Rewriting Legacy Code

Video: https://youtu.be/bTuvjjtGipY

Did you ever have to maintain a 20-year-old application? Dead code and tables everywhere, static methods, database queries in between HTML tags and some pages still in PHP3.

This presentation will lead you through a progressive rewrite from very old legacy to the latest shiny version of PHP. Learn how to automate legacy testing, how to pick the right rewrite strategy, and how to overcome other challenges that arise from dealing with legacy.

Transcript

1. 
   Rewriting
   Legacy Code
   THE LEGACY OF SOCRATES | MAY 13, 2020
   @afilina
   

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2. Anna Filina
   ‣ Coding since 1997.
   ‣ Legacy archaeology.
   ‣ Test automation.
   ‣ Talks and workshops.
   ‣ 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. Web in 2000
   ‣ Y2K bug.
   ‣ Internet Explorer 5.
   ‣ ICQ was starting to be cool.
   ‣ Background music.
   ‣ Rounded corners = status symbol.
   

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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 by a child 10 method calls ago.
   ‣ 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
    React
    CircleCI
    ElasticSearch Ansible
    Docker
    Symfony
    Ambitious new tech stack
    

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13. Other Ambitions
    ‣ SOLID.
    ‣ Test-driven development.
    ‣ Strong typing.
    Too many new things at once
    

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14. Expanding Scope
    ‣ Rethinking existing features.
    ‣ New features.
    ‣ New work methodology.
    

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15. Start With the Strategy
    

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

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17. Version Upgrade
    ‣ PHPCodeSniffer can detect incompatibilities.
    ‣ Some libs might be easier to swap than to fix (PHPExcel).
    ‣ Regexes are your friends.
    ‣ Rector can automatically refactor.
    ‣ Need to freeze development.
    

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

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20. Product/IndexHandler
    ProductController
    indexAction
    viewAction
    Product/ViewHandler
    Routes
    Routes
    Product/BaseHandler
    Example: Zend Framework 1 to Zend Expressive
    

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21. 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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22. Progressive Rewrite
    ‣ By class.
    ‣ By module
    • Can switch language.
    • Use design extraction.
    

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23. Old Code
    New Design
    Old Design
    New Code
    

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

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25. 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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26. Spot Problems Early
    ‣ PHPStorm inspections.
    ‣ Pslam: start with auto-detected level.
    

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27. Unknown
    Unknowns
    

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28. Docs
    ‣ No docs.
    ‣ Too many, little relevance.
    ‣ Outdated or misleading.
    

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29. Tests
    ‣ No tests.
    ‣ Not runnable.
    ‣ Failing.
    ‣ Brittle: specific environment or inter-dependencies.
    ‣ Unreliable.
    

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30. Dead Code
    ‣ Unused endpoints.
    ‣ Commented-out code.
    ‣ Unused classes or methods: exercise extreme caution.
    ‣ Merge duplicated code (bug repeated 80 times).
    

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31. Testing Approaches
    

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32. Version Upgrade
    Smoke tests Fix code
    Should work

    for old and new
    versions.
    Unit tests
    

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33. New Framework
    Smoke tests Migrate
    Should work

    for old and new
    versions.
    Unit tests
    

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34. Progressive Rewrite
    For new code only.
    Acceptance tests Unit tests
    Should work

    for old and new
    versions.
    New code
    

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35. Refactoring Only
    For the code that
    you want to write.
    Acceptance tests Unit tests
    Should work

    for old and new
    versions.
    Refactor
    

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36. Preparing the
    Environments
    

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

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38. services:
    php: //...
    mysql: //...
    acceptance: //...
    mysql_test: //...
    

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

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41. Credit: xebia.com
    

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42. 
    THANKS!
    @afilina
    

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