Effective debugging
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The document outlines effective debugging techniques emphasizing that developers should focus on fixing problems rather than just identifying them. It provides practical examples and tools for debugging in PHP, including utilizing error logs, configuration settings, and the use of debugging functions. Additionally, it highlights common pitfalls in debugging and encourages asking for help when needed while ensuring one has collected adequate information.
![READ THE WEBSERVER ERROR LOG
Location depends on configuration
Typically/var/log/*/
PHPParseerror: [...]in/var/www/example.dev/public/index.phponline4"](https://image.slidesharecdn.com/effectivedebugging-140823102847-phpapp01/85/Effective-debugging-8-320.jpg)
![GREP FOR IT
$grep-rl"NotFound"*
...
src/Really/Not/Obvious/File.php
$grep-r"NotFound"*
...
src/Really/Not/Obvious/File.php 404=>'NotFound',
$catsrc/Really/Not/Obvious/File.php
...
functionerror(){
die($this->_statusCodes['404']);
}](https://image.slidesharecdn.com/effectivedebugging-140823102847-phpapp01/85/Effective-debugging-11-320.jpg)
![CAKEPHP AUTOMODELS
$model=ClassRegistry::init('MyModel');
$model->methodName();
SQLError:1064:YouhaveanerrorinyourSQLsyntax;[...]
check[...]fortherightsyntaxtousenear'methodName'
MyModeldoes nothave the function methodName
MyModelhas no behaviour bound implementing
methodName
$modelis an instance of AppModel](https://image.slidesharecdn.com/effectivedebugging-140823102847-phpapp01/85/Effective-debugging-36-320.jpg)
Effective debugging
- 1. EFFECTIVE DEBUGGING SPEND TIME FIXING PROBLEMS, NOT FINDING THEM /AndyDawson @AD7six
- 2. THE WEB IS JUST PLUMBING WITH BYTES Image:Wikipedia
- 3. OF COURSE IT'S NOT REALLY THAT SIMPLE Image:Flickr.com โ The hardestproblems to fix existhere. In the developer's head.
- 4. FIND IT WITH A HAMMER Image:Codinghorror.com //pickone thrownewException('Madeithere!'); print_r(debug_backtrace()); die(__FILE__.':'.__LINE__);
- 5. MOST APPS AREN'T THAT SIMPLE Ahammer willwork, butitmighttake awhile. Image:energysystemsgroup.com
- 6. HELLO WORLD EXAMPLE "Where's mywebpage"
- 7. CHECK THE HTTP RESPONSE CODE Is itactually an error?
- 8. READ THE WEBSERVER ERROR LOG Location depends on configuration Typically/var/log/*/ PHPParseerror: [...]in/var/www/example.dev/public/index.phponline4"
- 10. NOT AN ERROR EXAMPLE Notan error response code so no (direct) logmessages:"
- 11. GREP FOR IT $grep-rl"NotFound"* ... src/Really/Not/Obvious/File.php $grep-r"NotFound"* ... src/Really/Not/Obvious/File.php 404=>'NotFound', $catsrc/Really/Not/Obvious/File.php ... functionerror(){ die($this->_statusCodes['404']); }
- 12. CAKEPHP HELLO WORLD EXAMPLE "Whythe Four Oh Four?"
- 13. CHECK THE ERROR LOG app/tmp/error.log
- 15. DEBUG BASICS Configand functions everydeveloper should know about
- 16. PHP CONFIG Inifile settings are the defaults if notchanged (duh) display_errors(On/Off) log_errors(On) display_startup_errors(Off) error_reporting(On/Off) Runtime settings override the inifile -no effectif the file theyare in has aparse error (duh) ini_set('display_errors', 0/1)
- 19. A WAY TO REPRODUCE Have awayto consistentlyreproduce the error $curl-Ihttp://example.dev/ HTTP/1.1500InternalServerError Server:nginx Date:Sat,23Aug201410:33:46GMT Content-Type:text/html gitbisect-find regression errors quickly
- 21. WEBGRIND Turn on xdebugprofiling, and look atwhatarequestis doing
- 22. CLI DEBUGGING Pause execution with read: print_r($somethingInteresting); `readfoo`; Usefulwhen debuggingaloop.
