Practical Event Sourcing

Traditionally, we create structural models for our applications, and store the state of these models in our databases.

But there are alternatives: Event Sourcing is the idea that you can store all the domain events that affect an entity, and replay these events to restore the object's state. This may sound counterintuitive, because of all the years we've spent building relational, denormalized database schemas. But it is in fact quite simple, elegant, and powerful.

In the past year, I've had the pleasure of building and shipping two event sourced systems. In this session, I will show practical code, to give you a feel of how you can build event sourced models using PHP.

Mathias Verraes is a recovering music composer turned programmer, consultant, blogger, speaker, and podcaster. He advises companies on how to build enterprise web applications for complex business domains . For some weird reason, he enjoys working on large legacy projects: the kind where there’s half a million lines of spaghetti code, and nobody knows how to get the codebase under control. He’s the founder of the Domain-Driven Design Belgium community. When he’s not working, he’s at home in Kortrijk, Belgium, helping his two sons build crazy Lego train tracks.

http://verraes.net

Transcript

1. Sourcing
   Event
   Practical
   @mathiasverraes
   

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2. Mathias Verraes
   Student of Systems
   Meddler of Models
   Labourer of Legacy
   verraes.net
   mathiasverraes
   

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3. Elephant in the Room
   Podcast with @everzet
   elephantintheroom.io
   @EitRoom
   

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4. DDDinPHP.org
   

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5. The Big Picture
   

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6. Client
   Write
   Model
   Read
   Model
   DTO
   Commands
   Events
   CQRS: http://verraes.net/2013/12/fighting-bottlenecks-with-cqrs/
   

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7. Write
   Model
   Events
   Events
   Read
   Model
   This talk
   

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8. Event Sourcing
   

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9. Using on object’s
   history
   to reconstitute its
   State
   

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10. Express
    history
    as a series of
    Domain Events
    

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11. Something that has
    happened in the past
    that is of
    interest to the business
    Domain Event
    

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12. !
    happened in the past
    !
    

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13. Express
    history
    in the
    Ubiquitous Language
    

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14. Relevant
    to the business.
    !
    First class citizens of the
    Domain Model
    

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15. Domain Events
    

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16. interface DomainEvent
    {
    /**
    * @return IdentifiesAggregate
    */
    public function getAggregateId();
    }
    

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17. final class ProductWasAddedToBasket implements DomainEvent
    {
    private $basketId, $productId, $productName;
    !
    public function __construct(
    BasketId $basketId, ProductId $productId, $productName
    ) {
    $this->basketId = $basketId;
    $this->productName = $productName;
    $this->productId = $productId;
    }
    !
    public function getAggregateId()
    {
    return $this->basketId;
    }
    !
    public function getProductId()
    {
    return $this->productId;
    }
    !
    public function getProductName()
    {
    return $this->productName;
    }
    }
    

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18. final class ProductWasRemovedFromBasket implements DomainEvent
    {
    private $basketId;
    private $productId;
    !
    public function __construct(BasketId $basketId, ProductId $productId)
    {
    $this->basketId = $basketId;
    $this->productId = $productId;
    }
    !
    public function getAggregateId()
    {
    return $this->basketId;
    }
    !
    public function getProductId()
    {
    return $this->productId;
    }
    }
    

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19. final class BasketWasPickedUp implements DomainEvent
    {
    private $basketId;
    !
    public function __construct(BasketId $basketId)
    // You may want to add a date, user, …
    {
    $this->basketId = $basketId;
    }
    !
    public function getAggregateId()
    {
    return $this->basketId;
    }
    }
    

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20. Domain Events
    are
    immutable
    

