Domain-Driven Design: Tackling Complexity in the Heart of Software

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• Copyright 2004
• Dimensions: 7" x 9-1/4"
• Pages: 560
• Edition: 1st

• Book
• ISBN-10: 0-321-12521-5
• ISBN-13: 978-0-321-12521-7

Incorporate effective domain modeling into the software development process

Software design thought leader and founder of Domain Language, Eric Evans, provides a systematic approach to domain-driven design, presenting an extensive set of design best practices, experience-based techniques, and fundamental principles that facilitate the development of software projects facing complex domains. Intertwining system design and development practice, this book incorporates numerous examples based on actual projects to illustrate the application of domain-driven design to real-world software modeling and development.

• Domain Model: Part I outlines the goals of domain-driven development, defines terms, and gives an overview of the implications of using the domain model to drive communication and design
• Model-Driven Design: Part II condenses a core of best practices in object-oriented domain modeling into a set of basic building blocks and focuses on the kinds of decisions that keep the model and implementation aligned with each other, each reinforcing the other’s effectiveness
• Refactoring: Part III delves into modeling principles that can guide choices along the way, and techniques that help direct the search
• Strategic Design: Part IV explores a triad of principles that apply to the system as a whole: context, distillation, and large-scale structure

 Throughout the book, discussions are illustrated not with over-simplified, “” problems, but with realistic examples adapted from actual projects. With this book in hand, object-oriented developers, system analysts, and designers will have the guidance they need to organize and focus their work, create rich and useful domain models, and leverage those models into quality, long-lasting software implementations.

 “The book is a fun read. Eric has lots of interesting stories, and he has a way with words. I see this book as essential reading for software developers—it is a future classic.”

—Ralph Johnson, author of Design Patterns

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Table of Contents

Foreword.


Preface.


Acknowledgments.

I. PUTTING THE DOMAIN MODEL TO WORK.

1. Crunching Knowledge.

Ingredients of Effective Modeling.

Knowledge Crunching.

Continuous Learning.

Knowledge-Rich Design.

Deep Models.

2. Communication and the Use of Language.

UBIQUITOUS LANGUAGE.

Modeling Out Loud.

One Team, One Language.

Documents and Diagrams.

Written Design Documents.

Executable Bedrock.

Explanatory Models.

3. Binding Model and Implementation.

MODEL-DRIVEN DESIGN.

Modeling Paradigms and Tool Support.

Letting the Bones Show: Why Models Matter to Users.

HANDS-ON MODELERS.

II. THE BUILDING BLOCKS OF A MODEL-DRIVEN DESIGN.

4. Isolating the Domain.

LAYERED ARCHITECTURE.

Relating the Layers.

Architectural Frameworks.

The Domain Layer Is Where the Model Lives.

THE SMART UI “ANTI-PATTERN”

Other Kinds of Isolation.

5. A Model Expressed in Software.

Associations.

ENTITIES (A.K.A. REFERENCE OBJECTS).

Modeling ENTITIES.

Designing the Identity Operation.

VALUE OBJECTS.

Designing VALUE OBJECTS.

Designing Associations That Involve VALUE OBJECTS.

SERVICES.

SERVICES and the Isolated Domain Layer.

Granularity.

Access to SERVICES.

MODULES (A.K.A. PACKAGES).

Agile MODULES.

The Pitfalls of Infrastructure-Driven Packaging.

Modeling Paradigms.

Why the Object Paradigm Predominates.

Nonobjects in an Object World.

Sticking with MODEL-DRIVEN DESIGN When Mixing Paradigms.

6. The Life Cycle of a Domain Object.

AGGREGATES.

FACTORIES.

Choosing FACTORIES and Their Sites.

When a Constructor Is All You Need.

Designing the Interface.

Where Does Invariant Logic Go?

ENTITY FACTORIES Versus VALUE OBJECT FACTORIES.

Reconstituting Stored Objects.

REPOSITORIES.

Querying a REPOSITORY.

Client Code Ignores REPOSITORY Implementation; Developers Do Not.

Implementing a REPOSITORY.

Working Within Your Frameworks.

The Relationship with FACTORIES.

Designing Objects for Relational Databases.

7. Using the Language: An Extended Example.

Introducing the Cargo Shipping System.

Isolating the Domain: Introducing the Applications.

