Domain Driven Design Quickly

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Domain-Driven
Design
Quickly

FREE ONLINE EDITION
(non-printable free online version)
If you like the book, please support
the author and InfoQ by
purch...

Domain-Driven Design
Quickly
Free Online Version.
Support this work, buy the print copy:
http://www.infoq.com/minibooks/do...

© 2006 C4Media Inc.
All rights reserved.
C4Media, Publisher of InfoQ.com Enterprise Software Development
Community
Part of...

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Contents
Preface: Why DDD Quickly?..................................................... iv
Introduction......................

Preface: Why DDD Quickly?
first heard about Domain Driven Design and met Eric Evans at
a small gathering of architects at ...

It is my hope that by publishing a short, quickly-readable
summary and introduction to the fundamentals of DDD and
making ...

1
Introduction
oftware is an instrument created to help us deal with the
complexities of our modern life. Software is just...

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2│DOMAIN DRIVEN DESIGN QUICKLY
method which has emerged and evolved over the last two
decades, but has crystallized more c...

1
3
What Is Domain-Driven Design
oftware development is most often applied to automating
processes that exist in the real ...

4│DOMAIN DRIVEN DESIGN QUICKLY
Software development is similar. We can’t just sit down and
type code. We can do that, and ...

WHAT IS DOMAIN DRIVEN DESIGN│5
keyboard into the computer to become code. We need to create
an abstraction of the domain. ...

6│DOMAIN DRIVEN DESIGN QUICKLY
model is the essence of the software, but we need to create ways
to express it, to communic...

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WHAT IS DOMAIN DRIVEN DESIGN│7
Another approach is the Agile methodologies, such as Extreme
Programming (XP). These method...

8│DOMAIN DRIVEN DESIGN QUICKLY
Building Domain Knowledge
Let’s consider the example of an airplane flight control system
p...

WHAT IS DOMAIN DRIVEN DESIGN│9
OK, the plane takes off from some place and touches down in
another. But what happens in th...

10│DOMAIN DRIVEN DESIGN QUICKLY
Talking with the controller about the routes airplanes follow,
you discover that actually ...

WHAT IS DOMAIN DRIVEN DESIGN│11
What is actually happening here? You and the domain experts
are talking, you are exchangin...

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12│DOMAIN DRIVEN DESIGN QUICKLY
domain during discussions with domain experts, and also help
construct a structure for fut...

2
13
The Ubiquitous Language
The Need for a Common Language
he previous chapter made the case that it is absolutely necess...

14│DOMAIN DRIVEN DESIGN QUICKLY
To overcome this difference in communication style, when we
build the model, we must commu...

THE UBIQUITOUS LANGUAGE│15
A core principle of domain-driven design is to use a language
based on the model. Since the mod...

16│DOMAIN DRIVEN DESIGN QUICKLY
Domain experts should object to terms or structures that are
awkward or inadequate to conv...

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THE UBIQUITOUS LANGUAGE│17
Developer: OK, then let’s call each of those points a fix,
because it’s a fixed point of Earth’...

18│DOMAIN DRIVEN DESIGN QUICKLY
Developer: That’s better. Now that I’m looking at it, I realize
something. When we are mon...

THE UBIQUITOUS LANGUAGE│19
It is also highly recommended for the developers to implement
the main concepts of the model in...

20│DOMAIN DRIVEN DESIGN QUICKLY
names, and show the relationships between them. It’s very easy
for everyone to follow what...

THE UBIQUITOUS LANGUAGE│21
It might be tempting to try to create one large diagram over the
entire model. However, most of...

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3
23
MODEL-DRIVEN DESIGN
he previous chapters underscored the importance of an approach
to software development that is ce...

24│DOMAIN DRIVEN DESIGN QUICKLY
final implementation. The good end result is not assured. Good
developers might pull toget...

MODEL-DRIVEN DESIGN│25
Developers will be forced to make some decisions on their own,
and will make design changes in orde...

26│DOMAIN DRIVEN DESIGN QUICKLY
something is wrong, it is identified at an early stage, and the
problem can be easily corr...

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MODEL-DRIVEN DESIGN│27
change to the model. Its effect must ripple through the rest of the
project’s activities accordingl...

28│DOMAIN DRIVEN DESIGN QUICKLY
The Building Blocks Of A Model-Driven Design
The following sections of this chapter will p...

