Roberto Hernandez-Pou presented "Working with Legacy Code" at the 2012 DC Agile Engineering Conference

1. Working Effectively with
Legacy Code
Roberto Hernandez-Pou
Managing Consultant
Excella Consulting
@hernandezrobert
http://www.overridethis.com
Excella Consulting – http://www.excella.com

2. Legacy Code: Characteristics
• Poor Architecture
• Non-Uniform coding styles
• Poor or Non-Existing Documentation
• Mythical “Oral” Documentation
• No Tests (or Minimal Test Coverage)
• Extremely Valuable!
– Only Successful code becomes Legacy Code.
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

3. Agenda
• How does code become Legacy Code?
• Reasons to work with Legacy Code
• What are my options?
• How do we approach the work?
– Edit and Pray „or‟ Cover and Modify
• The Legacy Code Change Algorithm
• Sensing and Separation
• Fake Collaborators
• Seams
• Tools
• TDD for Legacy Code
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

4. How does code become Legacy Code?
• New Features or New Requirements
– Developers usually turn to shortcuts and hacks to make deadlines
and deliver on time to market.
• Software Team Turnover or Large Development Teams
– Multiple styles of coding.
– Lack of vision.
– Code duplication.
• Poor Team Communication.
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

5. Reasons to “Work with Legacy Code”
• Four reasons to change software:
– Adding a feature
– Fixing a bug
– Refactoring
– Optimizing resource usage
Adding a Feature Fixing a Bug Refactoring Optimizing
resource usage
Structure Changes Changes Changes
New Functionality Changes
Functionality Changes
Resource Usage Changes
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

6. As a Developer what are my options?
• Look for a new Job?
• Start from scratch.
– Big bang approach!
• Have somebody else do the job:
– Members of the original development team
– Consultants ($$$)
• Deal with it!
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

7. How do we approach the work?
• Working with Feedback
– Edit and pray
(Industry standard)
– Cover and modify
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

8. Edit and Pray
• Carefully plan the changes
• Make sure you understand the code
• Start making changes
• Poke around to see if you broke anything
• Deploy to production environment
• Wait for feedback from customers

9. Cover and Modify
• Build a safety net (Covering = Testing)
• Unit Testing
• Higher level testing
The Legacy Code Dilemma
When we change code, we should have tests in place.
To put tests in place we need to change code
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

10. Cover and Modify
• Introduce Unit Tests
• Unit Tests are:
– Fast
– Localise Problems
– Do not talk to a database
– Do not communicate over the network
– Do not communicate with local resources
– Do not need config changes
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

11. The Legacy Code Change Algorithm
 Identify change points
 Find test points
 Break dependencies
 Write tests
 Make changes and refactor
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

12. Sensing & Separation
• Sensing: we break dependencies to sense when we can't access
values our code computes
• Separation: we break dependencies to separate when we can't
even get a piece of code into a test harness to run
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

13. Fake Collaborators
• Object that impersonates some collaborator of your class when its
being tested.
– Demo: Echo Console
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

14. The Seam Model
• Seam: a place where you can alter behavior of your program
without editing in that place.
• Enabling Point: Every seam has an enabling point, a place where
you can make the decision to use one behavior or another.
• Looking for existing seams in legacy code allow us to break
dependencies (for sensing or separation) without refactoring.
• Demo: Echo Console
• Demo: Fake Data Reader
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

15. Tools
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

16. Test Driven Development
• Test-Driven Development (TDD)
1. Write a failing test
2. Get it to compile
3. Make it pass
4. Remove duplication
5. Repeat
• Programming by difference
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

17. Test Driven Development
• Test-Driven Development (TDD) for legacy
0. Get the class you want to change under test
1. Write a failing test
2. Get it to compile
3. Make it pass (try not to change existing code)
4. Remove duplication
5. Repeat
• Programming by difference
Excella Consulting – http://www.excella.com #DCAEC12 @HernandezRobert
* Based on the Book “Working Effectively with Legacy Code” by Michael Feathers

18. Conclusions
• No “silver bullet” here, it's hard work but not impossible
• Overwhelming at first but things will get better as the number of
tests increase
• Be pragmatic!

19. Resources
“Working Effectively with Legacy Code” by
1.
Michael Feathers
2. http://www.objectmentor.com
3. http://www.overridethis.com
http://bitbucket.org/rhp_74/overridethis.legacycode
4.
sample
Excella Consulting – http://www.excella.com

20. Thank You!
1. Blog: http://www.overridethis.com
2. Email: roberto.hernandez@excella.com
3. @hernandezrobert
4.
Excella Consulting – http://www.excella.com