1. QoS stands for quality of service. It was introduced in Chapter 3, Getting Started with MQTT.
2. QoS 1 is a good compromise for our use case between reliability and performance. With QoS 1, we can be sure messages are delivered (if the other end also uses QoS 1 or 2).
3. MQTT is an open protocol, so there are libraries for every programming language, more or less. There are also a lot of pre-made third-party applications for Android, iOS, macOS, Windows, Linux, and the web, which you can use. All you have to do is specify the login credentials of the MQTT server you are using (in our case, https://shiftr.io/).
4. A client ID is basically the name of the MQTT client, as seen in the network. The MQTT server stores messages to be sent, as well as subscriptions for each client ID. Don't confuse this with the MQTT username and password: these are just for authentication.
5. There can be only one loop and one setup function for each Arduino sketch. If you combine various examples into one, you need to make sure that you integrate the code accordingly. Having two functions with the same name will result in a compile-time error.
6. There are many examples given in Chapter 3, Getting Started with MQTT, for MQTT apps on various platforms. You are missing out on all of the fun if you do not try them out. Third-party MQTT apps allow you to build your own user interfaces to control your smart devices without having to learn another programming language.
7. When using your own login on https://shiftr.io/, you can activate private namespaces, so other MQTT clients cannot see what you send and cannot interfere with your namespace.