MarkRedeman/presentation.org

## presentation.org

      
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              presentation.org
            
          
    Event sourcing in practice

@MarkRedeman

  Event Sourcing (ES)
  Command Query Responsibility Segregation (CQRS)
  Hexagonal architecture
  TestDrivenDevelopment (TDD)
  Domain Driven Design (DDD)
  Behavior Driven Development (BDD)

And lots of acronyms and other cool words [fn:1].
It’s popular

At the Dutch PHP Conference there was

  1 workshop on the basics of CQRS and Event Sourcing
  1 keynote about event sourcing
  4 talks that had something to do with Event Sourcing [fn:2]

What is event sourcing


  Domain Events as first class citizens
    (e.g. UserWasRegistered, ActivityWasPlanned, MembershipFeeWasPayed)
  Persist entities by only storing their domain events

Examples of events in our application


  AuthorWasInvitedByEditor.php
  AuthorWasRegistered.php
  EditorWasRegistered.php
  FormularyChangedName.php
  FormularyCreated.php
  IndicationWasArchived.php
  IndicationWasCreated.php
  IndicationWasPublished.php
  IndicationWasRenamed.php
  IndicationWasRestored.php
  TherapySchemesWereReordered.php
  TherapySchemeWasAddedToIndication.php
  TherapySchemeWasRemovedFromIndication.php
  TherapySchemeWasValidated.php
  GroupWasCreated.php
  GroupWasRenamed.php
  MedicineWasAddedToGroup.php
  MedicineWasCreated.php
  MedicineWasRemovedFromGroup.php
  PreferredMedicineWasChosen.php
  PreferredMedicineWasRemoved.php
  GroupWasAppendedToStep.php
  GroupWasRemovedFromStep.php
  MedicinesAndGroupsWereReordered.php
  StepWasArchived.php
  StepWasCreated.php
  StepWasRenamed.php
  StepWasRestored.php
  UnclassifiedMedicineWasAppendedToStep.php
  UnclassifiedMedicineWasRemoved.php
  StepsWereReordered.php
  StepWasAddedToTherapyScheme.php
  StepWasRemovedFromTherapyScheme.php
  TherapySchemeWasArchived.php
  TherapySchemeWasCreated.php
  TherapySchemeWasMadeInvalid.php
  TherapySchemeWasMadeReady.php
  TherapySchemeWasRenamed.php
  TherapySchemeWasRestored.php

Pros & Cons

Pros


  Tell Don’t Ask
  Loosly coupled code
  No loss of data
  Very testable

Cons


  More boilerplate
  More complex?
  Requires you to think before writing code (advantage!?)

Test Driven Development


  Write event based tests for aggregates & projections
  Use in memory repositories when testing aggregates & read models
  Integration tests for generic repositories

Common workflow

Adding new behaviour

Adding new views / reports

Adding new behaviour


  Write the test

use Broadway\EventSourcing\Testing\AggregateRootScenarioTestCase;

class BuildingTest extends AggregateRootScenarioTestCase
{
    /** @test */
    function a_user_can_check_in_to_a_building
    {
        $id = new BuildingId('8838aa4b-2fa2-4fdf-b1c0-53a8fb1f2c26');

        $this->scenario
            ->given([
                new BuildingWasRegistered($id)
            ])
            ->when(function (Building $building) {
                $building->checkInUser(string $username)
            })
            ->then([
                new UserCheckedIntoBuilding($id, $username)
            ]);
    }
}

  Implement the test

final class Building extends EventSourcedAggregateRoot
{
    // ...

    public function checkInUser(string $username)
    {
        $this->apply(
            new UserCheckedIntoBuilding($this->id, $username)
        );
    }
}

  Improve the test

use Broadway\EventSourcing\Testing\AggregateRootScenarioTestCase;

class BuildingTest extends AggregateRootScenarioTestCase
{
    /** @test */
    function a_user_can_check_in_to_a_building
    {
        $id = new BuildingId('8838aa4b-2fa2-4fdf-b1c0-53a8fb1f2c26');

        $this->scenario
            ->given([
                new BuildingWasRegistered($id)
            ])
            ->when(function (Building $building) {
                $building->checkInUser(string $username)
            })
            ->then([
                new UserCheckedIntoBuilding($id, $username)
            ])
            // A user can't check into a building twice
            ->expectExceptionWhen(
                CantCheckIntoBuilding::class,
                function (Building $building) {
                    $building->checkInUser(string $username);
                }
            );
    }
}