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21. RecordsEvents
    

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22. $basket = Basket::pickUp(BasketId::generate());
    $basket->addProduct(new ProductId('AV001'), “The Last Airbender");
    $basket->removeProduct(new ProductId('AV001'));
    !
    !
    $events = $basket->getRecordedEvents();
    !
    it("should have recorded 3 events",
    3 == count($events));
    !
    it("should have a BasketWasPickedUp event",
    $events[0] instanceof BasketWasPickedUp);
    !
    it("should have a ProductWasAddedToBasket event",
    $events[1] instanceof ProductWasAddedToBasket);
    !
    it("should have a ProductWasRemovedFromBasket event",
    $events[2] instanceof ProductWasRemovedFromBasket);
    !
    !
    // Output:
    ✔ It should have recorded 3 events
    ✔ It should have a BasketWasPickedUp event
    ✔ It should have a ProductWasAddedToBasket event
    ✔ It should have a ProductWasRemovedFromBasket event
    TestFrameworkInATweet https://gist.github.com/mathiasverraes/9046427
    

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23. final class Basket implements RecordsEvents
    {
    public static function pickUp(BasketId $basketId)
    {
    $basket = new Basket($basketId);
    $basket->recordThat(
    new BasketWasPickedUp($basketId)
    );
    return $basket;
    }
    !
    public function addProduct(ProductId $productId, $name)
    {
    $this->recordThat(
    new ProductWasAddedToBasket($this->basketId, $productId, $name)
    );
    }
    !
    public function removeProduct(ProductId $productId)
    {
    $this->recordThat(
    new ProductWasRemovedFromBasket($this->basketId, $productId)
    );
    }
    !
    // continued on next slide
    

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24. // continued: final class Basket implements RecordsEvents
    !
    private $basketId;
    !
    private $latestRecordedEvents = [];
    !
    private function __construct(BasketId $basketId)
    {
    $this->basketId = $basketId;
    }
    !
    public function getRecordedEvents()
    {
    return new DomainEvents($this->latestRecordedEvents);
    }
    !
    public function clearRecordedEvents()
    {
    $this->latestRecordedEvents = [];
    }
    !
    private function recordThat(DomainEvent $domainEvent)
    {
    $this->latestRecordedEvents[] = $domainEvent;
    }
    !
    }
    

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25. Protecting Invariants
    

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26. $basket = Basket::pickUp(BasketId::generate());
    !
    $basket->addProduct(new ProductId('AV1'), “The Last Airbender");
    $basket->addProduct(new ProductId('AV2'), "The Legend of Korra");
    $basket->addProduct(new ProductId('AV3'), “The Making Of Avatar”);
    !
    it("should disallow adding a fourth product",
    throws(‘BasketLimitReached’, function () use($basket) {
    $basket->addProduct(new ProductId('AV4'), “The Last Airbender Movie”);
    })
    !
    );
    

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27. final class Basket implements RecordsEvents
    {
    private $productCount = 0;
    !
    public function addProduct(ProductId $productId, $name)
    {
    $this->guardProductLimit();
    $this->recordThat(
    new ProductWasAddedToBasket($this->basketId, $productId, $name)
    );
    ++$this->productCount;
    }
    !
    private function guardProductLimit()
    {
    if ($this->productCount >= 3) {
    throw new BasketLimitReached;
    }
    }
    !
    public function removeProduct(ProductId $productId)
    {
    $this->recordThat(
    new ProductWasRemovedFromBasket($this->basketId, $productId)
    );
    --$this->productCount;
    }
    // ...
    }
    

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28. $basket = Basket::pickUp(BasketId::generate());
    !
    $productId = new ProductId(‘AV1');
    !
    $basket->addProduct($productId, “The Last Airbender");
    $basket->removeProduct($productId);
    $basket->removeProduct($productId);
    !
    it(“shouldn't record an event when removing a Product
    that is no longer in the Basket”,
    !
    count($basket->getRecordedEvents()) == 3
    !
    );
    1
    2
    3
    4
    

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29. final class Basket implements RecordsEvents
    {
    private $productCountById = [];
    !
    public function addProduct(ProductId $productId, $name)
    {
    $this->guardProductLimit();
    $this->recordThat(new ProductWasAddedToBasket(…));
    !
    if(!$this->productIsInBasket($productId)) {
    $this->productCountById[$productId] = 0;
    }
    !
    ++$this->productCountById[$productId];
    }
    !
    public function removeProduct(ProductId $productId)
    {
    if(! $this->productIsInBasket($productId)) {
    return;
    }
    !
    $this->recordThat(new ProductWasRemovedFromBasket(…);
    !
    --$this->productCountById;
    }
    private function productIsInBasket(ProductId $productId) {…}
    

