Practical Event Sourcing

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

330627d5f564b710721236077903ed60?s=128

Mathias Verraes

March 18, 2014
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Transcript

  1. 11.

    Something that has happened in the past that is of

    interest to the business Domain Event
  2. 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; } }
  3. 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; } }
  4. 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; } }
  5. 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
  6. 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
  7. 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; } ! }
  8. 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”); }) ! );
  9. 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; } // ... }
  10. 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
  11. 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) {…}
  12. 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 );
  13. 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); }
  14. 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]; }
  15. 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); } !
  16. 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() } }
  17. 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'); } } }
  18. 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
  19. 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
  20. 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
  21. 49.

    interface NaiveEventStore { public function commit(DomainEvents $events); ! /** @return

    AggregateHistory */ public function getAggregateHistoryFor(IdentifiesAggregate $id); ! /** @return DomainEvents */ public function getAll(); } !
  22. 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;
  23. 51.
  24. 57.
  25. 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() ]);