Product Details

• Amazon Sales Rank: #79962 in Books
• Published on: 2003-08-30
• Original language: English
• Number of items: 1
• Binding: Hardcover
• 560 pages

From the Back Cover

"This book belongs on the shelf of every thoughtful software developer."

--Kent Beck

"What Eric has managed to capture is a part of the design process that experienced object designers have always used, but that we have been singularly unsuccessful as a group in conveying to the rest of the industry. We've given away bits and pieces of this knowledge...but we've never organized and systematized the principles of building domain logic. This book is important."

The software development community widely acknowledges that domain modeling is central to software design. Through domain models, software developers are able to express rich functionality and translate it into a software implementation that truly serves the needs of its users. But despite its obvious importance, there are few practical resources that explain how to incorporate effective domain modeling into the software development process.

Domain-Driven Design fills that need. This is not a book about specific technologies. It offers readers 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 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 development.

Readers learn how to use a domain model to make a complex development effort more focused and dynamic. A core of best practices and standard patterns provides a common language for the development team. A shift in emphasis--refactoring not just the code but the model underlying the code--in combination with the frequent iterations of Agile development leads to deeper insight into domains and enhanced communication between domain expert and programmer. Domain-Driven Design then builds on this foundation, and addresses modeling and design for complex systems and larger organizations.Specific topics covered include:

• Getting all team members to speak the same language
• Connecting model and implementation more deeply
• Sharpening key distinctions in a model
• Managing the lifecycle of a domain object
• Writing domain code that is safe to combine in elaborate ways
• Making complex code obvious and predictable
• Formulating a domain vision statement
• Distilling the core of a complex domain
• Digging out implicit concepts needed in the model
• Applying analysis patterns
• Relating design patterns to the model
• Maintaining model integrity in a large system
• Dealing with coexisting models on the same project
• Organizing systems with large-scale structures
• Recognizing and responding to modeling breakthroughs

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.

About the Author

Eric Evans is an independent consultant specializing in object-oriented design and Extreme Programming. During his extensive career, he has worked on many projects developing large business systems. Eric has authored several articles on design patterns and has trained teams in Extreme Programming. His current focus is on leveraging a rich domain model to produce quality software.

0321125215AB04282003

Excerpt. © Reprinted by permission. All rights reserved.

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 ...

Customer Reviews

Re-introduces a new way to think about software design
If you have even been involved in a software project (a) as a developer and did not know what the end product is going to be used for or how it will be used or (b) as an architect who spent countless hours with your stakeholders and domain experts trying to figure out how to go about architecting your application, then you should read this book. Read it again after you have read it for the first time. This book is packed with pointers, information, tips, how-tos, "down to earth" practical samples, and even conversational examples that one could have while gathering requirements. Evans in his book fills a wide gap that we all tend to come across while designing software applications.

There are many software engineering processes out there, and each one tries to tackle the complexities of designing software applications for a given domain in its own way. Evans recognizes the tools and the processes that are popular in the industry, UML, Agile, and focuses on some aspects of the software engineering process that we tend to miss. He starts the book by talking about the importance of creating and having a Ubiquitous Language. There is a similar concept in the RUP, but not emphasizes as much - or at all. Evans goes into a great detail on why, from the inception of a project, it is important to have a common language and gives many pointers on what makes up the Ubiquitous Language for each project:

"Use the model as the backbone of a language. Commit the team to exercising that language relentlessly within the team and the in the code. Use the same language in diagrams, writing, and especially speech."

Parts II-IV of the book put domain-driven design in perspective, and show the reader thru examples and patterns, architectural patterns, design patterns and process patterns, the importance of having a consistent model that maps to the domain and how to go about achieving such model. In an essence, "Model-Driven Design discards the dichotomy of analysis model and design to search out a single model that serves both purposes".

Part II of the book, introduces the building blocks of a Model-Driven Design. This section, as with the others, takes popular patterns from the Gamma, Flower, or others and applies them to the topic at hand - Model-Driven Design. In that aspect, the reader can easily follow the text and relate to topic at hand. Evans uses the ever-popular Model-View-Controller (MVC) design pattern to get things going in part II. He then goes off to explain why the layered architecture approach is an important aspect of a Domain-Driven Design and how it would makes things simpler:

"[Layered Architecture] allows a model to evolve to be rich enough and clear enough to capture essential business knowledge and put it to work."

The author then goes into great detail in explaining the elements that express a model:
1) Entities: An object that is tracked thru different states or even across different implementations.
2) Value Objects: An attribute that describes the state of something else.
3) Services: Aspects of domain that are expressed as actions or operations, rather than objects.
4) Packages: Organize the objects and services.

