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Debugging Effectively

Colin O’Dell@colinodell

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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OverviewI. Importance of debuggingII. Debugging processIII. Tools & TechniquesIV. Q&A

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Debugging is...Photo by Ewan Cross // cc by-nc-nd 2.0 // https://flic.kr/p/7MCt3m

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ (adjective)

Debugging is...

important

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Debugging is...the single most

important skill in programming.

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PlanningCoding

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Planning

Testing & Debugging

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

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 tools and code

Experience

Process

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Develop a “sixth sense”Experience

Process

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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XY Problem

•I want to solve problem X•How do I solve X?

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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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1.Don’t parse HTML with regex2.Solve problems the right way

“I don’t know why”

“For some reason”

“Doesn’t make sense”

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Bugs arelogical

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“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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Assume your code is the problem

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Systematic Approach

1. Gather information2. Replicate the issue3. Identify the culprit4. Fix it & re-test5. Mitigate future occurrences

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• Expected behaviorvs. actual behavior• Error messages• Stack traces

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• Screenshots•Browser & OS•Date & time• Log entries

1. Gather Information

2. Replicate the Issue

Be able to replicate with 100% certainty

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• Be methodical• Make no assumptions• Understand the bug

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3. Identify the Culprit

• 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

5. Mitigate Future Occurrences•Add an automated test• Share your new knowledge• Project documentation• Blog post• StackOverflow

• Submit patch upstream

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Recap

1. Gather information2. Replicate the issue3. Identify the culprit4. Fix it & re-test5. Mitigate future occurrences

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Long-Term Results

• Gain experience• Learn how the system works• Build heuristics• Boost confidence

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Tools &Techniques

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Two essential tools

• Integrated development environment (IDE)

• Interactive debugger

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Integrated Development Environment

• Minimum features:• Syntax highlighting• Auto-completion• Fast code navigation• Debugger

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Interactive Debugger

• Pause code execution• Breakpoints• Conditional breakpoints

• Step through execution• Examine variables• Explore call stack

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Techniques1. Trace backwards from known issue2. Divide & conquer3. Use tools4. Get help5. Take a break

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1. Trace backwards•Use a debugger• Identify source of error•Establish context•Work backwards

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1. Trace backwards

a() b()

c() d()

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1. Trace backwards

a() b()

c() d()

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2. Divide & Conquer

• Identify different code sections• Set breakpoints at the boundaries• Isolate issue to one particular area• Focus efforts on that area

✓

X

3. Use tools• VarDumper• Debug toolbars• Console utility• Profilers• git bisect

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•netcat•curl• strace•etc.

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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/consoleDrupal: drush

Drupal ConsoleMagento: n98-magerunLaravel: artisan

Performance Profiling

Identify slowness:• Bottlenecks• Resource hogs• Inefficient code

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Tools:• Blackfire (freemium)• New Relic (freemium)• xhprof (open-source)

git bisect

v1.7 ? ? ? ? ? HEAD

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git bisect

v1.7 ? ? BAD ? ? HEAD

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git bisect

v1.7 ? ? BAD BAD BAD HEAD

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git bisect

v1.7 GOOD ? BAD BAD BAD HEAD

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git bisect

v1.7 GOOD X BAD BAD BAD HEAD

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abcd123 is the first bad commit

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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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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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Four things to walk away with

1. Computers aren’t random,and neither are bugs

2. Persistence will always pay off3. Don’t be afraid to dive deep4. Don’t make assumptions or

take things for granted

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Questions?Photo by Marcello Maria Perongini // cc by-nc-nd 2.0 // https://flic.kr/p/6KDtm

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/introduction.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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Thanks!

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