Message-passing applications are one of the main components of reliable distributed systems; above all, they make it feasible to decouple the "when" and "where" of a problem from "who does it how." Event-driven applications go one step further and give you the causal chain of your system as a first-class citizen.
While the difference between an event-driven system and one that’s not event-driven is pretty clear and obvious to most software engineers, there are a gazillion ways to design such a system. Each has its own trade-offs and is useful in a specific context. The one I’m going to talk about in this article is modeling applications as state machines that compose and form a fractal-like structure — as each component is an application, and composed ones are also the same kind of application.