Wrangle Cross-cutting Concerns with Event Driven Development

Event driven programming is becoming essential to many applications and frameworks that can be utilized to make your application more flexible and “plugin” ready.

Learn how to effectively use events in applications to reduce code complexity of cross-cutting concerns, how various frameworks implement events and make them available to the developer, and the benefits and drawbacks of utilizing aspect oriented development with real world examples.

We will also look at many popular frameworks (Symfony2, CakePHP, Zend, etc) to see how their event architecture is implemented at a bird’s-eye view and how developers can take advantage, including a demo using Symfony2’s dispatcher to illustrate the benefits of event driven design.

Transcript

1. Event Driven
   Applications
   Wrangle Cross-Cutting Concerns
   

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2. Chris Saylor
   Lead Engineer @zumbatech
   www.chris-saylor.com
   @cjsaylor
   

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3. A little about Zumba
   ● 180 Countries
   ● 15 million class participants
   ● 60 million service requests per month
   

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4. Indicates my opinion on a subject.
   Feel free to discuss over beers.
   

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5. What are cross-cutting concerns?
   Cross-cutting concerns are parts of a program
   that rely on or must affect many other parts of
   the system.
   

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6. Cross-cutting examples
   ● Logging
   ● Caching
   ● Product feature interaction
   ● Monitoring
   ● Record audit trail
   ● State Management
   

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7. class Cart
   {
   public function addItem(Item $item)
   {
   $this->cart->add($item);
   $this->cart->save();
   }
   }
   

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8. class Cart
   {
   public function addItem(Item $item)
   {
   $this->cart->add($item);
   if ($this->promo->isApplicable($this->cart, $item))
   {
   $this->promo->applyItemPromo($item);
   }
   $this->cart->save();
   }
   }
   

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9. class Cart
   {
   public function addItem(Item $item)
   {
   if (!$this->inventory->isAvailable($item))
   {
   throw new ItemUnavailable();
   }
   $this->cart->add($item);
   if ($this->promo->isApplicable($this->cart, $item))
   {
   $this->promo->applyItemPromo($item);
   }
   $this->cart->save();
   }
   }
   

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10. class Cart
    {
    public function addItem(Item $item)
    {
    if (!$this->inventory->isAvailable($item))
    {
    throw new ItemUnavailable();
    }
    $this->cart->add($item);
    if ($this->promo->isApplicable($this->cart, $item))
    {
    $this->promo->applyItemPromo($item);
    }
    $this->inventory->reduceEphemeral($item);
    $this->cart->save();
    }
    }
    

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11. class Cart
    {
    public function addItem(Item $item)
    {
    if (!$this->inventory->isAvailable($item))
    {
    throw new ItemUnavailable();
    }
    if (!$this->catalog->isUserAllowed($this->cart, $item))
    {
    throw new ItemNotAllowed();
    }
    $this->cart->add($item);
    if ($this->promo->isApplicable($this->cart, $item))
    {
    $this->promo->applyItemPromo($item);
    }
    $this->inventory->reduceEphemeral($item);
    $this->cart->save();
    }
    }
    

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12. Our cart is overloaded!
    

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13. Cyclomatic Complexity
    A measure of the complexity by the number of
    linearly independent paths through the
    program.
    M = E - N + 2P
    E = the number of edges of the graph
    N = the number of nodes of the graph
    P = The number of connected components
    

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14. class Cart
    {
    public function addItem(Item $item)
    {
    $this->cart->add($item);
    $this->cart->save();
    }
    }
    

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15. View Slide

16. 3 - 4 + 2 * 1 = 1
    

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17. class Cart
    {
    public function addItem(Item $item)
    {
    if (!$this->inventory->isAvailable($item))
    {
    throw new ItemUnavailable();
    }
    if (!$this->catalog->isUserAllowed($this->cart, $item))
    {
    throw new ItemNotAllowed();
    }
    $this->cart->add($item);
    if ($this->promo->isApplicable($this->cart, $item))
    {
    $this->promo->applyItemPromo($item);
    }
    $this->inventory->reduceEphemeral($item);
    $this->cart->save();
    }
    }
    

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18. View Slide

19. 13 - 11 + (2 * 1) = 4
    Four times as much complexity!
    

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20. Risks of cross-cutting concerns
    ● Coupling systems too tightly.
    ● Lots of code duplication.
    ● Brittle Test Cases.
    

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21. Cart
    Promo Inventory Catalog
    

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22. Cart
    Promo Inventory Catalog
    Changing Promo can affect Cart.
    

