Now that we understand how connection, subscription, and publication work in combination with topic names and topic filters with wildcards, we can dive deep into the QoS levels. So far, we have analyzed how both subscription and publication work with a QoS level equal to 0. Now, we will understand what this number means and how things work when we use the other available QoS levels for publication and subscription.
Remember that publication involves publishing from the MQTT client to the MQTT server and then from the server to the subscribed client. It is very important to understand that we can publish with a QoS level and subscribe with another QoS level. Hence, there is a QoS level for the publish process between the publisher and the MQTT server and another QoS level for the publish process between the MQTT server and the subscriber...