Elevating Tactical DDD Patterns Through Object Calisthenics

After immersing yourself in the blue book and its red counterpart, attending DDD-focused conferences, and applying tactical patterns, you're left with a crucial question: How do I ensure my design is effective? Tactical patterns within Domain-Driven Design (DDD) serve as guiding principles for creating clear and manageable domain models. However, achieving success with these patterns requires additional guidance. Interestingly, we've observed that a set of constraints initially designed for training purposes remarkably aligns with effective pattern implementation, offering a more ‘mechanical’ approach. Let's explore together how Object Calisthenics can elevate the design of your tactical DDD patterns, offering concrete help for those venturing into DDD for the first time!

2. Dorra Bartaguiz
•VP Tech
•Co-author & illustrator
•Different Roles
•DEV/Architect
•Coach/trainer
•Teacher
•Consulting company
•Training centre
•TDD, BDD, DDD
•Pragmatic Architecture
•Pragmatic API Design

3. Entity
Service
Repository
Value Object
Aggregate
DDD tactical patterns

4. How can we ensure
DDD tactical patterns
are well implemented

5. A set of constraints
aligns with effective
pattern implementation
Object calisthenics

6. Calisthenics – Street Workout
•Greek : Kallos sthenos
•Beauty and courage•The art of training and bodybuilding
with bodyweight and without
equipment

7. Object Calisthenics – Jeff Bay
? Only One Level Of Indentation Per Method
? Don’t Use The ELSE Keyword
? Wrap All Primitives And Strings
? First Class Collections
? One Dot Per Line
? Don’t Abbreviate
? Keep All Entities Small
? No Classes With More Than Two Instance Variables
? No Getters/Setters/Properties

8. After several years of
experience and experimentation

9. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

10. Wrap all primitives
and strings

11. •Drawbacks of Primitive obsession
•Lake of clarity
•Error-proneness
•Code duplication
•Reduced flexibility
Why?

12. Identify business
concepts &
domain language

13. Refactor

14. Wrap all primitives
and strings
Identity and secure
domain concepts

15. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

16. First class
collections

17. Cow

18. Cattle

19. House
&
Neighbourhood

20. Ship
&
Fleet

21. Do we have a
distinct noun for
plural forms

22. First class collections

23. First class collections
Same type!

24. First class collections
Can be static!

25. First class collections

26. First class
collections
Enriched business
language
Rich model

27. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

28. Don’t
abbreviate!

29. Estimated Time of Arrival

30. Emergency Transportation Assistance
Medical Emergencies

31. Educational Technology Assessment
Teacher training

32. Don’t abbreviate

33. Don’t
abbreviate!
Explicit domain
concepts

34. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

35. No
getters/setters/
properties

36. •Drawbacks of separating data and behaviour
•Anaemic domain model
•Lack of immutability
•Higher coupling
•“Tell don’t ask” principle!

37. Move the behaviour into domain concepts

38. Secure your
domain concepts

39. Domain Language in code

40. No
getters/setters/
properties
Transpose the domain
language into your code
Secure your
domain concepts

41. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

43. •Law of Demeter
•Don’t talk to strangers, only talk to your immediate friends
•Tell, don’t ask

44. Tom Thumb
“Le petit poucet” in French

45. Use a fluent interface

47. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

48. Don’t Use The ELSE Keyword

49. •Hard to read
•“Arrow” antipattern

50. •Linear code, easy to read
•“The faster you can fail, the faster you can succeed”

51. Don’t Use The ELSE Keyword
Design differently
•Strategy pattern
•Null object pattern

52. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

53. Only One Level Of
Indentation Per Method

54. Only One Level Of Indentation Per Method
+1
indentation
=
+1
cyclomatic complexity
(minimum)

55. Only One Level Of Indentation Per Method

56. Only One Level Of
Indentation Per Method
•Explicit
intention
•Enrich model
•Use domain
language

57. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

58. Keep All
Entities Small

59. Keep all entities small
Make big things from small things
in the same way. That’s the power
of composition.
Scott Wlaschin

60. Keep all entities
small, even
aggregates!

61. Cyrille’s Story

62. Family as an
aggregate

63. Keep all entities small
Don’t trust every use case.
Vaughn Vernon

64. An adult
alone?

65. Invariant
=
No child alone

66. Find invariants
correctly
Keep All
Entities Small,
even
aggregates

67. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

68. No Classes With More Than
Two Instance Variables

69. Period has a value ?
What is that value ?
No Classes With More Than
Two Instance Variables

70. Period has a value ?
What is that value ?
No Classes With More Than
Two Instance Variables

71. 3. Wrap All Primitives And Strings
Period has a value ?
What is that value ?
No Classes With More Than
Two Instance Variables

72. Payoff
amount
date
description
currency
label
Amount
amount
currency
Reason
description
label
Payoff
amount
date
reason
No Classes With More Than
Two Instance Variables

74. Allocation
Reservation
Performance
play
startDate
endDate
performanceNature
Topology
No Classes With More Than
Two Instance Variables
Performance nature: standard, pre-premiere

75. Allocation
TheaterSession
title
startDate
Reservation
PerformanceNature
value
Performance
play
date: {startDate, endDate}
nature
Topology
No Classes With More Than
Two Instance Variables

76. No Classes With More Than Two
Instance Variables
Discover and
learn more about
your domain
language to a
better design

77. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables

78. Object Calisthenics serving DDD
? Wrap All Primitives And Strings
? First Class Collections
? Don’t Abbreviate
? No Getters/Setters/Properties
? One Dot Per Line
? Don’t Use The ELSE Keyword
? Only One Level Of Indentation Per Method
? Keep All Entities Small
? No Classes With More Than Two Instance Variables Domain language
Domain language
Domain language
Domain language
Domain language
Domain language
Domain language
Domain language
Domain language
That’s the
DDD
obsession!

79. 9 simple, stupid rules
that work

81. Find the xmind file here

82. Object Calisthenics
=
Better
implementation of
DDD tactical
patterns

83. Enjoy your
Calisthenics
journey!

85. @DorraBartaguiz
Thank you!
French reader,
buy the book!