These are the slides of my "Fake it Outside-In TDD" session at the #XP2017 conference. 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
24th May @ XP 2017

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. 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
2016 SoCraTes DE Outside-In Fake It Session

7. 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
2016 SoCraTes DE Outside-In Fake It Session
2017 Refinement / Work in Progress

8. INFLUENCES
2003 Kent Beck’s "Fake It" Pattern
2009 #GOOS’s "Outside-In" Design
2013 Emily Bache
"Outside-In development with Double Loop TDD"
COMBINATION
2016 SoCraTes DE Outside-In Fake It Session
2017 Refinement / Work in Progress

9. 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
2016 SoCraTes DE Outside-In Fake It Session
2017 Refinement / Work in Progress

10. 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“
COMBINATION
2016 SoCraTes DE Outside-In Fake It Session
2017 Refinement / Work in Progress

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

12. BUILDING BLOCKS

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

14. 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);
}

15. 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. 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

18. 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

19. 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?

20. 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

21. 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
N UNITTESTS

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

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

24. GREEN BAR PATTERNS
DEMO

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

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

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

28. INTEGRATION
OPERATION
Fake it
Triangulation
GREEN BAR PATTERNS
Structure
Logic
IOSP

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

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

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

32. PREPARATORY REFACTORINGS
DEMO

33. Acceptance Test
UI
Persistence
OUTSIDE-IN

34. Unit test
Mock
OUTSIDE-IN & MOCKS

35. Unit Test
OUTSIDE-IN & MOCKS

36. Acceptance Test
UI
Persistence
OUTSIDE-IN & MOCKS

37. Comprehensive
Acceptance Test
OUTSIDE-IN & FAKE IT

38. Comprehensive
Acceptance Test
Faked Result
OUTSIDE-IN & FAKE IT

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

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

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

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

43. 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

44. 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

45. DIAMOND KATA

46. FAKE IT OUTSIDE-IN
DEMO

47. FAKE-IT VARIANT
CALCULATE BACKWARDS
DEMO

48. PRACTICE

49. CODING SESSION
KATA
Continue Diamond Kata
https://github.com/davidvoelkel/diamond-kata
CONSTRAINTS
Drive structure by refactoring
Drive conditionals by triangulation

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

51. 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";
}
}

52. COUPLING OUTSIDE-IN
Mocking Fake It
Decoupling Refactorability

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

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

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

56. TIME IN GREEN
Triangulation
Prep Refactoring
Triangulation
Fake It

57. DISCUSSION

58. LICENSE
Creative Commons Attribution-ShareAlike 3.0

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