Distinguishing ENTITIES and VALUE OBJECTS.

Role and Other Attributes.

Designing Associations in the Shipping Domain.

AGGREGATE Boundaries.

Selecting REPOSITORIES.

Walking Through Scenarios.

Sample Application Feature: Changing the Destination of a Cargo.

Sample Application Feature: Repeat Business.

Object Creation.

FACTORIES and Constructors for Cargo.

Adding a Handling Event.

Pause for Refactoring: An Alternative Design of the Cargo AGGREGATE.

MODULES in the Shipping Model.

Introducing a New Feature: Allocation Checking.

Connecting the Two Systems.

Enhancing the Model: Segmenting the Business.

Performance Tuning.

A Final Look.

III. REFACTORING TOWARD DEEPER INSIGHT.

8. Breakthrough.

Story of a Breakthrough.

A Decent Model, and Yet….

The Breakthrough.

A Deeper Model.

A Sobering Decision.

The Payoff.

Opportunities.

Focus on Basics.

Epilogue: A Cascade of New Insights.

9. Making Implicit Concepts Explicit.

Digging Out Concepts.

Listen to Language.

Scrutinize Awkwardness.

Contemplate Contradictions.

Read the Book.

Try, Try Again.

How to Model Less Obvious Kinds of Concepts.

Explicit Constraints.

Processes as Domain Objects.

SPECIFICATION

Applying and Implementing SPECIFICATION.

10. Supple Design.

INTENTION-REVEALING INTERFACES.

SIDE-EFFECT-FREE FUNCTIONS.

ASSERTIONS.

CONCEPTUAL CONTOURS.

STANDALONE CLASSES.

CLOSURE OF OPERATIONS.

DECLARATIVE DESIGN.

Domain-Specific Languages.

A Declarative Style of Design.

Extending SPECIFICATIONS in a Declarative Style.

Angles of Attack.

Carve Off Subdomains.

Draw on Established Formalisms, When You Can.

11. Applying Analysis Patterns.
12. Relating Design Patterns to the Model.

STRATEGY (A.K.A. POLICY).

COMPOSITE.

Why Not FLYWEIGHT?

13. Refactoring Toward Deeper Insight.

Initiation.

Exploration Teams.

Prior Art.

A Design for Developers.

Timing.

Crisis as Opportunity.

IV. STRATEGIC DESIGN.

14. Maintaining Model Integrity.

BOUNDED CONTEXT.

Recognizing Splinters Within a BOUNDED CONTEXT

CONTINUOUS INTEGRATION.

CONTEXT MAP.

Testing at the CONTEXT Boundaries.

Organizing and Documenting CONTEXT MAPS.

Relationships Between BOUNDED CONTEXTS.

SHARED KERNEL.

CUSTOMER/SUPPLIER DEVELOPMENT TEAMS.

CONFORMIST.

ANTICORRUPTION LAYER.

Designing the Interface of the ANTICORRUPTION LAYER.

Implementing the ANTICORRUPTION LAYER.

A Cautionary Tale.

SEPARATE WAYS.

OPEN HOST SERVICE.

PUBLISHED LANGUAGE.

Unifying an Elephant.

Choosing Your Model Context Strategy.

Team Decision or Higher.

Putting Ourselves in Context.

Transforming Boundaries.

Accepting That Which We Cannot Change: Delineating the External Systems.

Relationships with the External Systems.

The System Under Design.

Catering to Special Needs with Distinct Models.

Deployment.

The Trade-off.

When Your Project Is Already Under Way.

Transformations.

Merging CONTEXTS: SEPARATE WAYS—SHARED KERNEL.

Merging CONTEXTS: SHARED KERNEL—CONTINUOUS INTEGRATION.

Phasing Out a Legacy System.

OPEN HOST SERVICE—PUBLISHED LANGUAGE.

15. Distillation.

CORE DOMAIN.

Choosing the CORE.

Who Does the Work?

An Escalation of Distillations.

GENERIC SUBDOMAINS.

Generic Doesn't Mean Reusable.

Project Risk Management.

DOMAIN VISION STATEMENT.

HIGHLIGHTED CORE.

The Distillation Document.

The Flagged CORE.

The Distillation Document as Process Tool.

COHESIVE MECHANISMS.

GENERIC SUBDOMAIN Versus COHESIVE MECHANISM.

