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Debugging Effectively

Colin O’Dell

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Colin O’Dell• Lead Web Developer at Unleashed Technologies• PHP developer since 2002• Certifications: Symfony and Magento• PHP League Member• league/commonmark• league/html-to-markdown

• PHP 7 Upgrade Guide e-book• @colinodell / www.colinodell.com

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

important

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

the single most important skill in

programming.

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PlanningCoding

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Planning

Testing & Debugging

Coding

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”

“Sixth Sense

”Experience

Process

Junior Developers• Try the “usual” steps• Clear the cache• Re-install dependencies• chmod –R 777 *

• Google the error• Try every solution

• Ask somebody else• Co-worker• StackOverflow post

• Give up

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XY ProblemPhoto by Yoel Ben-Avraham, with changes // cc by--nd 2.0 // https://flic.kr/p/6pmtQL

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

XY ProblemPhoto by Yoel Ben-Avraham, with changes // cc by--nd 2.0 // https://flic.kr/p/6pmtQL

•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 are logical

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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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1. Gather information

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

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

2. Replicate the Issue

Be able to replicate with 100% certainty

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3. Identify the CulpritPhoto by U.S. Navy // cc 2.0 // https://flic.kr/p/n6Wgks

• Be methodical• Make no assumptions• Understand the bug

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

a()

b()

c()

d()

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

a()

b()

c()

d()

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

a()

b()

c()

d()

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

a()

b()

c()

d()

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1. Trace backwards•Use a debugger• Identify source of error•Establish context•Work 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• Automated tests• Kint / VarDumper• Debug toolbars• Console utility • Profilers

• git bisect•netcat•curl•etc.

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Automated TestsPhoto by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe

KintPhoto by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe

Kint::dump($var);

VarDumperPhoto by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe

Twig: {{ dump(foo) }}{% dump foo %}

PHP: dump($somevar);

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Debug toolbars

Console Utility

Symfony: app/consoleDrupal: drushMagento: n98-magerun.pharLaravel: artisan

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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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New RelicPhoto by Alan Stark // cc by-sa 2.0 // https://flic.kr/p/dn3qjx

git bisect

v1.7 ? ? ? ? ? ? ? HEAD

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git bisect start

git bisect bad

git bisect

v1.7 ? ? ? ? ? ? ? HEAD

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git bisect good

git bisect

v1.7 ? ? ? ? ? ? ? HEAD

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git bisect bad

git bisect

v1.7 ? ? ? BAD ? ? ? HEAD

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git bisect bad

git bisect

v1.7 ? ? ? BAD BAD BAD BAD HEAD

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git bisect good

git bisect

v1.7 ? GOOD ? BAD BAD BAD BAD HEAD

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git bisect good

git bisect

v1.7 GOOD GOOD ? BAD BAD BAD BAD HEAD

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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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netcat

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curl

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

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• Clear your mind; start fresh• Forget invalid assumptions• Recharge your batteries• Let your subconscious work on it

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?

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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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Thanks!Feedback?• https://joind.in/14752 •@colinodell

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