MODEL-DRIVEN DESIGN│29
Layered Architecture
When we create a software application, a large part of the
application is not ...

30│DOMAIN DRIVEN DESIGN QUICKLY
in each activity, a program must be kept very simple or it
becomes impossible to understan...

MODEL-DRIVEN DESIGN│31
should be focused on core domain issues. It should not be
involved in infrastructure activities. Th...

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32│DOMAIN DRIVEN DESIGN QUICKLY
information, like temperature, it is quite possible to have two
distinct instances of the ...

MODEL-DRIVEN DESIGN│33
same identity to be considered the same by the system. If that
condition is not met, then the entir...

34│DOMAIN DRIVEN DESIGN QUICKLY
Value Objects
We have discussed entities and the importance of recognizing
entities early ...

MODEL-DRIVEN DESIGN│35
It is necessary to distinguish between Entity Objects and Value
Objects. It is not helpful to make ...

36│DOMAIN DRIVEN DESIGN QUICKLY
any consequences. If there is no identity, you can make as many
copies as you wish, and de...

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MODEL-DRIVEN DESIGN│37
Services
When we analyze the domain and try to define the main objects
that make up the model, we d...

38│DOMAIN DRIVEN DESIGN QUICKLY
Services act as interfaces which provide operations. Services are
common in technical fram...

MODEL-DRIVEN DESIGN│39
which adds a supplementary level of complexity. Those services
are even more difficult to separate ...

40│DOMAIN DRIVEN DESIGN QUICKLY
to obtain the template which corresponds to a reportID. This
template is stored in a file ...

MODEL-DRIVEN DESIGN│41
between them. The functional cohesion is achieved when all
parts of the module work together to per...

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42│DOMAIN DRIVEN DESIGN QUICKLY
Aggregates
The last three patterns in this chapter will deal with a different
modeling cha...

MODEL-DRIVEN DESIGN│43
understandable as possible. Most of the time it pays of to
eliminate or simplify relations from the...

44│DOMAIN DRIVEN DESIGN QUICKLY
But if the model was not carefully designed, there will be a high
degree of database conte...

MODEL-DRIVEN DESIGN│45
aggregate, it is simple to enforce the invariants because the root
will do that. It is much harder ...

46│DOMAIN DRIVEN DESIGN QUICKLY
Factories
Entities and Aggregates can often be large and complex – too
complex to create i...

MODEL-DRIVEN DESIGN│47
Creation of an object can be a major operation in itself, but
complex assembly operations do not fi...

48│DOMAIN DRIVEN DESIGN QUICKLY
when a client wants to create an object which belongs to an
Aggregate. The solution is to ...

MODEL-DRIVEN DESIGN│49
following constraints: the shortest route, the fastest route, the
cheapest route. The routes create...

50│DOMAIN DRIVEN DESIGN QUICKLY
remain simple and will serve their specific purpose without the
clutter of complex constru...

MODEL-DRIVEN DESIGN│51
Repositories
In a model-driven design, objects have a life cycle starting with
creation and ending ...

52│DOMAIN DRIVEN DESIGN QUICKLY
such code is scattered throughout the entire domain. At that
point the domain model become...

MODEL-DRIVEN DESIGN│53
may use different storage locations for different type of objects.
The overall effect is that the d...

54│DOMAIN DRIVEN DESIGN QUICKLY
parameters which represent the selection criteria used to select
an object or a set of mat...

MODEL-DRIVEN DESIGN│55
Another option is to specify a selection criteria as a
Specification. The Specification allows defi...

56│DOMAIN DRIVEN DESIGN QUICKLY
objects. It is not a creation from scratch, but a reconstitution of
an object which existe...

4
57
Refactoring Toward Deeper Insight
Continuous Refactoring
o far we have been talking about the domain, and the importa...

58│DOMAIN DRIVEN DESIGN QUICKLY
There are many ways to do code refactoring. There are even
refactoring patterns. Such patt...

REFACTORING TOWARD DEEPER INSIGHT│59
leaves the challenge of actually finding an incisive model, one
that captures subtle ...

60│DOMAIN DRIVEN DESIGN QUICKLY
risky, because ample refactoring may introduce behavioral
changes in the application.
To r...

REFACTORING TOWARD DEEPER INSIGHT│61
When building knowledge it is possible to run into
contradictions. What a domain expe...

62│DOMAIN DRIVEN DESIGN QUICKLY
We can add books to a bookshelf, but we should never add more
than its capacity. This can ...

