These are the slides of my "Fake it Outside-In TDD" session at the I T.A.K.E unconference. Do not miss to check out the referenced screencasts that illustrate the approach very well: https://www.youtube.com/playlist?list=PL_ueet93U84VIy8O7U4dUV0GyGvuzFAt8

1. FAKE IT OUTSIDE-IN TDD
David Völkel
08.06.2018 @ IT.A.K.E Unconference

2. @DAVIDVOELKEL
@codecentric
@softwerkskammer
#TDD

3. INFLUENCES
2003 Kent Beck’s "Fake It" Pattern
2009 #GOOS’s "Outside-In" Design
2013 Emily Bache
"Outside-In development with Double Loop TDD"
2014 Justin Searls "The Failures of 'Intro to TDD'"
COMBINATION
2015 Dimitry Polivaev Outside-In with faked Data
2016 SoCraTes DE Outside-In Fake It Session
2017 Refinement

4. INFLUENCES
2003 Kent Beck’s "Fake It" Pattern
2009 #GOOS’s "Outside-In" Design
2013 Emily Bache
"Outside-In development with Double Loop TDD"
2014 Justin Searls "The Failures of 'Intro to TDD'"
COMBINATION
2015 Dimitry Polivaev Outside-In with faked Data
2016 SoCraTes DE Outside-In Fake It Session
2017 Refinement

5. INFLUENCES
2003 Kent Beck’s "Fake It" Pattern
2009 #GOOS’s "Outside-In" Design
COMBINATION
FAKE IT OUTSIDE-IN TDD

6. FAKE IT OUTSIDE-IN TDD
INFLUENCES
2003 Kent Beck’s "Fake It" Pattern
2009 #GOOS’s "Outside-In" Design
2013 Emily Bache
"Outside-In development with Double Loop TDD"
2014 Justin Searls "The Failures of 'Intro to TDD'"
2017 Llewelyn Falco „Extreme: Fake it Till you Make It“
FAKE IT OUTSIDE-IN TDD

7. WHY?
Outside-In => YAGNI free design

8. WHY?
Outside-In => YAGNI free design
Avoid mocking drawbacks
- Readability
- Refactorability

9. WHY?
Outside-In => YAGNI free design
Avoid mocking drawbacks
- Readability
- Refactorability
Later: Smaller steps

10. AGENDA
Theory
Building Blocks
Fake It Outside-In
Practice
Mob Programming Session
Reflect
Retro
Trade-Offs

11. BUILDING BLOCKS

12. INTEGRATION OPERATION
SEGREGATION PRINCIPLE
*"Integration Operation Segregation Principle", Ralf Westphal
"Die kniffligen Fälle beim Testen - Sichtbarkeit", Stefan Lieser
*

13. INTEGRATION OPERATION
SEGREGATION PRINCIPLE
public void sendDiscountMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
Account account = customer.account();
String discount = account == FREE ? "no" :
account == BASE ? "a 10%" :
account == PREMIUM ? "a 25%" :
"-ERROR-";
String content = "Hello " + customer.getName() + ",nn" +
"This week you get " + discount + " discount " +
"on all our products.nn" +
"Best Regards,n" + "ACME Customer Service";
mailService.sendMail(email, content);
}

14. OPERATION
public void sendDiscountMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
Account account = customer.account();
String discount = account == FREE ? "no" :
account == BASE ? "a 10%" :
account == PREMIUM ? "a 25%" :
"-ERROR-";
String content = "Hello " + customer.getName() + ",nn" +
"This week you get " + discount + " discount " +
"on all our products.nn" +
"Best Regards,n" + "ACME Customer Service";
mailService.sendMail(email, content);
}
INTEGRATION OPERATION
SEGREGATION PRINCIPLE

