debugging effectively - php uk 2017
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Debugging Effectively
Colin O’Dell
@colinodell
Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
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
OverviewI. Importance of debuggingII. Debugging processIII. Tools & TechniquesIV. Q&A
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@colinodell
Debugging is...Photo by Ewan Cross // cc by-nc-nd 2.0 // https://flic.kr/p/7MCt3m
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ (adjective)
@colinodell
Debugging is...Photo by Ewan Cross // cc by-nc-nd 2.0 // https://flic.kr/p/7MCt3m
important@colinodell
Debugging is...Photo by Ewan Cross // cc by-nc-nd 2.0 // https://flic.kr/p/7MCt3m
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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Photo by Matthew Paulson // cc by-nc-nd 2.0 // https://flic.kr/p/di35xa
Gain experience with code and tools
Experience
Process
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Develop your intuition
IntuitionExperience
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 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?
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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?•Solution Y might work•How can I do Y?
@colinodell
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1.Don’t parse HTML with regex2.Solve problems the right way
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“I don’t know why”
“For some reason”
“Doesn’t make sense”
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@colinodell
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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@colinodell
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
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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
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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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@colinodell
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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@colinodell
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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@colinodell
Techniques1. Trace backwards from known issue2. Trace forwards from start 3. Binary search4. Use tools5. Get help6. Take a break
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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()
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
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 lineisn’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 lineisn’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 lineisn’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 lineisn’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
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@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 dumpsPhoto by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
•var_dump()•kint()•file_put_contents()•$logger->debug()
@colinodell
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Debug toolbars
@colinodell
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Console Utility
Symfony: bin/consoleLaravel: artisan
Drupal: drushDrupal 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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Photo by Joseph B // cc by-nc-nd 2.0 // https://flic.kr/p/7GAMBe
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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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 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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@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-co
de/• 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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