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23. class Cart
    {
    public function addItem(Item $item)
    {
    if (!$this->inventory->isAvailable($item)) {
    throw new ItemUnavailable();
    }
    if (!$this->catalog->isUserAllowed($this->cart, $item))
    {
    throw new ItemNotAllowed();
    }
    $this->cart->add($item);
    if ($this->promo->isPromo($item)) {
    $this->promo->applyItemPromo($item);
    }
    $this->cart->save();
    $this->inventory->reduceEphemeral($item);
    }
    }
    

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24. class Cart
    {
    public function updateItem(Item $item, $line)
    {
    if (!$this->inventory->isAvailable($item)) {
    throw new ItemUnavailable();
    }
    if (!$this->catalog->isUserAllowed($this->cart, $item))
    {
    throw new ItemNotAllowed();
    }
    $this->cart->update($item, $line);
    if ($this->promo->isPromo($item)) {
    $this->promo->applyItemPromo($item);
    }
    $this->cart->save();
    $this->inventory->reduceEphemeral($item);
    }
    }
    

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25. class Cart
    {
    public function updateItem(Item $item, $line)
    {
    if (!$this->inventory->isAvailable($item)) {
    throw new ItemUnavailable();
    }
    if (!$this->catalog->isUserAllowed($this->cart, $item))
    {
    throw new ItemNotAllowed();
    }
    $this->cart->update($item, $line);
    if ($this->promo->isPromo($item)) {
    $this->promo->applyItemPromo($item);
    }
    $this->cart->save();
    $this->inventory->reduceEphemeral($item);
    }
    }
    11 of 13 lines (85%) are duplicated from addItem!
    

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26. Unit tests become unmaintainable and complex.
    class CartTest extends PHPUnit_Framework_TestCase
    {
    public function testCanAddItem() {
    // Mock promo
    // Mock inventory
    // Mock catalog
    }
    public function testUnableToAddItemFromCatalogUserLimit()
    {
    // More mocks
    }
    public function testAddItemWithPromotionRelated()
    {
    // Ugh...
    }
    public function testUnableToAddItemFromUnavailability()
    {
    // ...
    }
    }
    

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27. View Slide

28. Event Patterns
    Observer Pattern Mediator Pattern
    

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29. Subject
    Observer
    Observer
    Observer
    Observer
    Observer Pattern
    

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31. class Subject implements \SplSubject
    {
    private $observers;
    public function __construct()
    {
    $this->observers = new \SplObjectStorage();
    }
    public function attach(\SplObserver $observer)
    {
    $this->observers->attach($observer);
    }
    public function detach(\SplObserver $observer)
    {
    $this->observers->detach($observer);
    }
    public function notify()
    {
    foreach ($this->observers as $observer) {
    $observer->update($this));
    }
    }
    }
    

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33. class Observer implements \SplObserver
    {
    public function update(\SplSubject $subject)
    {
    echo 'Observer notified!';
    }
    }
    

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34. $observer = new Observer();
    $subject = new Subject();
    $subject->attach($observer);
    $subject->notify();
    // Observer notified!
    The subject has the responsibility of the Mediator.
    

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35. Mediator Pattern
    Subject Mediator
    Observer
    Observer
    Subscribe
    Subscribe
    

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36. Separation of Concerns
    Cart
    Promo Handler
    Inventory Handler
    Catalog Handler
    

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37. Separation of Concerns
    Cart
    Promo Handler
    Inventory Handler
    Catalog Handler
    Event Bus
    

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38. Synchronous
    Separation of Concerns
    ● Provides for modularity
    ● Allow for unit testing handlers separately
    ● Allows for priority and interrupts
    

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39. class Cart
    {
    public function addItem(Item $item)
    {
    if (!$this->inventory->isAvailable($item))
    {
    throw new ItemUnavailable();
    }
    if (!$this->catalog->isUserAllowed($this->cart, $item))
    {
    throw new ItemNotAllowed();
    }
    $this->cart->add($item);
    if ($this->promo->isApplicable($this->cart, $item))
    {
    $this->promo->applyItemPromo($item);
    }
    $this->inventory->reduceEphemeral($item);
    $this->cart->save();
    }
    }
    

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40. ● Always be descriptive with event names.
    ● Always name them in past tense.
    ● Don’t emit the same event from more than
    one location.
    Event Conventions
    

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41. class Cart
    {
    public function addItem(Item $item)
    {
    $this->dispatcher->dispatch(
    'cart.addItemStarted',
    new GenericEvent($this->cart, compact('item'));
    );
    $this->cart->add($item);
    if ($this->promo->isApplicable($this->cart, $item))
    {
    $this->promo->applyItemPromo($item);
    }
    $this->inventory->reduceEphemeral($item);
    $this->cart->save();
    }
    }
    

