There is a plethora of documentation on the request-reply (often called request-response) pattern, since it is the pattern that most of HTTP works with. In simple terms: a client requests something from a server and the server replies. That's it! Our automation system will be a very basic implementation of this pattern pattern. We are not, for example, implementing a web browser. We will be using monogamous clients and servers, sharing a common protocol, with synchronous behavior, and residing on the same machine. The extent of our required request-reply knowledge is minimal.

So that's all you need to know about request-reply right now: the client requests something from the server, and the server replies. Rinse and repeat.