Less Painful Legacy Code Replacement

Replacing legacy code is a challenge on every front, from managing stakeholder expectations to tackling the technical work. Thoughtful preparation and a pocket full of tools can make the experience a little less painful.

45min talk at Open Source Bridge 2016:
http://opensourcebridge.org/sessions/1746

20min talk at SF.rb meetup March 2016:
http://www.meetup.com/sf-dot-rb/events/229451240/

Contact me if you would like me to speak at your conference.

Transcript

1. Less Painful Legacy Code Replacement Jennifer Tu
2. Totally reasonable requests • Can you add this feature?
3. Photo credit: Johannes Bader, via Flickr @ https://www.flickr. com/photos/johannesbader/ CC BY-SA 2.0 Image overlaid second image • Can you add this feature? Totally reasonable requests
4. Totally reasonable requests • Can you add this feature? • Can you get rid of this timeout?
5. Photo credit: Erik, via Flickr @ https://www.flickr. com/photos/zebrapares/4529836138 CC BY-NC-SA 2.0 Image reversed and cropped Totally reasonable requests • Can you add this feature? • Can you get rid of this timeout?
6. • Select Your Option • Decision Factors • Tools for Rewriting • Design Patterns Outline
7. Select Your Option 1) Small change 2) Refactor and change 3) Rewrite
8. Select Your Option 1) Small change if foo … elsif bar … elsif baz … elsif YOUR_CHANGE_HERE # tiny change else … end Photo credit: adam nicholson, via Flickr @ https://www.flickr. com/photos/adamgn/5434050218/ CC BY 3.0
9. Select Your Option 1) Small change 2) Refactor and change
10. Select Your Option 1) Small change 2) Refactor and change 3) Rewrite
11. Refactor vs Rewrite
12. Refactor vs Rewrite
13. Refactor vs Rewrite
14. Refactor vs Rewrite
15. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new end end
16. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new end end > my_tree = Tree.new
17. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new end end > my_tree = Tree.new > my_tree.grow_fruits
18. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new create_stem @fruits << Orange.new end end
19. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new create_stem @fruits << Orange.new end end > my_tree = Tree.new
20. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new create_stem @fruits << Orange.new end end > my_tree = Tree.new > my_tree.grow_fruits
21. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new create_stem @fruits << Orange.new end end
22. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new create_stem @fruits << Orange.new end end
23. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new create_stem @fruits << Orange.new end def add_new_fruit(fruit:) create_stem @fruits << fruit end end
24. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new create_stem @fruits << Orange.new end def add_new_fruit(fruit:) create_stem @fruits << fruit end end
25. class Tree attr_accessor :fruits def grow_fruits add_new_fruit(fruit: Apple.new) create_stem @fruits << Orange.new end def add_new_fruit(fruit:) create_stem @fruits << fruit end end
26. class Tree attr_accessor :fruits def grow_fruits add_new_fruit(fruit: Apple.new) create_stem @fruits << Orange.new end def add_new_fruit(fruit:) create_stem @fruits << fruit end end
27. class Tree attr_accessor :fruits def grow_fruits add_new_fruit(fruit: Apple.new) add_new_fruit(fruit: Orange.new) end def add_new_fruit(fruit:) create_stem @fruits << fruit end end
28. class Tree attr_accessor :fruits def grow_fruits ... end def add_new_fruit(fruit) ... end end
29. class Tree attr_accessor :fruits def grow_fruits ... end def add_new_fruit(fruit) ... end end > my_tree = Tree.new
30. class Tree attr_accessor :fruits def grow_fruits ... end def add_new_fruit(fruit) ... end end > my_tree = Tree.new > my_tree.grow_fruits
31. class Tree attr_accessor :fruits def grow_fruits ... end def add_new_fruit(fruit) ... end end
32. class Tree attr_accessor :fruits def grow_fruits ... end end
33. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new create_stem @fruits << Orange.new end end
34. class Tree attr_accessor :fruits def grow_fruits create_stem @fruits << Apple.new create_stem @fruits << Orange.new end end > my_tree = Tree.new > my_tree.grow_fruits
35. class Tree attr_accessor :fruits def grow_fruits add_new_fruit(fruit: Apple.new) add_new_fruit(fruit: Orange.new) end def add_new_fruit(fruit:) create_stem @fruits << fruit end end
36. class Tree attr_accessor :fruits def grow_fruits add_new_fruit(fruit: Apple.new) add_new_fruit(fruit: Orange.new) add_new_fruit(fruit: Banana.new) end def add_new_fruit(fruit:) create_stem @fruits << fruit end end
37. Refactor is • Incremental change • No behavior change • Tests also don’t change (and still pass) • Easier to modify code (add_new_fruit(fruit: Banana.new)) Refactor vs Rewrite
38. Refactor vs Rewrite
39. Refactor vs Rewrite
40. Refactor vs Rewrite
41. Refactor vs Rewrite
42. Refactor vs Rewrite
43. Refactor vs Rewrite Rewrite dangers: • More opportunities to miss requirements • Longer time • All or nothing
44. • Choose refactor whenever possible • Why do you want to rewrite? • Are you actually rewriting while saying you’re refactoring? Refactor vs Rewrite
45. • Select Your Option (Simple / Refactor / Rewrite) • Decision Factors • Tools for Rewriting • Design Patterns Outline
46. Case Study An existing appointment searcher finds available appointment inventory, so patients can see doctors Improve performance so it stops hitting timeouts
49. 1. Go to the model (AppointmentInventory) 2. Build up a giant SQL statement based on parameters 3. Add or remove rows based on business rules 4. Repeat steps 1-3 for all the other business rules 5. Return results class LegacySearcher end