When a MECHANISM Is Part of the CORE DOMAIN.

Distilling to a Declarative Style.

SEGREGATED CORE.

The Costs of Creating a SEGREGATED CORE.

Evolving Team Decision.

ABSTRACT CORE.

Deep Models Distill.

Choosing Refactoring Targets.

16. Large-Scale Structure.

EVOLVING ORDER.

SYSTEM METAPHOR.

The “Naive Metaphor” and Why We Don't Need It.

RESPONSIBILITY LAYERS.

Choosing Appropriate Layers.

KNOWLEDGE LEVEL.

PLUGGABLE COMPONENT FRAMEWORK.

How Restrictive Should a Structure Be?

Refactoring Toward a Fitting Structure.

Minimalism.

Communication and Self-Discipline.

Restructuring Yields Supple Design.

Distillation Lightens the Load.

17. Bringing the Strategy Together.

Combining Large-Scale Structures and BOUNDED CONTEXTS.

Combining Large-Scale Structures and Distillation.

Assessment First.

Who Sets the Strategy?

Emergent Structure from Application Development.

A Customer-Focused Architecture Team.

Six Essentials for Strategic Design Decision Making.

The Same Goes for the Technical Frameworks.

Beware the Master Plan.

Conclusion.
Appendix: The Use of Patterns in This Book.
Glossary.
References.
Photo Credits.
Index. 0321125215T08272003

Preface

Leading software designers have recognized domain modeling and design as critical topics for at least twenty years, yet surprisingly little has been written about what needs to be done or how to do it. Although it has never been clearly formulated, a philosophy has developed as an undercurrent in the object community, which I call "domain-driven design".

I have spent the past decade focused on developing complex systems in several business and technical domains. I've tried best practices in design and development process as they have emerged from the leaders in the object-oriented development community. Some of my projects were very successful; a few failed. A feature common to the successes was a rich domain model that evolved through iterations of design and became part of the fabric of the project.

This book provides a framework for making design decisions and a technical vocabulary for discussing domain design. It is a synthesis of widely accepted best practices along with my own insights and experiences. Projects facing complex domains can use this framework to approach domain-driven design systematically.

Contrasting Three Projects

Three projects stand out in my memory as vivid examples of the dramatic effect domain design practice has on development results. Although all three delivered useful software, only one achieved its ambitious objectives and delivered complex software that continued to evolve to meet ongoing needs of the organization.

I watched one project get out of the gate fast with a useful, simple web-based trading system. Developers were flying by the seat of their pants, but simple software can be written with little attention to design. As a result of this initial success, expectations for future development were sky-high. It was at this point that I was approached to work on the second version. When I took a close look, I saw that they lacked a domain model, or even a common language on the project, and were saddled with an unstructured design. So when the project leaders did not agree with my assessment, I declined the job. A year later, they found themselves bogged down and unable to deliver a second version. Although their use of technology was not exemplary, it was the business logic that overcame them. Their first release had ossified prematurely into a high-maintenance legacy.

Lifting this ceiling on complexity calls for a more serious approach to the design of domain logic. Early in my career, I was fortunate to end up on a project that did emphasize domain design. This project, in a domain at least as complex as the one above, also started with a modest initial success, delivering a simple application for institutional traders. But this delivery was followed up with successive accelerations of development. Each successive iteration opened exciting new options for integration and elaboration of functionality. The team way able to respond to the needs of the traders with flexibility and expanding capability. This upward trajectory was directly attributable to an incisive domain model, repeatedly refined and expressed in code. As the team gained new insight into the domain, the model deepened. The quality of communication improved among developers and between developers and domain experts, and the design, far from imposing an ever-heavier maintenance burden, became easier to modify and extend.

Unfortunately, not all projects that start with this intention manage to arrive at this virtuous cycle. One project I joined started with lofty aspirations to build a global enterprise system based on a domain model, but finally had a disappointing result. The team had good tools, a good understanding of the business and gave serious attention to modeling. But a separation of developer roles led to a disconnect between the model and implementation, so the design did not reflect the deep analysis that was going on. In any case, the design of detailed business objects was not rigorous enough to support combining them in elaborate applications. Repeated iteration produced no improvement in the code, due to uneven skill-level among developers with no clear understanding of the particular kind of rigor needed. As months rolled by, development work became mired in complexity and the team lost its cohesive vision of the system. After years of effort, the project did produce modest, useful software, but had given up the early ambitions along with the model focus.

