A presentation about handling legacy code (based on Michael Feathers' book) for SAP Inside Track Hannover, 11 May 2019.

1. The
Hitchhikers
Guide to the
Legacy
sitHVR | 11 May 2019 | Laurens van Rijn
SAP Inside Track presentation

2. Agenda
Today’s topics
―What is Legacy Code?
―Is Legacy Code bad?
―Is Legacy Code good?
―The way forward
―Working with Legacy Code
―Want to know more?
―Q&A
HHGtt
Legacy

3. … in large friendly letters on its cover.
HHGtt
Legacy

4. What is Legacy Code? (1/2)
Some definitions…
―“Code where you can see the different programming styles of all the
developers who ever worked on it” – Ginny Hendry
―“Code that we don't like any more for whatever reason (typically because
it's not cool or fashionable but it works” – Dan Dyer
―“The code base or platform on which it is built is so old that getting qualified
or experienced developers for the system is both hard and expensive” –
Anonymous
―“ABAP code, riddled with deprecated statements (OCCURS etc.), usually
written under considerable time pressure and badly documented by
developers who have since left the company and who laugh at you upon
learning you’re supporting their code when meeting them at events” – Me
―“Simply code without tests” – Michael Feathers
HHGtt
Legacy

5. What is Legacy Code? (2/2)
Our working definition
“Code without tests is a bad code. It doesn’t matter how well written it is; how
well structured it is; how well encapsulated it is. Without tests there is no way
to tell if our code is getting better or worse.” – Michael Feathers
Please note: Your Legacy Code is not the same as your Code Legacy. The
latter partly determines your standing in the company as well as the likelihood
you’ll be invited back for another assignment.
HHGtt
Legacy

6. Is Legacy Code bad?
(Spoiler alert) Of course it’s not…
Legacy Code…
―Supports the company’s business processes
―Brings in revenue
―Provides learning moments and challenging puzzles
HHGtt
Legacy

7. Is Legacy Code good?
(Spoiler alert) Of course it’s not…
Legacy Code is…
―Complicated
―Likely to fall apart when touched
―Time consuming
―Bad for morale
The business may become hesitant to change due to the risks and
costs of changing legacy code.
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8. Legacy (Code) is everywhere…
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9. The way forward (1/2)
Why should we want to get a grip on Legacy Code?
―Improved efficiency
―Reduced development costs
―Improved product quality
―Lower learning curve
Thereby allowing the business the freedom to change as needed and be
supported by their software systems.
HHGtt
Legacy

10. The way forward (2/2)
How can we achieve this?
―Edit & Pray vs Cover & Modify
―Test Driven Development
The Legacy Code Dilemma: when we change code, we want to have tests
in place. To put tests in place, we need to change code. So, we…
―Identify change points
―Find test points
―Break dependencies
―Write tests
―Make changes and refactor
HHGtt
Legacy

11. Working with Legacy Code (1/5)
Identify change points
―I don’t understand the code well enough to change it
1. Sketching
2. Listing mark-up
3. Scratch refactoring
4. Delete unused code
―My application has no structure
1. Telling the story
2. Naked CRC
HHGtt
Legacy

12. Working with Legacy Code (2/5)
Find test points
―I need to make a change, what methods should I test?
1. Effect analysis → Effect sketch
2. Where are values changed, stored and who is using them?
―I need to make many changes in one area, do I have to break
dependencies for all the classes involved?
1. (High level) interception points
2. Pinch points
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13. Working with Legacy Code (3/5)
Break dependencies
―How do I know I’m not breaking anything?
1. Hyperaware & single goal editing
2. Preserve signatures
3. “Use the Compiler, Luke”
4. Pair programming
―I need to change a monster method and I can’t write tests for it
1. A plethora of monsters (bulleted, snarled, and mixed)
2. Refactoring: automated or manual
HHGtt
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14. Working with Legacy Code (4/5)
Write tests
―I need to make a change but I don’t know what tests to write
1. Use characterization tests (from failure to good result)
2. What if you encounter bugs?
―My test code is in the way
1. Naming conventions (FAKE… / …TEST / TESTING…)
2. Documentation
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15. Working with Legacy Code (5/5)
Make changes and refactor
―I don’t have much time and I have to change it
1. Sprout
2. Wrap
―I’m changing the same code all over the place
1. Necessary to remove redundancy?
2. Extract the difference from similar methods and use a common method
3. Open / Closed principle
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Legacy

16. Want to know more?
The book (contains so much more, worth reading)
―https://www.goodreads.com/book/show/44919.Working_Effectively_with_Le
gacy_Code
Literature
―https://en.wikipedia.org/wiki/Legacy_code
―https://stackoverflow.com/questions/479596/what-makes-code-legacy
―https://en.wikipedia.org/wiki/Test-driven_development
―https://en.wikipedia.org/wiki/Technical_debt
―https://en.wikipedia.org/wiki/Towel_Day
―https://open.sap.com/courses/wtc1
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17. Say hello
to Q&A

18. Thank you for the attention…
HHGtt
Legacy