fire and forget. Messages are sent to the broker without confirmation that it has been sent on by the broker to subscribers. It still has all the guarantees of the TCP protocol, as do all forms of standard MQTT.

As can be seen in the following diagram, there is minimal communication overhead, which translates into minimal power requirements. Once sent to the broker, the message is deleted on the device (publisher). The broker immediately sends on to subscribers that have an open connection. Unlike the other QoS levels, the message is not stored for offline subscribers when QoS is set to zero.

QoS 0 is often used when there is a stable connection and disruption is unlikely, like in the case of a wired connection. It is also used when power constraints are more important than message delivery. In this case, either the resulting data is acceptable even with some messages lost, or the frequency of messages is high enough that if one is lost, then no big deal, another will be on the way shortly.