  Fix the test

final class Building extends EventSourcedAggregateRoot
{
    // ...
    private $checkedInUsers = [];

    public function checkInUser(string $username)
    {
        // Don't let a user check in more than once
        if ($this->userIsCheckedIn($username)) {
            throw CantCheckIntoBuilding::user($username);
        }

        $this->apply(
            new UserCheckedIntoBuilding($this->id, $username)
        );
    }

    private function userIsCheckedIn(string $username) : bool
    {
        return in_array($username, $this->checkedInUsers, true);
    }

    private function whenUserCheckedIntoBuilding(UserCheckedIntoBuilding $event)
    {
        $this->checkedInUsers[] = $event->username();
    }
}

final class CantCHeckIntoBuilding extends \LogicException
{
    public static function user(string $username)
    {
        return new self(sprintf('User "%s" is not checked in', $username));
    }
}
Adding new views / reports

The customer wants a view of all checked in users of a specific building.

  Write a read model containing the username data for a building (make it serializable)

class CheckedInUsersTest extends TestCase
{
    /** @test */
    function it_keeps_track_of_the_users_checked_into_a_building()
    {
        $id = new BuildingId('943ed5e8-7058-4b2e-90cd-8b131152faa3');
        $readModel = CheckedInUsers::forBuilding($id);

        $this->assertInstanceOf(CheckedInUsers::class, $readModel);
        $this->assertEquals($id, $readModel->buildingId());
    }

    /** @test */
    function a_user_can_be_added_to_the_read_model()
    {
        $id = new BuildingId('943ed5e8-7058-4b2e-90cd-8b131152faa3');
        $readModel = CheckedInUsers::forBuilding($id);
        $readModel->addUsername("Mark");

        $this->assertEquals(["Mark"], $readModel->usernames());
    }

    // .. etc.
}
final class CheckedInUsers
{
    /** @var string[] */
    private $usersnames;
    /** @var string */
    private $buildingId;

    public static function forBuilding(BuildingId $id) : CheckedInUsers
    {
        $model = new self;
        $model->buildingId = $id;
        return $model;
    }

    public function addUsername(string $username)
    {
        $this->usernames[] = $username;
    }

    public function usernames() : array
    {
        return $this->usernames;
    }

    public function buildingId() : BuildingId
    {
        return new BuildingId($this->buildingId);
    }

    // some stuff to make it serializable
}

  Write a projection that populates the read model

class CheckedInUsersProjectorTest extends ProjectorScenarioTestCase
{
    protected function createProjector(InMemoryRepository $repository)
    {
        return new CheckedInUsersProjector($repository);
    }

    /** @test */
    function it_starts_keeping_track_of_a_buildings_checked_in_users()
    {
        $id = new BuildingId('8838aa4b-2fa2-4fdf-b1c0-53a8fb1f2c26');

        $this->scenario->when(
            new BuildingWasRegistered($id)
        )->then(function (InMemoryRepository $repo) use ($id) {
            // Check that our in memory repository contains the building
            $this->assertEquals(
                CheckedInUsers::forBuilding($id),
                $repo->find($id)
            );
        }]);
    }

    /** @test */
    function it_keeps_track_of_a_buildings_checked_in_users()
    {
        $id = new BuildingId('8838aa4b-2fa2-4fdf-b1c0-53a8fb1f2c26');

        $this->scenario->when(
            new BuildingWasRegistered($id),
            new UserCheckedIntoBuilding($id, "Mark"),
            new UserCheckedIntoBuilding($id, "Piet"),
            new UserCheckedIntoBuilding($id, "Klaas")
        )->then(function (InMemoryRepository $repo) use ($id) {
                $readModel = $repo->find($id);

                $this->assertEquals(
                    ["Mark", "Piet", "Klaas"],
                    $readModel->usernames()
                );
            }
        );
    }
}

  Persist the read model

Future stuff


  Improve testing of read models (lots of duplicated code)

Footnotes

Open source event sourced application: Professor Francken
[fn:1] See Matthias Noback’s talk “All the cool kids…”
[fn:2] Talks:

  Integrating Bounded Contexts (Carlos Buenosvinos)
  CQRS and Domain Events for integration (Giorgio Sironi)
  Event Sourcing: the good, the bad and the complicated (Marco Pivetta)
  How I Built A Video Game using Event Sourcing (Shawn McCool)

Other interesting talks


  Models & Service Layers; Hemoglobin & Hobgoblins (Ross Tuck) https://youtu.be/ajhqScWECMo
  Event Sourcing is actually just functional code» (Greg Young) https://www.youtube.com/watch?v=kZL41SMXWdM
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