50. Decision Factors • Goals • Risks • Tests
51. Decision Factors: Goals Some possible goals: • Add a feature • Fix a bug • Improve performance • Upgrade external dependency / add security
52. 1. Go to the model (AppointmentInventory) 2. Build up a giant SQL statement based on parameters 3. Add or remove rows based on business rules 4. Repeat steps 1-3 for all the other business rules 5. Return results class LegacySearcher end
53. 1. Go to the model (AppointmentInventory) 2. Build up a giant SQL statement based on parameters 3. Add or remove rows based on business rules 4. Repeat steps 1-3 for all the other business rules 5. Return results class LegacySearcher end
54. 1. Go to the model (AppointmentInventory) 2. Build up a giant SQL statement based on parameters 3. Add or remove rows based on business rules 4. Repeat steps 1-3 for all the other business rules 5. Return results class LegacySearcher end
55. 1. Go to the model (AppointmentInventory) 2. Build up a giant SQL statement based on parameters 3. Add or remove rows based on business rules 4. Repeat steps 1-3 for all the other business rules 5. Return results class LegacySearcher end
56. class LegacySearcher end 1. Go to the model (AppointmentInventory) 2. Build up a giant SQL statement based on parameters 3. Add or remove rows from results 4. Repeat steps 1-3 for all the other business rules 5. Return results
57. Decision Factors • Goals • Risks • Tests
58. Decision Factors: Risks How much risk is acceptable? • Business • Reputation internally • User pain • Accuracy
59. Decision Factors • Goals • Risks • Tests
60. Decision Factors: Tests • Legacy test suite? Useful? • Does anyone know what this is supposed to do?
61. Decision Factors: Tests • Legacy test suite? Useful? • Does anyone know what this is supposed to do?
63. Decision: Rewrite • Goals • Risks • Tests
64. • Select Your Option (Simple / Refactor / Rewrite) • Decision Factors (Goals / Risks / Tests) • Tools for Rewriting ◦ Team Commitment ◦ Stabilize Before Extending ◦ Scientist ◦ Feature Flags & Beta Users ◦ Tests! • Design Patterns Outline
65. Team Commitment
66. Stabilize
67. Stabilize Before Extending
68. Behavior Matching (Github Scientist)
69. Feature Flags Beta Users
70. Behavior Definition (Tests)
71. • Select Your Option (Simple / Refactor / Rewrite) • Decision Factors (Goals / Risks / Tests) • Tools for Rewriting (Team / Stabilize / Scientist / Beta Users / Tests) • Design Patterns Outline
72. 1) Strangler Application (Martin Fowler) 2) Sprout Method (Michael Feathers) 3) Wrap Method (Michael Feathers) Design patterns
73. 1) Strangler Application (Martin Fowler) Design patterns
74. 1) Strangler Application (Martin Fowler) Design patterns
75. 1) Strangler Application (Martin Fowler) Design patterns
76. 1) Strangler Application (Martin Fowler) Design patterns
77. 1) Strangler Application (Martin Fowler) Design patterns
78. 1) Strangler Application (Martin Fowler) Design patterns
79. 1) Strangler Application (Martin Fowler) Design patterns
80. 1) Strangler Application (Martin Fowler) Design patterns
81. 1) Strangler Application (Martin Fowler) Design patterns
82. 1) Strangler Application (Martin Fowler) Design patterns
83. “Gradually create a new system around the edges of the old, letting it grow slowly over several years until the old system is strangled.” 1) Strangler Application (Martin Fowler) Design patterns
84. 1) Strangler Application (Martin Fowler) 2) Sprout Method (Michael Feathers) Design Patterns
85. Design Patterns 1) Strangler Application (Martin Fowler) 2) Sprout Method (Michael Feathers) • Add call to new method • Test new method • New method is no longer legacy
86. class AppointmentsController def search LegacySearcher.new(params).results end end
87. class AppointmentsController def search LegacySearcher.new(params).results end end
88. class AppointmentsController def search if $feature_flag else LegacySearcher.new(params).results end end end
89. class AppointmentsController def search if $feature_flag else LegacySearcher.new(params).results end end end
90. class AppointmentsController def search if $feature_flag NewSearcher.new(params).results else LegacySearcher.new(params).results end end end
91. Design Patterns 1) Strangler Application (Martin Fowler) 2) Sprout Method (Michael Feathers) 3) Wrap Method (Michael Feathers)
92. Design Patterns 1) Strangler Application 2) Sprout Method 3) Wrap Method • Wrap legacy class in new class, slowly cut over • New class is no longer legacy • AKA Decorator Pattern
93. class AppointmentsController def search LegacySearcher.new(params).results end end
94. class AppointmentsController def search LegacySearcher.new(params).results end end
95. class AppointmentsController def search Searcher.new(params).results end end New class
96. class AppointmentsController def search Searcher.new(params).results end end class Searcher end New class
97. class AppointmentsController def search Searcher.new(params).results end end class Searcher def results if $feature_flag NewSearcher.new.results else LegacySearcher.new(params).results end end end New class Original legacy class
98. Design Patterns 1) Strangler Application (Martin Fowler) 2) Sprout Method (Michael Feathers) 3) Wrap Method (Michael Feathers) ? Sprout vs Wrap
99. • Select Your Option (Simple / Refactor / Rewrite) • Decision Factors (Goals / Risks / Tests) • Tools for Rewriting (Team / Stabilize / Scientist / Beta Users / Tests) • Design Patterns (Strangler / Sprout / Wrap) Outline
101. Thanks! Jennifer Tu @jtu