This recipe doesn't introduce any new modules or concepts. It's here to provide you with a general idea of how to combine all the things you've learned in this recipe in a somewhat realistic context. Specifically, our context consists of code that manages clients that connect with us in some way.
Client [8] holds all information relevant to a connection. As a basic example, it currently contains the client's IP address. Other possibilities would be the client's username, location, device, ping, and so on.
The ConnectionHandler [14] itself holds a list, more specifically a HashMap, of the active connections, indexed by a unique ID. Analogous to that, it also stores the ID for the next connection.
We are using unique IDs instead of a Vec<Client> because clients might be able to connect, multiple times, to whatever service we are providing on the same device. The easiest example for this is multiple tabs open in a browser, all accessing the...