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42. class Cart
    {
    public function addItem(Item $item)
    {
    $this->dispatcher->dispatch(
    'cart.addItemStarted',
    new GenericEvent($this->cart, compact('item'));
    );
    $this->cart->add($item);
    $this->dispatcher->dispatch(
    'cart.itemAdded',
    new GenericEvent($this->cart, compact('item'));
    );
    $this->cart->save();
    }
    }
    

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43. class Cart
    {
    private function dispatch($name, array $payload = [])
    {
    return $this->dispatcher->dispatch(
    'cart.' . $name,
    new GenericEvent($this->cart, $payload);
    );
    }
    public function addItem(Item $item)
    {
    $this->dispatch('addItemStarted', compact('item'));
    $this->cart->add($item);
    $this->dispatch('itemAdded', compact('item'));
    $this->cart->save();
    }
    }
    Almost back to the original simple addItem.
    

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44. 5 - 6 + (2 * 1) = 1
    

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45. Cart
    Promo Inventory Catalog
    

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46. Cart
    Promo Inventory Catalog
    Inversion of responsibility!
    

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47. Cart
    Promo Inventory Catalog
    Riskiest piece now changes less frequently.
    

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48. class CartTest extends PHPUnit_Framework_TestCase
    {
    public function testCanAddItem() {
    // Mock promo
    // Mock inventory
    // Mock catalog
    }
    public function testUnableToAddItemFromCatalogUserLimit()
    {
    // More mocks
    }
    public function testAddItemWithPromotionRelated()
    {
    // Ugh...
    }
    public function testUnableToAddItemFromUnavailability()
    {
    // ...
    }
    }
    

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49. class CartTest extends PHPUnit_Framework_TestCase
    {
    public function testAddItem() {
    // Mock dispatcher and spy on dispatch
    }
    }
    

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50. class InventoryHandler implements EventSubscriberInterface
    {
    }
    

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51. class InventoryHandler implements EventSubscriberInterface
    {
    // construct the inventory manager...
    public static function getSubscribedEvents()
    {
    return [
    ‘cart.addItemStarted’ => ‘checkAvailability’
    ];
    }
    }
    

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52. class InventoryHandler implements EventSubscriberInterface
    {
    // construct the inventory manager...
    public static function getSubscribedEvents()
    {
    return [
    ‘cart.addItemStarted’ => ‘checkAvailability’
    ];
    }
    public function checkAvailability(Event $e)
    {
    $item = $e->getArgument(‘item’);
    if (!$this->inventory->isAvailable($item)) {
    throw new ItemUnavailable();
    }
    }
    }
    

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53. Asynchronous
    ● Synchronous in order of execution.
    ● Asynchronous in order of completion.
    $('p')
    .on('click', function() {
    setTimeout(function() {
    console.log('first called, second to finish!');
    }, 1000);
    })
    .on('click', function() {
    console.log('second called, first to finish.');
    });
    

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54. Synchronous
    ● Predictable order of execution and
    completion.
    $event
    ->on('event1', function() {
    echo 'Always first.';
    sleep(1);
    })
    ->on('event1', function() {
    echo 'Always second.';
    });
    

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55. Events in the Wild
    

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56. ● Symfony 2
    ● Laravel
    ● CakePHP
    ● Lithium
    ● Yii
    ● Zend Framework
    ● Doctrine
    ● Guzzle
    ● Magento
    Events in the Wild
    

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57. Advantages & Drawbacks
    ● Modular design
    ● Separation of Concerns
    ● Side effects are not
    obvious from looking at
    code.
    ● Source of event is not
    always obvious when
    debugging.
    Decoupling Code
    

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58. Advantages & Drawbacks
    ● Allows third parties to
    hook in and modify
    behavior without
    modifying the core.
    ● Easy to add/remove
    hooks (even at runtime).
    ● No dependencies should
    exist between plugin
    hooks.
    Plugin Architecture
    

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59. Advantages & Drawbacks
    ● Test handlers separate
    from the core
    functionality.
    ● Allows for execution of
    events from manual
    triggers.
    ● Testing interaction of
    handlers is cumbersome
    (functional testing).
    ● Unbinding specific event
    callbacks can be difficult.
    Testing
    

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60. Conclusions
    ● We can take a relatively complex system
    and simplify key systems.
    ● We can remove concerns from key systems.
    ● We can modularize concerns into a
    containable / testable unit.
    ● We can allow for plugin architectures.
    

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61. Further Reading
    ● Use application events to hook in plugins
    ● Decoupling applications with domain events
    ● Domain Events (Martin Fowler)
    

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62. Questions?
    

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63. Thanks!
    

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64. Please leave feedback!
    https://joind.in/14244
    

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