The document provides an overview and instructions for practicing domain-driven design patterns including value objects, entities, aggregate roots, application services, and repositories. It includes examples of coding a money value object, post entity, post aggregate root, publish post application service, and post repository. The document recommends references for further reading on domain-driven design.

1. JUST POST IT!
CH4 - PRACTICE

2. TEXT
BEFORE WE START, YOU CAN CHECK THESE REFERENCES
▸ Java
▸ https://github.com/humank/EventStormingWorkShop
▸ https://github.com/VaughnVernon/IDDD_Samples
▸ https://github.com/ddd-by-examples/library
▸ C#
▸ https://github.com/ArthurChang01/DDDTW.CoffeeShop
▸ https://github.com/VaughnVernon/IDDD_Samples_NET
▸ https://www.wiley.com/go/domaindrivendesign Ch14, Ch15
▸ TypeScript
▸ https://github.com/stemmlerjs/ddd-forum
▸ Php
▸ https://github.com/justericgg/dddtw-laravel-coffee-shop

3. TEXT
ABOUT ME
▸ Fong
▸ Backend Engineer at Carousell
▸ DDDTW Speaker, Volunteer, Organizer
▸ IT 30-day blogs contest
▸ Think in GraphQL
▸ Think in DDD

4. OVERVIEW
▸ Value Object Practice
▸ Entity Practice
▸ Aggregate Root Practice
▸ Application Service
▸ Repository

5. VALUE OBJECT
PRACTICE
ALWAYS NEW

6. VALUE OBJECT
VALUE OBJECT KEY PROPERTIES
▸ Immutability
▸ Java:
fi
nal or @Value
▸ Java16: Record
▸ C#: readonly
▸ JS/TS: Object.freeze
▸ Attributed-Based Equality
▸ Java: @EqualsAndHashCode
▸ C#:
▸ JS/TS
▸ Replacibility (可替換性): always return new instances in state-changing methods

7. VALUE OBJECT
HOW TO CREATE VALUE OBJECTS IN VALID STATE (SELF-VALIDATING)
▸ Validate in the constructor
▸ Validate (or partial validate) in Factory method (useful when dealing with
different situations)
▸ Use code contracts to improve readability (tradeoff tech complexity)

8. VALUE OBJECT
VALID VALUE OBJECT CREATION - 1
▸ Validate in the constructor (before/after assignation)

9. VALUE OBJECT
VALID VALUE OBJECT CREATION - 2
▸ Validate (or partial validate) in Factory method (useful when dealing with
different situations)

10. VALUE OBJECT
VALID VALUE OBJECT CREATION - 3
▸ Use code contracts to improve readability (tradeoff tech complexity)

11. GET YOUR HANDS DIRTY👨💻

12. VALUE OBJECT
CODE YOUR FIRST VALUE OBJECT - MONEY OBJECT!
▸ Properties
▸ Currency (enum TWD, USD, JPY), Amount
▸ Invariants:
▸ 0 < Amount < INT_MAX
▸ Currency is not null
▸ Methods:
▸ Add(Money m): Money
▸ Subtract(Money m) Money
▸ Factories
▸ Static ofUSD(decimal amount): Money
▸ Static ofTWD(decimal amount): Money
https://github.com/tdd-best/tdd-by-example-the-money/blob/master/src/main/java/money/Money.java

13. VALUE OBJECT
CODE YOUR FIRST VALUE OBJECT - MONEY OBJECT! PART 1
▸ Properties
▸ Currency (enum TWD, USD, JPY)
▸ Amount
▸ Invariants:
▸ 0 < Amount < 1M
▸ Currency is not null
▸ Test Cases
▸ 10 TWD = 10 TWD
▸ 10 TWD != 20 TWD
▸ 10 TWD != 10 USD
▸ Hints
▸ Properties are immutable (java 16: record, java:
fi
nal, @value, c#: readonly, ts: readonly)
▸ Override equals function

14. VALUE OBJECT
CODE YOUR FIRST VALUE OBJECT - MONEY OBJECT! PART 2
▸ Methods:
▸ Add (Money augend) -> Money
▸ Cannot add money in different currency
▸ The result cannot be over 1 M
▸ Subtract (Money subtrahend) -> Money
▸ Cannot subtract money in different currency
▸ The result cannot be lower and equal to 0
▸ Test Cases
▸ 10 TWD + 10 TWD = 20 TWD
▸ 10 TWD + 20 USD -> Exception!
▸ 10 TWD - 5 TWD = 5 TWD
▸ 10 TWD - 1 USD -> Exception!
▸ 10 TWD - 20 TWD -> Exception!
https://github.com/tdd-best/tdd-by-example-the-money/blob/master/src/main/java/money/Money.java

