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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Colin O’Dell
• Lead Web Developer at Unleashed Technologies
• PHP League Member
• league/commonmark
• league/html-to-markdown
• PHP 7 Upgrade Guide e-book
@colinodell
3. Overview
I. Importance of debugging
II. Debugging process
III. Tools & Techniques
IV. Q&A
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@colinodell
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.
@colinodell
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PlanningCoding
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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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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
• bin/console cache:clear
• composer install
• 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
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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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@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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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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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
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
•Breakpoints
•Conditional breakpoints
•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()
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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()
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()
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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
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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•var_dump()
•kint()
•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
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)
@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. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
netcat
@colinodell
63. Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
curl
@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
According to the Mythical Man Month, 1/6th
½ is testing/debugging
How much - actually writing code vs getting it working
Area where it’s worthwhile to become efficient
READ SLOWLY
Shouldn’t be done haphazardly
Like intuition, but not
Understanding without proof
Clarity; accuracy; precision
--
Focus on the process
Experience and “sixth sense” will come with time
NEXT: Debugging process
BE NICE / CONSTRUCTIVE
TODO ADD NOTES
When you give up on the proper approach
Apply workaround instead
Improper solutions will cause issues later
NEXT - EXAMPLE
Base85-encoded
Don’t be clever
TRANSITION NEEDED!!
Magical thinking
Article
sneaky
Bugs are almost always based on faulty assumptions
95% in your code
If helping others, assume their code is problem
Challenge their assumptions
Take with grain of salt
Define the symptoms
Collect everything you can
Automated tests preferred, but manual is okay too
LEGOS
Understand the fundamental nature
Exactly why
Other nice-to-haves:
File syncing
Git integration
Run tools
PhpStorm, Sublime Text, vim
NOT Notepad++ or Dreamweaver
Debuggers are awesome tool
…
Better than primitive approaches
Examine in real time
Xdebug
Chrome developer tools
Click 4 times
Dive in
Look at call stack
Advanced breakpoints
Grouping
Conditions
Stack trace
Grep for error message
Set breakpoint
For example
Examine variables
How did I get here? – call stack
Log/output results at different steps
Identify where things start going wrong
Log/output results at different steps
Identify where things start going wrong
Log/output results at different steps
Identify where things start going wrong
Log/output results at different steps
Identify where things start going wrong
Debugger is critical
Not only tool
Tools are not a substitute for thinking