Chapter 4

1. MQTT, or Message Queue Telemetry Protocol, is a lightweight messaging protocol frequently used in distributed IoT networks.
2. Check the QoS levels, making sure they are either level 1 or 2.
3. A Will message will be published on behalf of a client if that client abruptly disconnects from the broker without cleanly closing the connection first.
4. Both the published message and subscribing clients must use at least QoS level 1, which ensures messages are delivered one or more times.

5. Ideally, nothing should need to change in your Python code other than perhaps the broker host and port because MQTT is an open standard. The proviso is that the new broker is configured similarly to the broker being replaced – for example, both brokers are configured similarly to provide message retention or durable connection features to clients.
6. You should subscribe to topics in an on successful connection-type handler. This way, if the client loses its broker connection, it can...