Introduction to CQRS and Event Sourcing

Samuel ROZE (@samuelroze)

The heart of software is its ability to solve domain-related

problems for its user— Eric Evans

Event Storming

Event Sourcing

Our domain

1. Create a "deployment"

2. Start a deployment

3. Realtime status of our deployment

Our model is this...

An entity +------------------+ | Deployment | +------------------+ |- uuid: Uuid | |- sha1: string | |- status: string | |- startedBy: User | +------------------+

What we could have doneA Doctrine mapping and.. a table!

+-------------------+ | deployment | +-------------------+ | uuid VARCHAR | | status VARCHAR | | sha1 VARCHAR | | startedBy VARCHAR | +-------------------+

An other approachA set of events as the reference!

+-------------------------------------+ |DeploymentCreated |DeploymentStarted | | uuid: [...] | uuid: [...] | ... | datetime: [...] | datetime: [...] | | sha1: [...] | startedBy: [...] | +------------------+------------------+

Why using events?1. Closer to the business language

2. Keep the information about what happened3. Easy to spread the logic across services

4. No coupling between domain and storage5. Append-only it's a LOT easier to scale

Let's get started !Scenario: A deployment need to have a valid SHA-1 When I create a deployment for "123" Then the deployment should not be valid

Scenario: Deployment for a valid SHA-1 When I create a deployment for "921103d" Then a deployment should be created

@When I create a deployment for :sha1

public function iCreateADeploymentFor(string $sha1){ try { $this->deployment = Deployment::create( Uuid::uuid4(), $sha1 ); } catch (\Throwable $e) { $this->exception = $e; }}

@Then the deployment should not be valid

public function theDeploymentShouldNotBeValid(){ if (!$this->exception instanceof \InvalidArgumentException) { throw new \RuntimeException( 'The exception found, if any, is not matching' ); }}

@Then a deployment should be created

public function aDeploymentShouldBeCreated(){ $events = $this->deployment->raisedEvents(); $matchingEvents = array_filter($events, function(DeploymentEvent $event) { return $event instanceof DeploymentCreated; });

if (count($matchingEvents) === 0) { throw new \RuntimeException('No deployment created found'); }}

Erm...$ bin/behat -fprogressFFFF

2 scenarios (0 passed)4 steps (0 passed)0m0.12s (40.89Mb)

An eventinterface DeploymentEvent{ public function getDeploymentUuid() : UuidInterface;

public function getDateTime(): \DateTimeInterface;}

'DeploymentCreated' eventfinal class DeploymentCreated implements DeploymentEvent{ public function __construct(UuidInterface $uuid, string $sha1) { /* .. */ }

public function getDeploymentUuid() : UuidInterface { return $this->uuid; }

public function getSha1() : string { return $this->sha1; }}

Event capabilitytrait EventsCapability{ private $events = [];

protected function raise(DeploymentEvent $event) { $this->events[] = $event; }

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

The aggregatefinal class Deployment{ use EventsCapability;

private function __construct() {}}

Creating the object from eventsfinal class Deployment{ // ... public static function fromEvents(array $events) { $deployment = new self();

foreach ($events as $event) { $deployment->apply($event); }

return $deployment; }}

Building the object statefinal class Deployment{ private $uuid;

// ...

private function apply(DeploymentEvent $event) { if ($event instanceof DeploymentCreated) { $this->uuid = $event->getDeploymentUuid(); } }}

Create... from the beginning!final class Deployment{ // ...

public static function create(Uuid $uuid, string $sha1) { if (strlen($sha1) < 7) { throw new \InvalidArgumentException('It is not a valid SHA-1'); }

$createdEvent = new DeploymentCreated($uuid, $sha1);

$deployment = self::fromEvents([$createdEvent]); $deployment->raise($createdEvent);

return $deployment; }}

Wourah!$ bin/behat -fprogress....

2 scenarios (2 passed)4 steps (4 passed)0m0.12s (40.89Mb)

Starting a deployment !Scenario: A successfully created deployment can be started Given a deployment was created When I start the deployment Then the deployment should be started

Scenario: A deployment can be started only once Given a deployment was created and started When I start the deployment Then I should be told that the deployment has already started

@Given a deployment was created and started

public function aDeploymentWasCreatedAndStarted(){ $uuid = Uuid::uuid4();

$this->deployment = Deployment::fromEvents([ new DeploymentCreated($uuid, '921103d'), new DeploymentStarted($uuid), ]);}

@When I start the deployment

public function iStartTheDeployment(){ try { $this->deployment->start(); } catch (\Throwable $e) { $this->exception = $e; }}

'start'ing a deploymentfinal class Deployment{ private $uuid; private $started = false;

// ...

public function start() { if ($this->started) { throw new \RuntimeException('Deployment already started'); }

$this->raise(new DeploymentStarted($this->uuid)); }}

Keeping trace of the statusfinal class Deployment{ private $started = false; // ...

public function apply(DeploymentEvent $event) { // ... if ($event instanceof DeploymentStarted) { $this->started = true; } }}

That's too fast...$ bin/behat -fprogress.........

