Rewriting Legacy Code

Did you ever have to maintain a 15-year-old application? Dead code everywhere, hard dependencies, null pointer exceptions, database queries mixed with HTML, no tests and most libraries have been discontinued.

This presentation will show you how to modernize your application using a variety of strategies. You will learn how to avoid common pitfalls, automate legacy testing, manage your development environment and overcome various challenges that arise from dealing with legacy.

Transcript

1. 
   Rewriting
   Legacy Code
   SKILLS MATTER | AUG 20, 2020
   @afilina
   

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2. Anna Filina
   ‣ Coding since 1997.
   ‣ Legacy archaeology.
   ‣ Test automation.
   ‣ Talks and workshops.
   ‣ Twitter advice (@afilina)
   ‣ YouTube videos.
   

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3. You Inherited a

   10-Year-Old Codebase
   

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4. You Inherited a

   15-Year-Old Codebase
   

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5. You Inherited a

   20-Year-Old Codebase
   

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7. History Since 2000
   2004
   PHP 5.0
   2005
   Symfony
   2006
   Zend Framework
   2008
   Clean Code
   2011
   Laravel
   2001
   PHPUnit
   

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8. Legacy Sadness
   

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9. The Methods
   ‣ Can easily exceed a thousand lines.
   ‣ Mixing SQL, PHP and HTML.
   ‣ Receiving dozens of arguments of arbitrary type.
   ‣ Accessing hundreds of potentially undefined properties.
   ‣ Expect properties to have been set 10 children down.
   ‣ methodName_new, methodName_new2
   

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10. The Dynamic Types
    ‣ Null pointer exceptions of every flavor.
    ‣ Attempting to split an array.
    ‣ Attempting to divide by a null.
    ‣ Attempting to count the number of elements in a float.
    ‣ Attempting to max($array1, $array2),

    where array items are of mixed types.
    

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11. How to Fail a Rewrite
    

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12. Microservices
    AWS
    GitHub
    Flutter
    CircleCI
    ElasticSearch Ansible
    Docker
    Symfony 5
    

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13. Start With the Strategy
    

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14. Strategy
    ‣ Version upgrade.
    ‣ New framework.
    ‣ Progressive rewrite.
    ‣ Refactoring only.
    ‣ Full rewrite.
    

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15. Version Upgrade
    ‣ No need to freeze development.
    ‣ PHPCodeSniffer can detect incompatibilities.
    ‣ Also need to patch libs & framework.
    ‣ Some libs might be easier to swap than to fix (PHPExcel).
    ‣ Regexes are your friends.
    ‣ Rector can automatically refactor.
    

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17. New Framework
    ‣ Use dependency injection.
    ‣ Automate large chunks with Rector.
    ‣ Use adapters.
    ‣ Need to freeze development.
    

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18. Product/IndexHandler
    ProductController
    indexAction
    viewAction
    Product/ViewHandler
    Routes
    Routes
    Product/BaseHandler
    

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19. class IndexHandler extends BaseHandler implements RequestHandlerInterface
    {
    public function __construct(
    TemplateRendererInterface $template,
    ProductModel $productModel
    ) {
    //...
    }
    public function handle(ServerRequestInterface $request): ResponseInterface
    {
    return new HtmlResponse(

    $this->template->render('admin::product/index', $view)

    );
    }
    }
    

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20. Progressive Rewrite
    ‣ By endpoint/feature
    • Can switch language.
    • Use design extraction.
    

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21. Old Code
    New Design
    Old Design
    New Code
    

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22. ‣ Remove dead code.
    ‣ Move logic to a better location.
    ‣ Split long functions.
    Techniques
    

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23. ‣ Some criteria for refactoring:
    • Need to change.
    • Hard to understand, but need to work with it.
    Refactoring Only
    

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24. Full Rewrite
    ‣ Discouraged.
    ‣ My criteria:
    • Can disregard existing features.
    • Few or no integrations.
    • Can trash old database.
    • If not, need a team experienced in full rewrites.
    

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25. Testing Approaches
    

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26. Version Upgrade
    Characterization tests Fix code
    Should work

    for old and new
    versions.
    Unit tests
    New/refactored code.
    What failed in E2E.
    

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27. New Framework
    Characterization tests Migrate
    Should work

    for old and new
    versions.
    Unit tests
    New/refactored code.
    What failed in E2E.
    

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28. Progressive Rewrite
    For new code only.
    Characterization tests Unit tests
    Should work

    for old and new
    versions.
    New code
    

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29. Refactoring Only
    For the 

    upcoming code.
    Characterization tests Unit tests
    Should work

    for old and new
    versions.
    Refactor
    

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30. Preparing the
    Environments
    

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31. Local Environment
    ‣ Docker.
    ‣ Baseline image.
    ‣ Migration image.
    ‣ Match server / vagrant.
    

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32. services:
    php_apache: //...
    mysql: //...
    acceptance: //...
    mysql_test: //...
    

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33. Gotchas
    ‣ Pay attention to MySQL mode.
    ‣ Run composer inside Docker, commit composer.lock
    ‣ Anonymized production database.
    ‣ Send e-mails to relay: DebugMail, MailTrap, etc.
    

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34. Staging Environment
    ‣ Baseline server.
    ‣ Migration server.
    ‣ Staging config: point to sandboxes.
    

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35. 
    @afilina
    @afilina
    

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