Of course many things can put a project off course, bureaucracy, unclear objectives, lack of resources, to name a few, but it is the approach to design that largely determines how complex software can become. When complexity gets out of hand, the software can no longer be understood well enough to be easily changed or extended. By contrast, a good design can make opportunities out of those complex features.

Some of these design factors are technological, and a great deal of effort has gone into the design of networks, databases, and other technical dimension of software. Books have been written about how to solve these problems. Developers have cultivated their skills.

Yet the most significant complexity of many applications is not technical. It is in the domain itself, the activity or business of the user. When this domain complexity is not dealt with in the design, it won't matter that the infrastructural technology is well-conceived. A successful design must systematically deal with this central aspect of the software.

The premise of this book is that

1. For most software projects, the primary focus should be on the domain and domain logic.
2. Complex domain designs should be based on a model.
3. Domain-driven design is a way of thinking and a set of priorities, aimed at accelerating software projects that have to deal with complicated domains. To accomplish that goal, this book presents an extensive set of design practices, techniques and principles.

Design vs. Development Process

Design books. Process books. They seldom even reference each other. Each is a complex topic in its own right. This is a design book. But I believe that these two issues are inextricable if design concepts are to be put into successful practice and not dry up into academic discussion. When people learn design techniques, they feel excited by the possibilities, but then the messy realities of a real project descend on them. They don't see how to fit the new design ideas with the technology they must use. Or they don't know when to worry about a particular design aspect and when to let go in the interest of time. While it is possible to talk with other team members about the application of a design principle in the abstract, it is more natural to talk about the things we do together. So, while this is a design book, I'm going to barge right across that artificial boundary when I need to. This will place design in the context of a development process.

This book is not specific to a particular methodology, but it is oriented toward the new family of "Agile Development Processes". Specifically, it assumes a couple of process practices are in place on the project. These two practices are prerequisites for applying the approach in this book.

• Iterative development. The practice of iterative development has been advocated and practiced for decades, and is a corner stone of the Agile development methods. There are many good discussions in the literature of Agile development and Extreme Programming, among them, Cockburn1998 and Beck 1999.
• A close relationship between developers and domain experts. Domain-driven design crunches a huge amount of knowledge into a model that reflects deep insight into the domain and a focus on the key concepts. This is a collaboration between those who know the domain and those who know how to build software. Because it is iterative, this collaboration must continue throughout the project's life.

Extreme Programming (XP), conceived by Kent Beck, Ward Cunningham and others Beck2000, is the most prominent of the agile processes and the one I have worked with most. To make the discussion concrete, I will use XP throughout the book as the basis for discussion of the interaction of design and process. The principles illustrated are easily adapted to other Agile Processes.

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.

XP recognizes the importance of design decisions, but strongly resists upfront design. Instead, it puts an admirable effort into increasing communication, and increasing 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 has been a much-needed antidote to some of the excesses of design enthusiasts. Projects have bogged down in cumbersome documents that provided little value. They have suffered "analysis paralysis", so afraid of an imperfect design that they made no progress at all. Something had to change.

Unfortunately, some of these new process ideas can be easily misinterpreted. Each person has a different definition of "simplest". Continuous refactoring without design principles to guide these small redesigns developers can produce a code base hard to understand or change - the opposite of agility. And, while fear of unanticipated requirements often leads to over-engineering, the attempt to avoid over-engineering can develop into another fear: The fear of 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 design choices make refactoring itself easier or harder. The XP process attempts to increase team communication. But model and design choices clarify or confuse communication. What is needed is an approach to domain modeling and design that pulls its weight.

This book intertwines design and development practice and illustrates how domain-driven 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.

The Structure of This Book

The book is divided into four major sections:

Part I: Putting the Domain Model to Work presents the basic goals of domain-driven development that motivate the practices in later sections. Since there are so many approaches to software development, Part I defines terms, and gives an overview of the implications of placing the domain model in the role of driving communication and design.

Part II: The Building Blocks of Model-driven Design condenses a core of best practices in object-oriented domain modeling into a set of basic building blocks. The focus of this section is on bridging the gap between models and practical, running software. Sharing these standard patterns brings order to the design and makes it easy for team members to understand each other's work. Using standard patterns also establishes a common language, which all team members can use to discuss model and design decisions.

