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QoS: The Quality of Service indicates the delivery guarantee of each message:
QoS = 0: The delivery of the message is not guaranteed. It is a kind of fire-and-forget delivery, where the receipt of the message by the client is not checked.
QoS = 1: The delivery of the message is guaranteed at least once, but the message can be delivered more than once if there are several delivery attempts. However, it is the most recommended QoS due to its reliability and resource consumption.
QoS = 2: The delivery of the message is guaranteed exactly once, so it is not necessary to worry about the receipt of duplicate messages.
Retained Message: The Broker stores the last retained message with its corresponding QoS for that topic. This way, each client subscribed to that topic will receive the retained message immediately after subscribing. Only one retained message can be stored for each topic.
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