Discovering the value of
Domain-Driven Design
presented by Herman Lintvelt
(c) 2011 Polymorph Systems
Do the right thing, and do the thing right.
“When we first set out to write software, we never
know enough.”
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What are some of the prominent problems
you struggle with in your software
development projects?
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When faced with these problems or
complex requirements, how do you go
about to resolve them?
An Experiment
In which we make use of volunteers to do
some role-playing.
Scenario A
Scenario A
Scenario B
Scenario B
Scenario B
So what was the difference?
• Build up a model of the domain to
enhance understanding
• Based on this a common language was
being developed
• New understanding immediately fed into
model and language
Coming to terms...
• Domain:
• subject area to which solution must be
applied
• Domain Model:
• simplification of real world
• abstracts aspects relevant to solving
problem at hand
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not just a particular diagram
• Ubiquitous Language:
• everywhere the same language
What is DDD?
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DDD is both way of thinking and set of
priorities
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aimed to accelerate software dealing with
complex domains
primary focus on domain and domain logic
complex domain designs should be based on a
model
Two development practices prerequisites:
1. Development is iterative
2. Developers and domain experts have a close
relationship
Where does DDD fit?
• DDD is not a new idea!
• Central part of OO Analysis & Design
Model
Code
Langua
ge
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What are different nouns that can be used
for “car”?
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car
vehicle
bakkie
automobile
SUV
minibus
HMUV (High Mobility Utility Vehicle)
jamjar (Cockney slang)
Case Study 0
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5.1.7 [Req-XXXX] - Car Finder
Allow the end user to locate itself and
the vehicle.
Case Study 0
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- (SoapRequest*) GetLastKnownVehiclePosition:
(id <SoapDelegate>) handler
uiTokenID: (NSString*) uiTokenID
vehicleRegistrationNr: (NSString*) vehicleRegistrationNr;
- (SoapRequest*) UnitPlayback:
(id <SoapDelegate>) handler
uiTokenID: (NSString*) uiTokenID
dtStartDate: (NSDate*) dtStartDate
dtEndDate: (NSDate*) dtEndDate
liCustomerVehicleID: (long) liCustomerVehicleID;
Case Study 0
Case Study 1
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As a user of the XXXX iphone
application
I want to view my current location and
my car’s location on a map, with the
shortest route indicated visually
So that I can easily find my car
Case Study 1
Case Study 1
Case Study 1
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The direct benefits we gained in project:
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Simpler code
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Faster development
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better separation in layers
requirements, code & tests could be verified against
domain model
easier to debug & refactor in domain layer than in UI,
Persistence, Web-service layers
Focused on correct user stories
Better tested
Team interaction & communication improved dramatically
Utility of a Model
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In DDD, the Model has 3 basic uses:
1. The model and the heart of the design
shape each other
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makes model relevant, ensures the
analysis applies to final product
2. The model is a backbone of a language
used by all team members
3. The model is distilled knowledge
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team’s agreed way of structuring
domain knowledge and key domain
elements
DDD and DSLs
• DSL “allows solutions to be expressed
more clearly and closer to the domain”
• Model can be expressed as DSL Model
• DSL can be used to
Langu
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generate scaffolding code
Code
generate specification and test docs
define business (domain) rules
age
Ingredients of Effective
Modeling
1. Binding the model and the implementation
2. Cultivating a language based on the model
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can also include technical terms to be taught to
domain experts
3. Developing a knowledge-rich model
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Objects not just data scheme (anemic objects),
but has behaviour and enforce rules
4. Distilling the model
5. Brainstorming and experimenting
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Agile modeling, Model out Loud
Deep Domain Model
• When faced with complexity, do you
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deepen your domain knowledge
OR deepen your technology stack?
•Client:
Sad Example
“Our system is very expensive to maintain,
and performance is horrible”
•Me:
•Client:
“Why do you think?”
“Well, cause we’re using Java and it is
really slow and has a high learning curve”
•Me:
“Interesting. Tell me about the business
domain of the system...”
•Client:
“Well, we’re integrating with a legacy
system and use Java EE and added ESB to help
with that....”
Modeling out Loud
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People has a genius for spoken language
Unfortunately they usually don’t use the language
of domain model when speaking
We must harness linguistic ability to the modeling
effort
It complements the visual/spatial reasoning done
by sketching diagrams
As we model out loud, we find easier ways to say
what we need to say
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NB to then take those new ideas back to the
diagrams and code
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Discuss what you see as benefits of DDD
that applies to your current projects of
software development circumstances?
What benefits will you gain if you start
thinking and designing more domainfocused? (short, medium, long term?)
DDD Benefits
• Allows effective collaboration
• Earlier experience feed back into model
and understanding of whole team
• Helps focus requirement analysis
• Allows evolving of model over time
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“When we first set out to write software,
we never know enough.”
References
• Applying UML and Patterns, Prentice
Hall - Craig Larman
• Domain Driven Design, Addison-Wesley
- Eric Evans
If you want...
• contact me:
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herman@polymorph.co.za
http://www.polymorph.co.za
twitter: hermanlintvelt
blog:
http://blog.richclientgui.com
training: http://jtraining.co.za
•
OO A&D Course, CapeTown, end of
July
• Thank you