1. Debugging Effectively
Colin O’Dell
@colinodell

2. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
Colin O’Dell / @colinodell
• Lead Web Developer & DevOps Engineer
at Unleashed Technologies
• Primary language is PHP
• PHP developer for 16 years
• PHP League Leadership Team
• Author/maintainer of PHP CommonMark library
• PHP 7 Upgrade Guide e-book author
• Also experienced in JavaScript, Java, C#, and Python
@colinodell

3. Overview
I. Importance of debugging
II. Debugging process
III. Tools & Techniques
IV. Q&A
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@colinodell

4. Debugging is...
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_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
(adjective)
@colinodell

5. Debugging is...
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important
@colinodell

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

7. Photo by florianric // cc by 2.0 // https://flic.kr/p/c4QJzC
PlanningCoding
@colinodell

8. Photo by florianric // cc by 2.0 // https://flic.kr/p/c4QJzC
Planning
Testing &
Debugging
Coding
@colinodell

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 code and tools
Experience
Process

12. Photo by Matthew Paulson // cc by-nc-nd 2.0 // https://flic.kr/p/di35xa
Develop your intuition
Intuition
Experience
Process

13. Junior Developers
• Try the “usual” steps
• Clear caches
• Re-install dependencies
• chmod –R 777 *
• Ask somebody else
• Co-worker
• Google
• StackOverflow post
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@colinodell

14. XY Problem
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•I want to solve problem X
•How do I solve X?
@colinodell

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?
@colinodell

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 @colinodell

17. “I don’t know why”
“For some reason”
“Doesn’t make sense”
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@colinodell

18. Bugs are logical
Photo by Photofest
@colinodell

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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@colinodell

20. Assume your
code is the
problem
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@colinodell

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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@colinodell

22. 1. Gather information
• Expected behavior vs.
actual behavior
• Error messages
• Stack traces
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•Screenshots
•Browser & OS
•Date & time
•Log entries
@colinodell

23. 2. Replicate the Issue
Be able to replicate with 100% certainty
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@colinodell

24. 3. Identify the Culprit
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• Be methodical
• Make no assumptions
• Understand the bug @colinodell

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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@colinodell

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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@colinodell

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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@colinodell

28. Long-Term Results
• Gain experience
• Learn how the system works
• Build heuristics
• Boost confidence
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@colinodell

29. Tools & Techniques
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@colinodell

30. Two essential tools
• Integrated development
environment (IDE)
• Interactive debugger
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@colinodell

31. Integrated Development Environment
•Minimum features:
•Syntax highlighting
•Auto-completion
•Fast code navigation
•Debugger
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@colinodell

32. Interactive Debugger
•Pause code execution
•Step through execution
•Examine variables
•Explore call stack
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@colinodell

33. Techniques
1. Trace backwards from known issue
2. Trace forwards from start
3. Binary search
4. Use tools
5. Get help
6. Take a break
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@colinodell

34. 1. Trace backwards
•Error is thrown from known location
•Use a debugger
•Establish context
•Work backwards
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@colinodell

35. 1. Trace backwards
•Error is thrown from known location
•Use a debugger
•Establish context
•Work backwards
a()
b()
c()
d()
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36. 1. Trace backwards
•Error is thrown from known location
•Use a debugger
•Establish context
•Work backwards
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

37. 1. Trace backwards
•Error is thrown from known location
•Use a debugger
•Establish context
•Work backwards
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

38. 1. Trace backwards
•Error is thrown from known location
•Use a debugger
•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
•Error is thrown from known location
•Use a debugger
•Establish context
•Work backwards
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

40. 2. Trace forwards
•Opposite direction
•Problematic line
isn’t known
•Use debugger or logging
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

41. 2. Trace forwards
•Opposite direction
•Problematic line
isn’t known
•Use debugger or logging
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

42. 2. Trace forwards
•Opposite direction
•Problematic line
isn’t known
•Use debugger or logging
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

43. 2. Trace forwards
•Opposite direction
•Problematic line
isn’t known
•Use debugger or logging
a()
b()
c()
d()
Photo by Shawn Harquail // cc by-nc 2.0 // https://flic.kr/p/npxFVw

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

45. @colinodell

46. @colinodell

47. @colinodell

48. ✓
X
@colinodell

49. @colinodell

50. 4. Use tools
•Variable dumps
•Debug toolbars
•Console utility
•Profilers
•git bisect
•netcat
•curl
•strace
•etc.
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@colinodell

51. Variable dumps
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•Print to output:
•var_dump()
•pprint()
•puts()
•Log to file
•file_put_contents()
•Logger.debug()
@colinodell

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

53. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
Console Utility
Drupal: drush
Drupal Console
Symfony: bin/console
Laravel: artisan
Magento: n98-magerun
@colinodell

54. Performance Profiling
Identify slowness:
• Bottlenecks
• Resource hogs
• Inefficient code
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Tools:
• Blackfire (freemium)
• New Relic (freemium)
• xhprof (open-source)
@colinodell

55. git bisect
v1.7 ? ? ? ? ? ? ? HEAD
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@colinodell

56. git bisect
v1.7 ? ? ? ? ? ? ? HEAD
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@colinodell

57. git bisect
v1.7 ? ? ? BAD ? ? ? HEAD
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@colinodell

58. git bisect
v1.7 ? ? ? BAD BAD BAD BAD HEAD
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@colinodell

59. git bisect
v1.7 ? GOOD ? BAD BAD BAD BAD HEAD
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@colinodell

60. git bisect
v1.7 GOOD GOOD ? BAD BAD BAD BAD HEAD
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@colinodell

61. git bisect
v1.7 GOOD GOOD
X BAD BAD BAD BAD HEAD
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abcd123 is the first bad commit
@colinodell

62. git bisect
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@colinodell
# of Commits # of Tests
9 4
90 8
900 16
Scales logarithmically – runs in O(log N)

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

64. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
strace
@colinodell

65. 5. 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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@colinodell

66. 6. Take a break
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• Clear your mind; start fresh
• Forget invalid assumptions
• Recharge your batteries
• Let your subconscious work on it
@colinodell

67. 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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@colinodell

68. Questions?
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@colinodell

69. 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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@colinodell

70. Thanks!
@colinodell / colinodell@gmail.com
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