- 23. NOT CAKEPHP CODE? Aghetto debugfunction: functiondebug($var,$showHtml=null){ if(!defined('DEBUG')||!DEBUG){return;} if($showHtml===null){ $showHtml=php_sapi_name()==='cli'?false:true; } $var=var_export($var,true); if(!$showHtml){ echo$var; return; } echo'<pre>'.htmlspecialchars($var).'</pre>'; } createatracefunction(Debugger::trace)tooifneeded
- 24. LOGIC AIDES Justvoicingaproblem can find the solution/error Image:Wikipedia
- 25. ERROR MESSAGES DON'T LIE PHP Parse error: syntax error, unexpected '$bar'(T_VARIABLE) in parse.php on line 3 $catfoo.php <?php ... if($foo||bar){ There's an accidentalnone-breakingspace on thatline
- 26. NETWORKING PROBLEMS If the problem is noton the webserver -where is it?
- 27. NAMESERVER PROBLEMS No response from nameservers is the same as adomain not existsing $digcakefest.org ;<<>>DiG9.8.3-P1<<>>cakefest.org ;;globaloptions:+cmd ;;Gotanswer: ;;->>HEADER<<-opcode:QUERY,status:SERVFAIL,id:15266 ;;flags:qrrdra;QUERY:1,ANSWER:0,AUTHORITY:0,ADDITIONAL:0 ;;QUESTIONSECTION: ;cakefest.org. IN A ;;Querytime:4142msec ;;SERVER:8.8.8.8#53(8.8.8.8) ;;WHEN:TueAug1916:26:592014 ;;MSGSIZE rcvd:30 ... ;;ANSWERSECTION: cakefest.org. 1568 IN A 50.56.232.22 ...
- 28. DNS PROBLEMS "No route to host"means the ip requested isn'taccessible $traceroutecakefest.org traceroutetocakefest.org(50.56.232.22),64hopsmax,52bytepackets 1 172.26.81.1(172.26.81.1) 1.032ms 0.912ms 0.912ms 2 192.168.0.1(192.168.0.1) 2.777ms 1.267ms 2.338ms ... 12 rackspace-ic-302090-dls-bb1.c.telia.net(62.115.33.78) 152.340ms 15 9.367ms 146.339ms 13 *** 14 *** 15 *** If there are stars -there be problems
- 29. BACKEND SERVER IS DOWN 502 Bad Gateway E.g. php-fpm or hhvm is notrunning Willbe in the webserver error log There willnotbe anyapplication logentries
- 30. PITFALLS TO AVOID Whatnotto do when debuggingcode
- 31. FIX PROBLEMS, NOT SYMPTOMS Don'tignore errors/warnings/notices Fix them in order, some errors willbe the concequence of earlier problems.
- 32. COMMON MISTAKES Notlookingfor error logs Readingor focussingon the wrongerror message Notreadingthe error message Notreadingthe error message aloud Misinterprettingthe error message Stoppingdebuggingtoo early. If "the problem"is aclass/function with source code -debug the source code of thatfunction New user: Ifound the problem, it's app/webroot/index.php!
- 33. IMPLICIT ASSUMPTIONS Be waryof implictassumptions You're debuggingthe same file the browser is loading You're debuggingthe same application the browser is loading You're debuggingthe same server the browser is loading You're debuggingthe same requestthe browser is loading Conclusions so far are accurate WTF?Backup, and re-verifyeverything.
- 34. COMMON PROBLEMS And how to debug/identifythem
- 35. NO MODREWRITE Don'tlook atphp files -the error is atthe webserver. Enable mod rewrite Restartthe webserver
- 36. CAKEPHP AUTOMODELS $model=ClassRegistry::init('MyModel'); $model->methodName(); SQLError:1064:YouhaveanerrorinyourSQLsyntax;[...] check[...]fortherightsyntaxtousenear'methodName' MyModeldoes nothave the function methodName MyModelhas no behaviour bound implementing methodName $modelis an instance of AppModel
- 37. CAKEPHP AUTOMODELS - IDENTIFICATION $model=ClassRegistry::init('MyModel'); debug(get_class($model)); ##########DEBUG########## 'MyModel' ########################### $model=ClassRegistry::init('MyModel'); debug(get_included_files()); ##########DEBUG########## array( ... ...app/Model/MyModel.php ... ) ###########################
- 38. CAN'T FIX IT (โฏยฐโกยฐ๏ผโฏ๏ธต โปโโป Can'tfind the problem/solution?-gethelp. Butfirst: Collectthe information you've got Write astandalone, reproducible example if possible Reduce the question to it's simplest, form Don'tover simplifyor make acontrived example Ask colleagues/the internet Brace for impact Stack overflow, the google group and irc as greatplaces to get help
- 39. SUMMARY Identifythe rightpartof arequestto debug Fix errors in order Check your assumptions/conclusions ateveryWTF Formulate aquestion, and Ask for help Profit!