15. INTEGRATION
INTEGRATION
OPERATION
public void sendDiscountMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
Account account = customer.account();
String discount = account == FREE ? "no" :
account == BASE ? "a 10%" :
account == PREMIUM ? "a 25%" :
"-ERROR-";
String content = "Hello " + customer.getName() + ",nn" +
"This week you get " + discount + " discount " +
"on all our products.nn" +
"Best Regards,n" + "ACME Customer Service";
mailService.sendMail(email, content);
}
INTEGRATION OPERATION
SEGREGATION PRINCIPLE

16. INTEGRATION
INTEGRATION
OPERATION
public void sendDiscountMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
Account account = customer.account();
String discount = account == FREE ? "no" :
account == BASE ? "a 10%" :
account == PREMIUM ? "a 25%" :
"-ERROR-";
String content = "Hello " + customer.getName() + ",nn" +
"This week you get " + discount + " discount " +
"on all our products.nn" +
"Best Regards,n" + "ACME Customer Service";
mailService.sendMail(email, content);
}
INTEGRATION OPERATION
SEGREGATION PRINCIPLE

17. OPERATION
INTEGRATION
public void sendDiscountMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String content = renderMessage(customer, customer.account());
mailService.sendMail(email, content);
}
private String renderMessage(Customer customer, Account account) {
String discount = account == FREE ? "no" :
account == BASE ? "a 10%" :
account == PREMIUM ? "a 25%" :
"-ERROR-";
return "Hello " + customer.getName() + ",nn" +
"This week you get " + discount + " discount " +
"on all our products.nn" +
"Best Regards,n" + "ACME Customer Service";
}
INTEGRATION OPERATION
SEGREGATION PRINCIPLE

18. public void sendDiscountMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String content = renderMessage(customer, customer.account());
mailService.sendMail(email, content);
}
private String renderMessage(Customer customer, Account account) {
String discount = account == FREE ? "no" :
account == BASE ? "a 10%" :
account == PREMIUM ? "a 25%" :
"-ERROR-";
return "Hello " + customer.getName() + ",nn" +
"This week you get " + discount + " discount " +
"on all our products.nn" +
"Best Regards,n" + "ACME Customer Service";
}
TESTS?

19. OPERATION
public void sendDiscountMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String content = renderMessage(customer, customer.account());
mailService.sendMail(email, content);
}
private String renderMessage(Customer customer, Account account) {
String discount = account == FREE ? "no" :
account == BASE ? "a 10%" :
account == PREMIUM ? "a 25%" :
"-ERROR-";
return "Hello " + customer.getName() + ",nn" +
"This week you get " + discount + " discount " +
"on all our products.nn" +
"Best Regards,n" + "ACME Customer Service";
}
TESTS?
N UNITTESTS

20. INTEGRATION
OPERATION
public void sendDiscountMailingTo(String email) {
Customer customer = customerDB.findCustomerBy(email);
String content = renderMessage(customer, customer.account());
mailService.sendMail(email, content);
}
private String renderMessage(Customer customer, Account account) {
String discount = account == FREE ? "no" :
account == BASE ? "a 10%" :
account == PREMIUM ? "a 25%" :
"-ERROR-";
return "Hello " + customer.getName() + ",nn" +
"This week you get " + discount + " discount " +
"on all our products.nn" +
"Best Regards,n" + "ACME Customer Service";
}
TESTS?1 INTEGRATION TEST (or MOCKING TEST)
N UNITTESTS

21. GREEN BAR PATTERNS*
Obvious Implementation
Fake it (until you make it)
Triangulation
* Kent Beck in "TDD by Example"

22. GREEN BAR PATTERNS
Obvious Implementation
Fake it
Triangulation
Trade-Off
Complexity

23. GREEN BAR PATTERNS
DEMO

24. SWEET SPOT
Trivial
GREEN BAR PATTERNS
Obvious Implementation
Fake it
Triangulation

25. SWEET SPOT
Trivial
Structure
GREEN BAR PATTERNS
Obvious Implementation
Fake it
Triangulation

26. SWEET SPOT
Trivial
Structure
Logic
GREEN BAR PATTERNS
Obvious Implementation
Fake it
Triangulation

27. PREPARATORY REFACTORINGS*
*"An example of preparatory refactoring", Martin Fowler

28. PREPARATORY REFACTORINGS*
*"An example of preparatory refactoring", Martin Fowler

29. "GREEN" PHASE TRIANGULATION
Implementation Run Test
"Green Phase"

