Debugging Effectively
Colin O’Dell
@colinodell

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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

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

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

Debugging is...
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important
@colinodell

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

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PlanningCoding
@colinodell

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Planning
Testing &
Debugging
Coding
@colinodell

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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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

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

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

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

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

Bugs are logical
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@colinodell

“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

Assume your
code is the
problem
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@colinodell

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

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

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

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

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

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

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

Long-Term Results
• Gain experience
• Learn how the system works
• Build heuristics
• Boost confidence
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Tools & Techniques
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@colinodell

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

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

Interactive Debugger
•Pause code execution
•Breakpoints
•Conditional breakpoints
•Step through execution
•Examine variables
•Explore call stack
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@colinodell

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

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

1. Trace backwards
•Error is thrown from known location
•Use a debugger
•Establish context
•Work backwards
a()
b()
c()
d()
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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

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

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

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

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

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

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

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

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

@colinodell

@colinodell

@colinodell

✓
X
@colinodell

@colinodell

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

Variable dumps
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•var_dump()
•kint()
•file_put_contents()
•$logger->debug()
@colinodell

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Debug toolbars
@colinodell

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

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

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

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

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

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

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

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

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

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netcat
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curl
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strace
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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

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

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

Questions?
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@colinodell

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

Thanks!
Feedback Appreciated!
• https://joind.in/talk/549fd
• @colinodell / colinodell@gmail.com
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