The document discusses debugging effectively. It begins with an overview of the importance of debugging and the debugging process. It then covers specific debugging tools and techniques, including using an integrated development environment (IDE) and interactive debugger. It outlines a systematic debugging approach and provides tips for junior developers. Specific techniques like tracing code backwards and forwards are presented. The importance of gathering information, replicating issues, and ruling out assumptions is emphasized.

1. Debugging Effectively
Colin O’Dell
@colinodell

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Colin O’Dell
• Lead Web Developer at Unleashed Technologies
• Primary language is PHP
• PHP developer for 15 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)
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5. Debugging is...
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important
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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
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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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Gain experience with code and tools
Experience
Process

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

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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@colinodell

32. Interactive Debugger
•Pause code execution
•Step through execution
•Examine variables
•Explore call stack
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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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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()
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37. 1. Trace backwards
•Error is thrown from known location
•Use a debugger
•Establish context
•Work backwards
a()
b()
c()
d()
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38. 1. Trace backwards
•Error is thrown from known location
•Use a debugger
•Establish context
•Work backwards
a()
b()
c()
d()
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39. 1. Trace backwards
•Error is thrown from known location
•Use a debugger
•Establish context
•Work backwards
a()
b()
c()
d()
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40. 2. Trace forwards
•Opposite direction
•Problematic line
isn’t known
•Use debugger or logging
a()
b()
c()
d()
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41. 2. Trace forwards
•Opposite direction
•Problematic line
isn’t known
•Use debugger or logging
a()
b()
c()
d()
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42. 2. Trace forwards
•Opposite direction
•Problematic line
isn’t known
•Use debugger or logging
a()
b()
c()
d()
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43. 2. Trace forwards
•Opposite direction
•Problematic line
isn’t known
•Use debugger or logging
a()
b()
c()
d()
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44. 3. Divide & Conquer
• Identify different code sections
• Set breakpoints at the boundaries
• Isolate issue to one particular area
• Focus efforts on that area
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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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51. Variable dumps
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•Print to output:
•var_dump()
•pprint()
•puts()
•Log to file
•file_put_contents()
•Logger.debug()
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Debug toolbars
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53. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
Console Utility
Symfony: bin/console
Laravel: artisan
Drupal: drush
Drupal Console
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)
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55. git bisect
v1.7 ? ? ? ? ? ? ? HEAD
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56. git bisect
v1.7 ? ? ? ? ? ? ? HEAD
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@colinodell

57. git bisect
v1.7 ? ? ? BAD ? ? ? HEAD
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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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60. git bisect
v1.7 GOOD GOOD ? BAD BAD BAD BAD HEAD
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61. git bisect
v1.7 GOOD GOOD
X BAD BAD BAD BAD HEAD
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abcd123 is the first bad commit
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62. git bisect
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# of Commits # of Tests
9 4
90 8
900 16
Scales logarithmically – runs in O(log N)

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netcat
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strace
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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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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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68. Questions?
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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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