Software bugs are inevitable; some are especially difficult to track down, causing you to waste countless hours before throwing your hands up in defeat. It doesn't have to be this way! The mental fatigue and wasted time can be avoided by using strategies like identifying the most-appropriate tool, taking a logical & objective approach, challenging assumptions, listening to variables, isolating the code path, and reinforcing code with automated tests. Attendees will learn how to combine these techniques with the right mindset and attitude in order to debug their code quickly and effectively.

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Debugging Effectively
Colin O’Dell
@colinodell

2. Colin O’Dell
• Lead Web Developer at Unleashed Technologies
• PHP developer since 2002
• PHP League Member
• league/commonmark
• league/html-to-markdown
• PHP 7 Upgrade Guide e-book
• @colinodell / www.colinodell.com
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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...
important
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6. Debugging is...
the single most
important skill in
programming.
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7. PlanningCoding
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8. 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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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
• app/console cache:clear
• composer install
• 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
•I want to solve problem X
•How do I solve X?
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15. XY Problem
•I want to solve problem X
•How do I solve X?
•Solution Y might work
•How can I do Y?
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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. •Expected behavior
vs. actual behavior
•Error messages
•Stack traces
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•Screenshots
•Browser & OS
•Date & time
•Log entries
1. Gather Information

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

25. • 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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4. Fix & Re-test

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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33. Photo by florianric // cc by 2.0 // https://flic.kr/p/c4QJzC

34. Techniques
1. Trace backwards from known issue
2. Divide & conquer
3. Use tools
4. Get help
5. Take a break
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35. 1. Trace backwards
•Use a debugger
•Identify source of error
•Establish context
•Work backwards
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36. 1. Trace backwards
a()
b()
c()
d()
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37. 1. Trace backwards
a()
b()
c()
d()
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38. 2. Divide & Conquer
• Identify different code sections
• Set breakpoints at the boundaries
• Isolate issue to one particular area
• Focus efforts on that area

42. ✓
X

44. 3. Use tools
• VarDumper
• Debug toolbars
• Console utility
• Profilers
• git bisect
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•netcat
•curl
•strace
•etc.

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

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Debug toolbars

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Console Utility
Symfony: bin/console
Drupal: drush
Drupal Console
Magento: n98-magerun
Laravel: artisan

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

49. git bisect
v1.7 ? ? ? ? ? HEAD
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50. git bisect
v1.7 ? ? BAD ? ? HEAD
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51. git bisect
v1.7 ? ? BAD BAD BAD HEAD
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52. git bisect
v1.7 GOOD ? BAD BAD BAD HEAD
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53. git bisect
v1.7 GOOD
X BAD BAD BAD HEAD
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abcd123 is the first bad commit

54. netcat
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55. 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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56. 5. Take a break
• Clear your mind; start fresh
• Forget invalid assumptions
• Recharge your batteries
• Let your subconscious work on it
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57. 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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58. Questions?
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59. 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://symfony.com/doc/current/components/var_dumper/introducti
on.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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60. Thanks!
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