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Not all of a large system will be well designed.
Strategic Design
Partnered in Scandinavia with
Eric Evans
Making Models Work in Large Projects
Eric EvansDomain Language, Inc.
Not all of a large system will be well designed.
Ground Up Rebuild
Ground Up Rebuild
Ground Up Rebuild
Ground Up Rebuild
Year 1
Year 2
Year 3
Legacy Replaced
Exciting New Features
Let’s Refactor
Let’s Refactor
domain A sphere of knowledge, influence, or activity.
Define Domain
domain A sphere of knowledge, influence, or activity.
The subject area to which the user applies a program is the domain of the software.
Define Domain
Why bother with models?
The critical complexity of most software projects is in understanding the domain itself.
Critical Complexity Is
What is a model?
origindestinationcustoms clearance (opt)
Route Specification
Itinerary
{ordered}
*
0..1
{Itinerary must satisfy specification}
Routing Servicea Route Specification
an Itinerary satisfyingthe RouteSpecification
Cargo
vessel Voyage IDload dateload locationunload dateunload location
Leg
Chinese Map
Mercator Map
model A system of abstractions that describes selected aspects of a domain and can be used to solve problems related to that domain.
Define Model
model A system of abstractions that describes selected aspects of a domain and can be used to solve problems related to that domain.
Define Model
model A system of abstractions that describes selected aspects of a domain and can be used to solve problems related to that domain.
Define Model
A Model Serves a Particular Use.
A Model Serves a Particular Use.
• Not “as realistic as possible”
A Model Serves a Particular Use.
• Not “as realistic as possible”
• Useful relative to specific set of domain scenarios
ubiquitous languageA language structured around the domain model and used by all team members to connect all the activities of the team with the software.
Define Ubiquitous Language
context The setting in which a word or statement appears that determines its meaning.
Define Context
There are always multiple models.
Rowing 8
“Context Map”
communications commitment/capability of team(s)
cont
rol o
f all
asso
ciat
ed s
yste
ms
SEPARATE WAYS
ANTI-CORRUPTIONLAYER
Single BOUNDEDCONTEXT
CUSTOMER/SUPPLIER TEAMS
SHARED KERNEL
CONFORMIST
OPEN HOSTSERVICE
communications commitment/capability of team(s)
cont
rol o
f all
asso
ciat
ed s
yste
ms
SEPARATE WAYS
ANTI-CORRUPTIONLAYER
Single BOUNDEDCONTEXT
CUSTOMER/SUPPLIER TEAMS
SHARED KERNEL
CONFORMIST
OPEN HOSTSERVICE
cooperative upstream/downstream
communications commitment/capability of team(s)
cont
rol o
f all
asso
ciat
ed s
yste
ms
SEPARATE WAYS
ANTI-CORRUPTIONLAYER
Single BOUNDEDCONTEXT
CUSTOMER/SUPPLIER TEAMS
SHARED KERNEL
CONFORMIST
OPEN HOSTSERVICE
uncooperative upstream/downstream
Custom House Currency Exchange System
early 2006
Custom House Context Map
SPOT
TBS
Custom House Context Map
SPOT
TBS
D
U
Custom House Context Map
SPOT
TBS
D
U
Custom House “SPOT” Modelof Currency Exchange Deal
Object Interpretation of Implicit “TBS” Model
Explicit Translation
Custom House Currency Exchange System
late 2006
Map What Is
Push Translation to the Borders
Single, Unified Model within Any One Context
Single, Unified Model within Any One Context
(Where careful design is going on.)
Single, Unified Model within Any One Context
No duplication...
Single, Unified Model within Any One Context
No duplication...within a context!
Duplication between...
Single, Unified Model within Any One Context
No duplication...within a context!
Duplication between...AOK!
Ground Up Rebuild
Let’s Refactor
The Enterprise Model
One Ring to rule them all, One Ring to find them, One Ring to
bring them all, and in the darkness bind them
Not all of a large system will be well designed.
Distilling the Core Domain
Distilling the Core Domain
Generic Subdomains
Distilling the Core Domain
Generic Subdomains
Supporting Subdomains
Distilling the Core Domain
Generic Subdomains
Supporting Subdomains
Core Domain
Distilling the Core Domain
– What makes your system worth writing?
Distilling the Core Domain
– What makes your system worth writing?
– Why not buy it off the shelf?
Distilling the Core Domain
– What makes your system worth writing?
– Why not buy it off the shelf?
– Why not outsource it?
Distilling the Core Domain
– What makes your system worth writing?
– Why not buy it off the shelf?
– Why not outsource it?
Effort Distribution
Effort Distribution
Effort Distribution
Effort Distribution
Ground Up Rebuild
Year 1
Year 2
Year 3
Legacy Replaced
Exciting New Features
Which Part is Core?
Which Part is Core?
Responsibility Traps
…Or, Why do irresponsible programmers become heroes?
Let’s Hack
Let’s Hack
Which Parts are Core?
Which Parts are Core?
Responsibility Traps
• Building a platform to make other (lesser) programmers more productive.
• Cleaning up other people’s mess.
What’s a responsible designer to do?!!
How could we just build the core part?
Build Platform Based on Model
Build Platform Based on Model
Strategy
• Draw a Context Map.• Work with business leadership to define
Core Domain.• Design a platform that supports work in
the Core Domain.• Work with management to give freedom to
the Core Domain Platform Context.• Develop and model in the Core Domain.
Strategy
• Keep an eye on your Context Map.• Keep working with business leadership as
Core Domain narrows and shifts.• Guard your platform that supports work in the
Core Domain.• Keep working with management to maintain
freedom of the Core Domain Platform Context.• Keep developing and evolving the model in the
Core Domain.
Strategic DDD Resources
www.domaindrivendesign.org
Domain-Driven Design by Eric Evans
PART 4!
www.domainlanguage.com
