Introduction to DDD as presented at #sitnl 2018

1. Domain Driven
Design
Fred Verheul

2. DDD Europe 2018
2

3. Agenda
• What is Domain Driven Design?
• Strategic patterns
• Tactical patterns
• Toolkit
3

4. What is Domain Driven Design (DDD)
• Not a technology or methodology
• Set of principles & patterns for focusing the design effort where it matters
most
• It’s all about:
• Understanding of the domain (subject area) where the software will be applied
• Creating a highly expressive model of that domain
• Distilling “Ubiquitous Language”
• Prerequisite: on-going collaboration between developers + domain experts
• When to apply: complex problems, high expectations
4

5. Ubiquitous Language
5
Domain Expert
Analyst Developer
Developer
Ubiquitous
Language
Application
code
Test code
Specs and
Documentation
Whiteboard
discussions

6. Strategic Patterns: Bounded Context
6

7. Context Map(-ping)
7

8. Tactical patterns: Building Blocks
• Value Objects
• Entities
• Aggregate Roots
• Repositories
• Services
• Factories
• Domain Events
8

9. Value Objects vs Entities
Entities:
Objects with an identity and a state
State evolves as long as application lives
Generally with a DB counterpart
Identifier Equality
9
Value Objects:
Have no conceptual identity
Can be designed as immutable
Can be shared between different entities
Structural Equality

10. Aggregates
• Cluster of objects
• Consistency Boundary
• Master object = aggregate root
10

11. Example, part 1 (Alberto Brandolini)
User Story 1:
As a Customer
I want to place an order
In order to purchase some goods
11

12. Example, part 2 (Alberto Brandolini)
User story 2, Returning
Customer:
As a customer
I want to retrieve my profile
in order to place more orders
12

13. Example, part 3 (Alberto Brandolini)
User story 3, Different
shipping address:
As a customer
I want to specify a valid
shipping address
In order to ship to a different
destination
13

14. Example, part 4 (Alberto Brandolini)
User story 4, Editable
customer profile:
As a customer
I want to edit my profile
In order to update it if
needed
14

15. Example, part 5 (Alberto Brandolini)
User story 5, Specify Billing
and Shipping Address:
As a customer
I want to specify
independent billing and
shipping addresses
In order to deliver goods to
different locations
15

16. Example, part 6 (Alberto Brandolini)
User story 6, Track past
orders:
As a Legal Department
I want to track orders
In order to manage
litigations
16

17. Result of modeling to deeper insights
17

18. Toolkit
• Tactical patterns (discussed above)
• Property based testing
• CQRS (Command Query Responsibility Segregation)
• Event sourcing
• Hexagonal Architecture
• Refactoring (to domain objects)
• Event Storming
18

19. Domain Driven Design
19

20. Further reading
• Udi Dahan: http://udidahan.com
• On CQRS: http://udidahan.com/2009/12/09/clarified-cqrs/
• Martin Fowler: https://martinfowler.com/intro.html
• On Event Sourcing: https://martinfowler.com/eaaDev/EventSourcing.html
• Greg Young:
• On CQRS: https://cqrs.files.wordpress.com/2010/11/cqrs_documents.pdf
• Vaughn Vernon: https://vaughnvernon.co/
• On designing aggregates: https://vaughnvernon.co/?p=838
20

21. Further reading on property based testing
• http://blog.jessitron.com/2013/04/property-based-testing-what-is-it.html
• https://hypothesis.works/articles/what-is-property-based-testing/
• https://fsharpforfunandprofit.com/posts/property-based-testing/
• https://propertesting.com/book_foundations_of_property_based_testing.html
• http://www.erikschierboom.com/2016/02/22/property-based-testing/
21

22. Further reading
• Hexagonal Architecture:
• http://alistair.cockburn.us/Hexagonal+architecture
• https://www.infoq.com/news/2014/10/exploring-hexagonal-architecture
• Refactoring:
• https://refactoring.com/
22

23. Thank You
www.soapeople.com
info@soapeople.com
@SOAPEOPLE
Fred Verheul
Development Consultant
+31 6 3919 2986
fred.verheul@soapeople.com