15. VALUE OBJECT
CODE YOUR FIRST VALUE OBJECT - MONEY OBJECT! PART 3
▸ Factories
▸ Static ofUSD(decimal amount): Money
▸ Static ofTWD(decimal amount): Money
▸ Static ofJYP(decimal amount): Money
https://github.com/tdd-best/tdd-by-example-the-money/blob/master/src/main/java/money/Money.java

16. GOOD JOB!
NEXT ROUND

17. VALUE OBJECT
POSTDATA
▸ Location: domain/model/PostData
▸ PostData
▸ Properties
▸ Subject
▸ Subject cannot be empty
▸ Body
▸ Body should below 100k words
▸ Methods:
▸ Change(Subject, Body): PostData
https://github.com/tdd-best/tdd-by-example-the-money/blob/master/src/main/java/money/Money.java

18. ENTITY PRACTICE
KEEP TRACKING

19. ENTITY
ENTITY KEY PROPERTIES
▸ Must have an ID that can be tracked down the path, and the ID is unique and
cannot be changed
▸ Usually has states
▸ Methods with rich behaviors
▸ Public getter/private setter

20. ENTITY
PROBLEMS WHEN YOU CREATE AN ENTITY
▸ How to make ID more explicit?
▸ When to generate the Entity ID?
▸ How to make sure

21. GET YOUR HANDS DIRTY👨💻

22. ENTITY
POST
▸ Location: domain/model/Post
▸ Properties
▸ ForumId: class ForumId (id: int)
▸ ID: class PostID (id: int)
▸ PostData
▸ creationDate
▸ modi
fi
edDate
▸ Public getter/private setter

23. ENTITY
POST FACTORY
▸ Static publishPost(forumId, postId, postData) -> Post

25. THEN, WHO’S THE AGGREGATE ROOT?

26. AGGREGATE ROOT &
REPOSITORY PRACTICE
ENTRANCE FOR ALL

27. AGGREGATE ROOT
AGGREGATE ROOT KEY PROPERTIES
▸ Must have an Entity acting as an aggregate root
▸ You can only use aggregate root to update the objects inside the aggregate
root

28. GET YOUR HANDS DIRTY👨💻

29. AGGREGATE & REPOSITORY
POST AS AGGREGATE ROOT
▸ Requirements:
▸ updateContent(PostData): void

30. APPLICATION SERVICE
THE CONDUCTOR
Aggregate Root
Repository
External Service

31. GET YOUR HANDS DIRTY👨💻

32. APPLICATION SERVICE
PUBLISH NEW POST
▸ Location: application/PostApplicationSvc
▸ Class PostApplicastionSvc
▸ publishNewPost(String subject, String body, int forumId)
▸ Post post = Post.publishPost(… … …. );
▸ You can just assign a random number to id for now

33. APPLICATION SERVICE
WHAT DO WE NEED NEXT?
▸ ID generator to encapsulate the implementation detail
▸ Persistent the aggregate root

34. REPOSITORY

35. REPOSITORY
REPOSITORY KEY PROPERTIES
▸ Relate to aggregate root, one aggregate root per repository
▸ Only create new methods when you need it in business!!!!
▸ Repository Interface in domain layer, Repository Implements in infra layer

36. GET YOUR HANDS DIRTY👨💻

37. REPOSITORY
POST REPOSITORY
▸ Location: domain/model/PostRepository
▸ Interface PostRepository
▸ nextId() -> PostId
▸ save(Post) -> void
▸ Location: infrastructure/persistence/InMemoryPostRepository
▸ Class InMemoryPostRepository implements PostRepository
▸ HashMap<PostId, Post> maps
▸ nextId() -> PostId
▸ save(Post) -> void

38. REPOSITORY
BACK TO APPLICATION SERVICES
▸ Id = postRepo.nextId();
▸ Post = Post.publishPost(id)
▸ postRepo.save(post)

39. THANKS

40. TEXT
REFERENCES
▸ Implementing Domain-Driven Design
▸ Practice, Pattern, Principle of Domain-Driven Design
▸ https://github.com/tdd-best/tdd-by-example-the-money/tree/master/src/main/
java/money
▸ https://www.petrikainulainen.net/programming/testing/junit-5-tutorial-writing-
nested-tests/
▸ https://softwareengineering.stackexchange.com/questions/367145/is-the-
builder-pattern-applicable-in-domain-driven-design