Domain-driven design

In recent years there has been a rebellion against elaborate development methodologies that burden projects with useless, static documents and obsessive upfront planning and design. Instead, the Agile processes, such as XP, emphasize the ability to cope with change and uncertainty.

Extreme Programming recognizes the importance of design decisions, but it strongly resists upfront design. Instead, it puts an admirable effort into communication and improving the project's ability to change course rapidly. With that ability to react, developers can use the "simplest thing that could work" at any stage of a project and then continuously refactor, making many small design improvements, ultimately arriving at a design that fits the customer's true needs.

This minimalism has been a much-needed antidote to some of the excesses of design enthusiasts. Projects have been bogged down by cumbersome documents that provided little value. They have suffered from "analysis paralysis" with team members so afraid of an imperfect design that they made no progress at all. Something had to change.

Unfortunately, some of these process ideas can be misinterpreted. Each person has a differente definition of "simplest". Continuous refactoring is a series of small redesigns; developers without solid design principles will produce a code base that is hard to understand or change - the opposite of agility. And although fear of unantecipated requirements often leads to overengineering, the attempt to avoid overengineering can develop into another fear: a fear of doing any deep design thinking at all.

In fact, XP works best for developers with a sharp design sense. The XP process assumes that you can improve a design by refactoring, and that you will do this often and rapidly. But past design choices make refactoring itself either easier or harder. The XP process attempts to increase team communication, but model and design choices clarify or confuse communication.

This book intertwines design and development practice and illustrates how domain-drive design and Agile development reinforce each other. A sophisticated approach to domain modeling within the context of an Agile development process will accelerate development. The interrelationship of process with domain development makes this approach more practical than any treatment of "pure" design in a vacuum.

Eric Evans, "Preface", in Domain-Driven Design: Tackling Complexity in the Heart of Software, xxiii.