Reactive Microsystems

Chapter 4. Events-First Domain-Driven Design

The term Events-First Domain-Driven Design was coined by Russ Miles, and is the name for set of design principles that has emerged in our industry over the last few years and has proven to be very useful in building distributed systems at scale. These principles help us to shift the focus from the nouns (the domain objects) to the verbs (the events) in the domain. A shift of focus gives us a greater starting point for understanding the essence of the domain from a data flow and communications perspective, and puts us on the path toward a scalable event-driven design.

Focus on What Happens: The Events

Here you go, Larry. You see what happens? You see what happens, Larry?!

Walter Sobchak, Big Lebowski

Object-Oriented Programming (OOP) and later Domain-Driven Design (DDD) taught us that we should begin our design sessions focusing on the things—the nouns—in the domain, as a way of finding the Domain Objects, and then work from there. It turns out that this approach has a major flaw: it forces us to focus on structure too early.

Instead, we should turn our attention to the things that happen—the flow of events—in our domain. This forces us to understand how change propagates in the system—things like communication patterns, workflow, figuring out who is talking to whom, who is responsible for what data, and so on. We need to model the business domain from a data dependency and communication perspective.

As Greg Young, who coined Command ...