REFACTORING TOWARD DEEPER INSIGHT│63
public class Bookshelf {
private int capacity = 20;
private Collection content;
publi...

64│DOMAIN DRIVEN DESIGN QUICKLY
The domain layer contains business rules which are applied to
Entities and Value Objects. ...

REFACTORING TOWARD DEEPER INSIGHT│65
Customer customer =
customerRepository.findCustomer(customerIdentiy);
…
Specification...

5
Preserving Model Integrity
his chapter is about large projects which require the combined
efforts of multiple teams. We ...

68│DOMAIN DRIVEN DESIGN QUICKLY
internal consistency of a model is called unification. An
enterprise project could have on...

PRESERVING MODEL INTEGRITY│69
Bounded Context
Each model has a context. When we deal with a single model,
the context is i...

70│DOMAIN DRIVEN DESIGN QUICKLY
A Bounded Context is not a Module. A Bounded Context
provides the logical frame inside of ...

PRESERVING MODEL INTEGRITY│71
the task at hand more carefully, we discover that the e-shop
application is not really relat...

72│DOMAIN DRIVEN DESIGN QUICKLY
However, breaking down the system into ever-smaller contexts
eventually loses a valuable l...

PRESERVING MODEL INTEGRITY│73
implementation. Continuous Integration applies to a Bounded
Context, it is not used to deal ...

74│DOMAIN DRIVEN DESIGN QUICKLY
together, and the entire system must work properly. If the
contexts are not clearly define...

PRESERVING MODEL INTEGRITY│75
Shared Kernel
Translation
Map
shared
When functional integration is limited, the overhead of...

76│DOMAIN DRIVEN DESIGN QUICKLY
Integrate a functional system frequently, but somewhat less often
than the pace of Continu...

PRESERVING MODEL INTEGRITY│77
a pure browser experience, while the other could be a rich GUI
application. Even if the repo...

78│DOMAIN DRIVEN DESIGN QUICKLY
changes to be done to the database, hurting the reporting team’s
activity. If the e-shoppi...

PRESERVING MODEL INTEGRITY│79
Conformist
A Customer-Supplier relationship is viable when both teams are
interested in the ...

80│DOMAIN DRIVEN DESIGN QUICKLY
model, and design without having to give a thought to the
supplier’s model. But this is no...

PRESERVING MODEL INTEGRITY│81
entangled hard to understand and hard to work with. Even if it
was well done, the legacy app...

82│DOMAIN DRIVEN DESIGN QUICKLY
achievement is that the client model remains pure and consistent
without being contaminate...

PRESERVING MODEL INTEGRITY│83
This can be a very simple object, with little functionality,
serving the basic need of data ...

84│DOMAIN DRIVEN DESIGN QUICKLY
from the user’s perspective this is one application, but from a
modeling and design point ...

PRESERVING MODEL INTEGRITY│85
The solution is to see the external subsystem as a provider of
services. If we can wrap a se...

86│DOMAIN DRIVEN DESIGN QUICKLY
When working with a large model, we should try to separate the
essential concepts from gen...

PRESERVING MODEL INTEGRITY│87
system will consider the Route as a generic subdomain. The
Core Domain of an application may...

88│DOMAIN DRIVEN DESIGN QUICKLY
Some parts of the model add complexity without capturing or
communicating specialized know...

PRESERVING MODEL INTEGRITY│89
of another domain to deal with. There is still the
inconvenience of integrating the outsourc...

6
DDD Matters Today:
An interview with Eric Evans
nfoQ.com interviews Domain Driven Design founder Eric Evans
to put Domai...

92│DOMAIN DRIVEN DESIGN QUICKLY
But now that basic level of web usage has largely been
assimilated, and projects are start...

DDD MATTERS TODAY│93
get/set prefixed accessors for all variables, etc.) produced terrible
objects. The tools were so cumb...

94│DOMAIN DRIVEN DESIGN QUICKLY
That is the Java world. Then you have the new-comers like
Ruby. Ruby has a very expressive...

DDD MATTERS TODAY│95
Among other things, they took context mapping and applied it to
evaluation of off-the-shelf software ...

96│DOMAIN DRIVEN DESIGN QUICKLY
About Eric Evans
Eric Evans is the author of "Domain-Driven Design: Tackling
Complexity in...

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We help ambitious software projects realize the potential of
domain-dri...

Domain Driven Design Quickly

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