What do you want to do after you have designed such elements? The creation and life cycle management of objects are discussed next in this book. Three patterns, mostly from the Gamma book, are used to manage the life cycle of objects:
1) Aggregates.
2) Factories.
3) Repositories.

Aggregates represent the hierarchy of objects or services and their interactions. Factories and Repositories operate of Aggregates and encapsulate the complexity of specific life cycle transitions.

Part III of the book talks about the things developers and architects need to do to achieve a Supple Design. Refactoring over and over represents the topic in this section:

"Each refinement of code and model gives developers a clearer view"

The author talks about a breakthrough point during the design that the "designers see the light" and both the domain experts and the designers, after many iterations, have finally come to this higher level of understanding of the domain and the value of refactoring exponentially increases after that.

Part IV of this book talks about a very important topic that we all have struggled with one time or another: the ability of the model and the modeling process to scale up to very complicated domains. It is great that we can model a small domain, but one goes about modeling an enterprise, which is most likely, too complex to model as a single unit? Low-coupling and high cohesion still applies here, but the goal is to not loose anything during the integration process. The author goes in to a great detail in this part to emphasize that even in large circumstances such as modeling an enterprise, every decision must have a direct impact on system development. Three different themes are covered in this section in order to assist with modeling of large units:
1) Context: the model has to be logically consistent throughout, without contradictory or overlapping definitions. For this theme, the author introduces the concept of a Bonded Context- a way that relationship to other context are defined a overlapping is then avoided.
2) Distillation: Reducing the clutter and focusing attention appropriately.
3) Large-scale Structure. The concept of Responsibility layers are introduced

In summary, Evans did a great job in writing this book, and filling it with useful ways of designing and architecting software applications that target a domain, which in most cases we do not know much about.

Reality Check - Yet Another Patterns Book
I bought this book due in part to the glowing reviews here on Amazon so I feel a duty to inject a bit of skepticism, now that I've read it.

5 stars for a technical book indicates to me a book of profound quality that really breaks through with penetrating insights -- The kind of book that makes me think, "Wow, this book has really brought my development practice into a renewed, sharper focus." It doesn't necessarily have to provide radically new material, but it does have to package whatever material it contains in a way that causes the gears in my head to shift around and reorganize themselves. Design Patterns is such a book. XP Explained is such a book. I don't think this one qualifies.

Some good points: The author makes a good case for agile development/extreme programming (close relationship with the customer, unit tests, refactoring...). He seems to believe there may be a tendency to over-emphasize the importance of code and to neglect design in such practices, which may or may not be true in industry at large. But in any case, his major thesis is that it is also important to consider the overall domain model and how well-aligned it is to the goals of the business. He proposes developing a common ("ubiqitous") language between developers and business users, and to unify the various traditional views of a software system (requirements, analysis model, design model, etc..) into one. The advice is quite wholesome and will hopefully promote bringing some harmony between the agile camp and the adherents of high-ceremony approaches such as RUP and CMM.

Some bad points: The book is rather wordy, and a lot of common-sense ideas are repeated at length. I don't feel that the patterns in the book are much more than re-statements of basic principles of OO design. I am not convinced that giving every possible variation on OO programming a fancy name is particularly helpful. Most of the patterns in this book come down to "produce a clean design that removes duplication and attempts to match the business domain." If you're new to OO, I suggest you'd be much better off reading some other books, such as GoF's Design Patterns, Fowler's Refactoring, Page-Jones' Object-Oriented Design in UML, and Kent Beck's XP Explained.

I give this book 3 stars because it's not a bad thing to read a book that makes you think about the importance of the business domain when programming. It's true that this emphasis, while fairly basic, does get lost in a world where specific technologies dominate good design and common sense. I don't think this book can really hurt -- although I have found the "declarative" approach it mentions can be very dangerous in inexperienced hands and can produce utterly unmaintainable code. It's not a bad effort, but it's not an earth shattering revelation either.

Read this book to graduate from programmer to designer
I think that this book along with Robert Martin's "Agile Software Development, Principles, Patterns, and Practices" and Martin Fowler's "Refactoring" are perhaps the three most fundamental prerequisites for making the leap in knowledge and maturity from object-oriented programming to true proficiency in object-oriented design. The books from Martin and Fowler cover the software solution design space and the core principles and patterns for making code that is resilient to change and easy to maintain. Eric Evans book covers the problem domain space and the abstraction skills that free programmers to "break out of the box" of the implementation domain and solution objects into the critical area the business domain and corresponding domain objects.

I once led a young software team and tried to convey the need for and essence of these skills to them, but I didnt have the right words and terms to do it for their level of experience. I wish this book had been available to me then because I think it would have made a real difference for that team.