30. LIMIT YOUR TIME IN RED
"Green Phase"
Implementation Run Test

31. LIMIT YOUR TIME IN RED
"Green Phase"
Implementation Run Test
Implementation Run TestPreparatory Refactoring

32. PREPARATORY REFACTORINGS
DEMO

33. CALCULATE
BACKWARDS
DEMO

34. Acceptance Test
UI
Persistence
OUTSIDE-IN

35. Acceptance Test
UI
Persistence
OUTSIDE-IN & MOCKS

36. Unit test
Mock
OUTSIDE-IN & MOCKS

37. Unit Test
OUTSIDE-IN & MOCKS

38. Acceptance Test
UI
Persistence
OUTSIDE-IN & MOCKS

39. Non trivial
Acceptance Test
OUTSIDE-IN & FAKE IT

40. Acceptance Test
Faked Result
OUTSIDE-IN & FAKE IT

41. Acceptance Test
Fake
Fake
OUTSIDE-IN & FAKE IT

42. Acceptance Test Drive Structure
through Refactoring
OUTSIDE-IN & FAKE IT

43. OUTSIDE-IN & FAKE IT
Acceptance Test
Unit Test
Fill logic with
triangulation

44. OUTSIDE-IN & FAKE IT
Acceptance Test Unit Test

45. OUTSIDE-IN & FAKE IT
INTEGRATION
Fake It
Triangulation
OPERATION
Start with
• comprehensive
Acceptance Test
• faked result
Drive structure by refactoring
Drive logic by unit tests

46. OUTSIDE-IN & FAKE IT
INTEGRATION
Fake It
Triangulation
OPERATION
Start with
• comprehensive
Acceptance Test
• faked result
Drive structure by refactoring
Drive logic by unit tests

47. DIAMOND KATA

48. FAKE IT OUTSIDE-IN
DEMO

49. MOB PROGRAMMING

50. DIAMOND KATA

51. MOB SESSION
KATA
Continue Diamond Kata ("Upper Left Corner“)
Fake it Outside-In constraint
https://github.com/davidvoelkel/diamond-kata

52. RETROSPECTIVE
Observations?

53. RETROSPECTIVE
Observations?
Strenghts / Weaknesses?

54. TRADE-OFFS
FAKE IT VS
OUTSIDE-IN & MOCKS
TRIANGULATION

55. CONDITIONAL INTERACTIONS
Mocking when IOSP not possible
public String signup(String username) throws Exception {
if(userDB.findUserBy(username) == null) {
userDB.createUser(new User(username));
return "Welcome " + username;
} else {
return "Username ' " + username + "' "
+ "already taken, please choose another";
}
}

56. COUPLING OUTSIDE-IN
Mocking Fake It
Decoupling Refactorability

57. DATA GUIDES STRUCTURE
Triangulation Fake It
Structure Cumbersome Data guides well

58. # BRANCHES
Triangulation Fake It
# N 1
More confidence
More effort

59. # BRANCHES
Triangulation Fake It
# N 1
Fake data easy to forget
Omitting cases is
tempting

60. TIME IN GREEN
Triangulation
Prep Refactoring
Triangulation
Fake It

61. WHY?
Outside-In
=> YAGNI free design
Without mocking drawbacks

62. WHY?
Outside-In
=> YAGNI free design
Without mocking drawbacks
Vs Triangulation
More time in GREEN
Better Design Guidance

63. THANKS

64. LICENSE
Creative Commons Attribution-ShareAlike 3.0

65. IMAGES
Most are Public Domain except theses
Creative Commons with attributions:
"Unstruttalbrücke" by Störfix
From State Library of Queensland