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30. Aggregates
    record events
    

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31. Aggregates
    protect invariants
    

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32. Possible outcomes
    !
    nothing
    one or more events
    exception
    

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33. Aggregates do not
    expose state
    

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34. Reconstituting
    Aggregates
    

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35. !
    $basket = Basket::pickUp($basketId);
    $basket->addProduct($productId, “The Last Airbender");
    !
    $events = $basket->getRecordedEvents();
    !
    // persist events in an event store, retrieve at a later time
    !
    $reconstitutedBasket = Basket::reconstituteFrom(
    new AggregateHistory($basketId, $retrievedEvents)
    );
    !
    it("should be the same after reconstitution",
    $basket == $reconstitutedBasket
    );
    

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36. final class Basket implements RecordsEvents, IsEventSourced
    {
    public function addProduct(ProductId $productId, $name)
    {
    $this->guardProductLimit();
    $this->recordThat(new ProductWasAddedToBasket(…));
    !
    // No state is changed!
    }
    !
    public function removeProduct(ProductId $productId)
    {
    if(! $this->productIsInBasket($productId)) {
    return;
    }
    !
    $this->recordThat(new ProductWasRemovedFromBasket(…));
    !
    // No state is changed!
    }
    !
    private function recordThat(DomainEvent $domainEvent)
    {
    $this->latestRecordedEvents[] = $domainEvent;
    !
    $this->apply($domainEvent);
    }
    

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37. private function applyProductWasAddedToBasket(
    ProductWasAddedToBasket $event)
    {
    !
    $productId = $event->getProductId();
    !
    if(!$this->productIsInBasket($productId)) {
    $this->products[$productId] = 0;
    }
    !
    ++$this->productCountById[$productId];
    !
    }
    !
    private function applyProductWasRemovedFromBasket(
    ProductWasRemovedFromBasket $event)
    {
    $productId = $event->getProductId();
    --$this->productCountById[$productId];
    }
    

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38. public static function reconstituteFrom(
    AggregateHistory $aggregateHistory)
    {
    $basketId = $aggregateHistory->getAggregateId();
    $basket = new Basket($basketId);
    !
    foreach($aggregateHistory as $event) {
    $basket->apply($event);
    }
    return $basket;
    }
    !
    private function apply(DomainEvent $event)
    {
    $method = 'apply' . get_class($event);
    $this->$method($event);
    }
    !
    

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39. Projections
    

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40. final class BasketProjector
    {
    public function projectProductWasAddedToBasket(
    ProductWasAddedToBasket $event)
    {
    INSERT INTO baskets_readmodel
    SET
    `basketId` = $event->getBasketId(),
    `productId` = $event->getProductId(),
    `name` = $event->getName()
    }
    public function projectProductWasRemovedFromBasket(
    ProductWasRemovedFromBasket $event)
    {
    DELETE FROM baskets_readmodel
    WHERE
    `basketId` = $event->getBasketId()
    AND `productId` = $event->getProductId()
    }
    }
    

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41. Fat events
    The good kind of duplication
    

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42. Individual read models for
    every unique
    use case
    

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43. final class BlueProductsSoldProjection
    {
    public function projectProductWasIntroducedInCatalog(
    ProductWasIntroducedInCatalog $event)
    {
    if($event->getColor() == 'blue') {
    $this->redis->sAdd('blueProducts', $event->getProductId());
    }
    }
    !
    public function projectProductWasAddedToBasket(
    ProductWasAddedToBasket $event)
    {
    if($this->redis->sIsMember($event->getProductId())) {
    $this->redis->incr('blueProductsSold');
    }
    }
    !
    public function projectProductWasRemovedFromBasket(
    ProductWasRemovedFromBasket $event)
    {
    if($this->redis->sIsMember($event->getProductId())) {
    $this->redis->decr('blueProductsSold');
    }
    }
    }
    

