Up to speed in domain driven design

Up to Speed in
Domain Driven Design
Rick van der Arend
rvdarend@sogyo.nl

Agenda
 What is Domain Driven Design?
 An example of a domain model
 The building blocks of a domain model
 Refactoring towards deeper insight
 What has been learned since the book?
 Wrap-up
SOFTWARE INNOVATORS 2

What is DDD?
“Putting the Domain Model to work”

DDD in one sentence
 Domain Driven Design is a style of software
development which focuses on removing translation
barriers between domain, development team and
design of working software using a ubiquitous
language supported by a well refined livingmodel.

Translation barriers
 Team talks to users and experts (if lucky)
 Write software that behaves as requested
 Names get translated back and forth
 Concepts diverge without anyone noticing
?

Domain, Team and Model
Ubiquitous Language

Origins of the acronym DDD
 Eric Evans wrote the book that gave the
acronym DDD an impulse in 2005. Aptly named:
Domain Driven Design
“Tackling Complexity in the heart of software”

An Example of a domain model

Domain model
Parallel to the equator
Parallel to the earth axis
Orthogonal to the sun’s plain of movement Turning during the year

From Phased to Iterations
Analysis Design Build
Iteration Iteration
Test

Start with the Model, don’t stop
Iteration Iteration
Domain Model
effort as part
of an iteration

The building blocks of a
Model-Driven Design

Entities & Value objects
 Entities have an unmistakeable identity
 Value objects are indentified by.. their value
 a Person
 a Name
 an Address
 This depends on.. of course.. Context!

Factories , Repositories, Aggregates
 Factories create, Repositories store
 The unit of work they work on is an aggregate
 The thing they create of find and hand over will
be a reference to the aggregate root

Services
 Services encapsulate... well… Services
 They are not part of the core domain, but linked
 Objects change into other objects

Layers.. and more
 UI > domain model > Data access
 Or.. domain model depends on nothing

Refactoring towards deeper
insight

Refactoring towards deeper insight
 Use patterns when appropriate
 Make Unit Tests
 Better yet, do TDD
 Better yet, do BDD
 DRY and YAGNI
 But: all a bit technical

Making Implicit Concepts explicit
 Listen to Language
 Scrutinize Awkwardness
 Contemplate Contradictions
 Read the Book
 Try, Try again
 Model the less-than-obvious: constraints,
processes, commands, relations, etc.

Strategic Design

Contents of chapter 4 as a model

Strategic Design: context rules!
 Be aware of different contexts
 Make a context map
 See how different bounded contexts relate
 Shared Kernel, Customer/Supplier development
teams, Conformist, Separate Ways, Partner
 Use an anti-corruption layer

Context Maps step-by-step
1. What models (or BBoM) do we know of? (draw
blob each and name it.)
2. Where does each apply? (Boundary in words.)
3. Where is information exchanged? (Connect)
4. What is the relationship?
(Upstream/downstream? Partner? Etc.)

What has been learned since
the book?

Highlights that Evans learned
 Building blocks are overemphasized
 But domain events added as a new type of block
 General advice
▫ Don’t spread modeling too thin
▫ Focus on the core domain
▫ Clean, bounded context (CM steps available)
▫ Iterative process
▫ Don’t bore your domain experts

Domain events taken further
 Greg Young has been promoting the idea of
Command and Query separation

Techniques to explore
 Exploratory Modeling (xM)
 Model Driven Development environments
 Language Workbenches
 Naked Objects and other such platforms

Quick Wrap-up

Summary
 DDD highlights aspects of software design
 It focuses on the ‘core domain’ and uses a
ubiquitous language, based on a domain model
 The ubiquitous language brings out flaws
 A Domain Model is normally made up of certain
types of building blocks
 Thinks about context too: take control where and
whenever possible

Resources for Further study
 http://www.domaindrivendesign.org
 Numerous articles on http://www.infoq.com
 http://www.slideshare.net/devnology/unleash-
your-domain-with-greg-young

DDD in C#
 Jimmy Nilsson applied it to a C# context. His
book is has an informal tone, but a bit noisy.
Applying DDD and Patterns
With Examples in C# and .NET

Contact
Rick van der Arend
rvdarend@sogyo.nl
030 - 220 22 16
Web: www.sogyo.nl
Blog: www.software–innovators.nl

Up to speed in domain driven design

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