1. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
Debugging Effectively
Colin O’Dell

2. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
Colin O’Dell
• Lead Web Developer at Unleashed Technologies
• PHP developer since 2002
• Certifications: Symfony and Magento
• PHP League Member
• league/commonmark
• league/html-to-markdown
• PHP 7 Upgrade Guide e-book
• @colinodell / www.colinodell.com

3. Overview
I. Importance of debugging
II. Debugging process
III. Tools & Techniques
IV. Q&A
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4. Debugging is...
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_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
(adjective)

5. Debugging is...
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important

6. Debugging is...
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the single most
important skill in
programming.

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PlanningCoding

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Planning
Testing &
Debugging
Coding

9. Debugging is the process of finding and resolving bugs
or defects that prevent correct operation of computer
software or a system.
– Wikipedia
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10. Process is the foundation of effective debugging
Process
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11. Photo by Matthew Paulson // cc by-nc-nd 2.0 // https://flic.kr/p/di35xa
Gain experience with tools and code
Experience
Process

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Develop a “sixth sense”
“Sixth Sense”
Experience
Process

13. Junior Developers
• Try the “usual” steps
• Clear the cache
• Re-install dependencies
• chmod –R 777 *
• Google the error
• Try every solution
• Ask somebody else
• Co-worker
• StackOverflow post
• Give up
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14. XY Problem
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•I want to solve problem X
•How do I solve X?

15. XY Problem
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•I want to solve problem X
•How do I solve X?
•Solution Y might work
•How can I do Y?

16. Photo by quattrostagioni // cc by 2.0 // https://flic.kr/p/aGjVq8
1.Don’t parse HTML with regex
2.Solve problems the right way

17. “I don’t know why”
“For some reason”
“Doesn’t make sense”
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18. Bugs are logical
Photo by Photofest

19. “The bug is not moving around in your code,
trying to trick or evade you. It is just siting in
one place, doing the wrong thing in the same
way every time.” – Nick Parlante, Debugging Zen
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20. Assume your
code is the
problem
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21. Systematic Approach
1. Gather information
2. Replicate the issue
3. Identify the culprit
4. Fix it & re-test
5. Mitigate future occurrences
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22. 1. Gather information
• Expected behavior vs.
actual behavior
• Error messages
• Stack traces
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•Screenshots
•Browser & OS
•Date & time
•Log entries

23. 2. Replicate the Issue
Be able to replicate with 100% certainty
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24. 3. Identify the Culprit
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• Be methodical
• Make no assumptions
• Understand the bug

25. 4. Fix it & Re-test
• Attempt to replicate again
• Avoid XY problem
• No temporary workarounds!
• Add technical debt
• May introduce other issues
• Never get replaced with true solutions
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26. 5. Mitigate Future Occurrences
• Add an automated test
• Share your new knowledge
• Project documentation
• Blog post
• StackOverflow
• Submit patch upstream
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27. Recap
1. Gather information
2. Replicate the issue
3. Identify the culprit
4. Fix it & re-test
5. Mitigate future occurrences
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28. Long-Term Results
• Gain experience
• Learn how the system works
• Build heuristics
• Boost confidence
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29. Tools & Techniques
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30. Two essential tools
• Integrated development
environment (IDE)
• Interactive debugger
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31. Integrated Development Environment
•Minimum features:
•Syntax highlighting
•Auto-completion
•Fast code navigation
•Debugger
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32. Interactive Debugger
•Pause code execution
•Breakpoints
•Conditional breakpoints
•Step through execution
•Examine variables
•Explore call stack
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34. Photo by florianric // cc by 2.0 // https://flic.kr/p/c4QJzC

35. Techniques
1. Trace backwards from known issue
2. Divide & conquer
3. Use tools
4. Get help
5. Take a break
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36. 1. Trace backwards
•Use a debugger
•Identify source of error
•Establish context
•Work backwards
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37. 1. Trace backwards
•Use a debugger
•Identify source of error
•Establish context
•Work backwards
a()
b()
c()
d()
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38. 1. Trace backwards
•Use a debugger
•Identify source of error
•Establish context
•Work backwards
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

39. 1. Trace backwards
•Use a debugger
•Identify source of error
•Establish context
•Work backwards
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

40. 1. Trace backwards
•Use a debugger
•Identify source of error
•Establish context
•Work backwards
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

41. 1. Trace backwards
•Use a debugger
•Identify source of error
•Establish context
•Work backwards
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

42. 2. Divide & Conquer
• Identify different code sections
• Set breakpoints at the boundaries
• Isolate issue to one particular area
• Focus efforts on that area

46. ✓
X

48. 3. Use tools
• Automated tests
• Kint / VarDumper
• Debug toolbars
• Console utility
• Profilers
• git bisect
•netcat
•curl
•etc.
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49. Automated Tests
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50. Kint
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Kint::dump($var);

51. VarDumper
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Twig:
{{ dump(foo) }}
{% dump foo %}
PHP:
dump($somevar);

52. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
Debug toolbars

53. Console Utility
Symfony: app/console
Drupal: drush
Magento: n98-magerun.phar
Laravel: artisan
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54. Performance Profiling
Identify slowness:
• Bottlenecks
• Resource hogs
• Inefficient code
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Tools:
• Blackfire (freemium)
• New Relic (freemium)
• xhprof (open-source)

55. Blackfire
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56. New Relic
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57. git bisect
v1.7 ? ? ? ? ? ? ? HEAD
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git bisect start
git bisect bad

58. git bisect
v1.7 ? ? ? ? ? ? ? HEAD
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git bisect good

59. git bisect
v1.7 ? ? ? ? ? ? ? HEAD
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git bisect bad

60. git bisect
v1.7 ? ? ? BAD ? ? ? HEAD
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git bisect bad

61. git bisect
v1.7 ? ? ? BAD BAD BAD BAD HEAD
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git bisect good

62. git bisect
v1.7 ? GOOD ? BAD BAD BAD BAD HEAD
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git bisect good

63. git bisect
v1.7 GOOD GOOD ? BAD BAD BAD BAD HEAD
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64. git bisect
v1.7 GOOD GOOD
X BAD BAD BAD BAD HEAD
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abcd123 is the first bad commit

65. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
netcat

66. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
curl

67. 4. Get help
• RTFM / RTFD
• Project forums or issue queue
• StackOverflow, IRC, etc.
• Ask a colleague
• Expert in that area
• Senior developer
• Rubber ducking
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68. 5. Take a break
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• Clear your mind; start fresh
• Forget invalid assumptions
• Recharge your batteries
• Let your subconscious work on it

69. Four things to walk away with
1. Computers aren’t random,
and neither are bugs
2. Persistence will always pay off
3. Don’t be afraid to dive deep
4. Don’t make assumptions or
take things for granted
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70. Questions?
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71. Learn More
• https://web.duke.edu/cps001/notes/Debugging.pdf
• http://www.fiveminutegeekshow.com/20
• http://blog.codeunion.io/2014/09/03/teaching-novices-how-to-debug-
code/
• https://www.jetbrains.com/phpstorm/help/debugging.html
• http://www.sitepoint.com/debugging-git-blame-bisect/
• http://unix.stackexchange.com/a/50099/80744
• http://codeception.com/docs/01-Introduction
• http://chadfowler.com/blog/2014/01/26/the-magic-of-strace/
• http://c2.com/cgi/wiki?RubberDucking
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72. Thanks!
Feedback?
• https://joind.in/14752
•@colinodell
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