But the main point of this section is on the kind of decisions that keep the model and implementation aligned with each other, reinforcing each other's effectiveness. This alignment requires attention to the detail of individual elements. Careful crafting at this small scale gives developers a steady platform to apply the modeling approaches of Parts III and IV.

Part III: Refactoring Toward Deeper Insight goes beyond the building blocks to the challenge of assembling them into practical models that provide the payoff. Rather than jumping directly into esoteric design principles, this section emphasizes the discovery process. Valuable models do not emerge immediately. They require a deep understanding of the domain. That understanding comes from diving in, implementing an initial design based on a probably naive model, and then transforming it again and again. Each time the team gains insight, the model is transformed to reveal that richer knowledge, and the code is refactored to reflect the deeper model and make it's potential available to the application. Then, once in a while, this onion pealing leads to an opportunity to break through to a much deeper model, attended by a rush of profound design changes.

Exploration is inherently open-ended, but it does not have to be random. Part III delves into modeling principles that can guide choices along the way, and techniques that help direct the search.

Part IV: Strategic Design deals with situations that arise in complex systems, larger organizations, interactions with external systems and legacy systems. This section explores a triad of principles that apply to the system as a whole: Bounded Context, Distillation, and Large-Scale Structure. Strategic design decisions are made by teams, or even between teams. Strategic design enables the goals of Part I to be realized on a larger scale, for a big system or in an application that fits in an enterprise-wide network.

Throughout the book, discussions are illustrated with realistic examples, drawn from actual projects, rather than oversimplified "toy" problems.

Much of the book is written as a set of "patterns." The reader should be able to fully understand the material without concern about this device, but those who are interested in the style and format of the patterns can read Appendix A.

Who This Book Is Written For

This book is primarily written for developers of object-oriented software. Most members of a software project team can benefit from some parts of it. It will make most sense to people who are on a project, trying to do some of these things as they go through, or who have deep experience already to relate it to.Some knowledge of object-oriented modeling is necessary to benefit from this book. The examples include UML diagrams and Java code, so the ability to read those languages at a basic level is important, but it is unnecessary to have mastered the details of either UML or Java. Knowledge of Extreme Programming will add perspective to the discussions of development process, but the discussion should be understandable without background knowledge.

For an intermediate software developer, a reader who already knows something of object-oriented design and may have read one or two software design books, this book will fill in gaps and provide perspective on how object modeling fits into real life on a software project. It will help an intermediate developer make the jump to applying sophisticated modeling and design skills to practical problems.An advanced or expert software developer should be interested in the comprehensive framework for dealing with the domain. The systematic approach to design will help them in leading teams down this path. The coherent terminology will help them communicate with peers.

Readers of various backgrounds may wish to take different paths through the book, shifting emphasis to different points. I recommend all readers to start with the introduction to Part I, and Chapter 1. This book is a narrative, and can be read beginning to end, or from the beginning of any chapter. A skimmer who already has some grasp of a topic should be able to pick up the main points by reading headings and bolded text. A very advanced reader may want to skim Parts I and II, and will probably be most interested in Parts III and IV.

In addition to this core readership, the book will be of interest to analysts and to relatively technical project managers. Analysts can draw on the connection between model and design to make more effective contributions in the context of an "Agile" project. Analysts may also use some of the principles of strategic design to better focus and organize their work.

Project managers should be interested in the emphasis on making a team more effective and more focused on designing software meaningful to business experts and users. And, since, strategic design decisions are interrelated with team organization and work styles, these design decisions necessarily involve the leadership of the project, and have a major impact on the project's trajectory.

While an individual developer who understands domain-driven design will gain valuable design techniques and perspective, the biggest gains come when a team joins to apply a domain-driven design approach and move the domain model to the center of discourse of the project. The team members will share a language that enriches their communication and keeps it connected to the software. They will produce an implementation in step with the model, giving leverage to application development. They will share a map of how design work of different teams relates, and will systematically focus attention on the most features most distinctive and valuable to the organization.

A domain-driven design is a difficult technical challenge that can pay off big, opening opportunities just at the stage when most software projects begin to ossify into legacy.

Eric Evans, San Francisco, California, March 2003
http://domainlanguage.com

0321125215P04092003

Foreword

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 cannot 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

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