Applying Quality of Service to messages

MQTT provides three Quality of Service (QoS) levels for individual message delivery—I am emphasizing individual message delivery because QoS levels apply to the delivery of individual messages and not to a topic. This will become clearer as you work through the examples.

While you, as the developer, stipulate the QoS for your messages, it's the broker that is responsible for ensuring that the message delivery adheres to the QoS. Here is the QoS you can apply to a message and what they mean for delivery:

QoS level	Meaning	Number of messages delivered
Level 0	The message will be delivered at most once, but maybe not at all.	0 or 1
Level 1	The message will be delivered at least once, but perhaps more.	1 or more
Level 2	The message will be delivered exactly once.	1

Table 2 – Message QoS levels

You might be asking the question: Level...