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44. LessonWasScheduled
    { SchoolId, GroupId, TeacherId, Subject, WeekDay, Timeslot }
    !
    =>
    !
    GroupScheduleProjector
    Group 1A Monday Tuesday Wednesday Thursday Friday
    09:00
    Math
    Ada
    German
    Friedrich
    Math
    Ada
    Chemistry
    Niels
    Economy
    Nicholas
    10:00
    French
    Albert
    Math
    Ada
    Physics
    Isaac
    PHP
    Rasmus
    History
    Julian
    11:00
    Sports
    Felix
    PHP
    Rasmus
    PHP
    Rasmus
    German
    Friedrich
    Math
    Ada
    

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45. LessonWasScheduled
    { SchoolId, GroupId, TeacherId, Subject, WeekDay, Timeslot }
    !
    =>
    !
    TeacherScheduleProjector
    Ada!
    Math
    Monday Tuesday Wednesday Thursday Friday
    09:00
    Group 1A
    School 5
    Group 1A
    School 5
    Group 6C
    School 9
    Group 5B
    School 9
    10:00
    Group 1B
    School 5
    Group 1A
    School 5
    Group 6C
    School 9
    Group 5B
    School 9
    11:00
    Group 2A
    School 5
    Group 5B
    School 9
    Group 1A
    School 5
    

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46. PupilWasEnlistedInGroup
    { PupilId, SchoolId, GroupId }
    LessonWasScheduled
    { SchoolId, GroupId, TeacherId, Subject, WeekDay, Timeslot }
    !
    =>
    !
    TeacherPermissionsProjector
    Ada Pupil 1
    Ada Pupil 3
    Friedrich Pupil 1
    Friedrich Pupil 7
    Ada Pupil 8
    Julian Pupil 3
    

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47. Event Store
    

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48. Immutable
    Append-only
    You can’t change history
    

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49. interface NaiveEventStore
    {
    public function commit(DomainEvents $events);
    !
    /** @return AggregateHistory */
    public function getAggregateHistoryFor(IdentifiesAggregate $id);
    !
    /** @return DomainEvents */
    public function getAll();
    }
    !
    

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50. CREATE TABLE `buttercup_eventstore` (
    `streamId` varbinary(16) NOT NULL,
    `streamVersion` bigint(20) unsigned NOT NULL,
    `streamContract` varchar(255) NOT NULL,
    `eventDataContract` varchar(255) NOT NULL,
    `eventData` text NOT NULL,
    `eventMetadataContract` varchar(255) NOT NULL,
    `eventMetadata` text NOT NULL,
    `utcStoredTime` datetime NOT NULL,
    `correlationId` varbinary(16) NOT NULL,
    `causationId` varbinary(16) NOT NULL,
    `causationEventOrdinal` bigint(20) unsigned,
    PRIMARY KEY (`streamId`,`streamVersion`,`streamContract`)
    ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
    

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52. Performance
    

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53. The Event Store is an
    immutable, append-only
    database:
    infinite caching
    

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54. Querying events happens by
    aggregate id only
    

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55. Read models are
    faster than joins
    

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56. Aggregate snapshots,
    if need be
    

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57. Testing
    

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58. // it should disallow evaluating pupils without planning them first
    !
    $scenario->given([
    new EvaluationWasPlanned(…)
    ]);
    !
    $scenario->when(
    new EvaluatePupil(…)
    );
    !
    $scenario->then([
    $scenario->throws(new CantEvaluateUnplannedPupil(…))
    ]);
    !
    ——————————————————————————————————————————————————————————————————————————-
    !
    $scenario->given([
    new EvaluationWasPlanned(…),
    new PupilWasPlannedForEvaluation(…)
    ]);
    !
    $scenario->when(
    new EvaluatePupil(…)
    );
    !
    $scenario->then([
    new PupilWasEvaluated()
    ]);
    

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59. verraes.net
    !
    joind.in/10911
    !
    buttercup-php/protects
    !
    mathiasverraes
    

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60. verraes.net
    !
    joind.in/10911
    !
    buttercup-php/protects
    !
    mathiasverraes
    

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