by Martin Fowler
April 2003

There are many things that make software development complex. But the heart of this complexity is the essential intricacy of the problem domain itself. If you’re trying to add automation to complicated human enterprise, then your software cannot dodge this complexity— all it can do is control it.

The key to controlling complexity is a good domain model, a model that goes beyond a surface vision of a domain by introducing an underlying structure, which gives the software developers the leverage they need. A good domain model can be incredibly valuable, but it’s not something that’s easy to make. Few people can do it well, and it’s very hard to teach.

Eric Evans is one of those few who can create domain models well. I discovered this by working with him—one of those wonderful times when you find a client who’s more skilled than you are. Our collaboration was short but enormous fun. Since then we’ve stayed in touch, and I’ve watched this book gestate slowly. It’s been well worth the wait.

This book has evolved into one that satisfies a huge ambition: To describe and build a vocabulary about the very art of domain modeling. To provide a frame of reference through which we can explain this activity as well as teach this hard-to-learn skill. It’s a book that’s given me many new ideas as it has taken shape, and I’d be astonished if even old hands at conceptual modeling don’t get a raft of new ideas from reading this book.

Eric also cements many of the things that we’ve learned over the years. First, in domain modeling, you shouldn’t separate the concepts from the implementation. An effective domain modeler can not only use a whiteboard with an accountant, but also write Java with a programmer. Partly this is true because you cannot build a useful conceptual model without considering implementation issues. But the primary reason why concepts and implementation belong together is this: The greatest value of a domain model is that it provides a ubiquitous language that ties domain experts and technologists together.

Another lesson you’ll learn from this book is that domain models aren’t first modeled and then implemented. Like many people, I’ve come to reject the phased thinking of “design, then build.” But the lesson of Eric’s experience is that the really powerful domain models evolve over time, and even the most experienced modelers find that they gain their best ideas after the initial releases of a system. I think, and hope, that this will be an enormously influential book. One that will add structure and cohesion to a very slippery field while it teaches a lot of people how to use a valuable tool. Domain models can have big consequences in controlling software development— in whatever language or environment they are implemented.

One final yet important thought. One of things I most respect about this book is that Eric is not afraid to talk about the times when he hasn’t been successful. Most authors like to maintain an air of disinterested omnipotence. Eric makes it clear that like most of us, he’s tasted both success and disappointment. The important thing is that he can learn from both—and more important for us is that he can pass on his lessons.

Martin Fowler
April 2003