4 scenarios (4 passed)10 steps (10 passed)0m0.31s (41.22Mb)

We are done!...with our domain

Repositories & Persistence

Event Storeinterface EventStore{ public function findByDeploymentUuid(UuidInterface $uuid) : array;

public function add(DeploymentEvent $event);}

Implementation detail: InMemory / Doctrine / Redis / GetEventStore / ...

Our repository contractinterface DeploymentRepository{ public function find(UuidInterface $uuid) : Deployment;}

The event-based implementationfinal class EventBasedDeploymentRepository implements DeploymentRepository{ public function __construct(EventStore $eventStore) { /** .. **/ }

public function find(UuidInterface $uuid) : Deployment { return Deployment::fromEvents( $this->eventStore->findByDeploymentUuid($uuid) ); }}

CQRS?

Projections!The "read model"

· Creates a read-optimized view of our model· As many projection as you want

· Any kind of backend (database, API, queue, ...)

final class DeploymentFirebaseProjector{ public function __construct( DeploymentRepository $repository, FirebaseStorage $storage ) { /* ... */ }

public function notify(DeploymentEvent $event) { $uuid = $event->getDeploymentUuid(); $deployment = $this->repository->find($uuid);

$this->storage->store('deployments/'.$uuid, [ 'started' => $deployment->isStarted(), ]); }}

We've done it!1. Create a "deployment"

2. Start a deployment

3. Realtime status of our deployment

Thank you!@samuelroze

continuouspipe.iohttps://joind.in/talk/03af6

SimpleBus· Written by Matthias Noback

http://simplebus.github.io/SymfonyBridge/

# app/config/config.ymlevent_bus: logging: ~

command_bus: logging: ~

final class DeploymentController{ private $eventBus;

public function __construct(MessageBus $eventBus) { /* ... */ }

public function createAction(Request $request) { $deployment = Deployment::create( Uuid::uuid4(), $request->request->get('sha1') );

foreach ($deployment->raisedEvents() as $event) { $this->eventBus->handle($event); }

return new Response(Response::HTTP_CREATED); }}

final class DeploymentController{ private $commandBus;

public function __construct(MessageBus $commandBus) { /* ... */ }

public function createAction(Request $request) { $uuid = Uuid::uuid4();

$this->commandBus->handle(new CreateDeployment( $uuid, $request->request->get('sha1') ));

return new Response(Response::HTTP_CREATED); }}

final class CreateDeploymentHandler{ private $eventBus;

public function __construct(MessageBus $eventBus) { /* ... */ }

public function handle(CreateDeployment $command) { $deployment = Deployment::create( $command->getUuid(), $command->getSha1() );

foreach ($deployment->raisedEvents() as $event) { $this->eventBus->handle($event); } }}

The plumbing<service id="app.controller.deployment" class="AppBundle\Controller\DeploymentController"> <argument type="service" id="command_bus" /></service>

<service id="app.handler.create_deployment" class="App\Deployment\Handler\CreateDeploymentHandler"> <argument type="service" id="event_bus" />

<tag name="command_handler" handles="App\Command\CreateDeployment" /></service>

What do we have right now?1. Send a command from an HTTP API

2. The command handler talks to our domain3. Domain raise an event

4. The event is dispatched to the event bus

Storing our eventsfinal class DeploymentEventStoreMiddleware implements MessageBusMiddleware{ private $eventStore;

public function __construct(EventStore $eventStore) { $this->eventStore = $eventStore; }

public function handle($message, callable $next) { if ($message instanceof DeploymentEvent) { $this->eventStore->add($message); }

$next($message); }}

We <3 XML<service id="app.event_bus.middleware.store_events" class="App\EventBus\Middleware\StoreEvents"> <argument type="service" id="event_store" />

<tag name="event_bus_middleware" /></service>

Our events are stored!...so we can get our

Deployment from the repository

Let's start our deployment!final class StartDeploymentWhenCreated{ private $commandBus; public function __construct(MessageBus $commandBus) { /* ... */ }

public function notify(DeploymentCreated $event) { // There will be conditions here...

$this->commandBus->handle(new StartDeployment( $event->getDeploymentUuid() )); }}

The handlerfinal class StartDeploymentHandler{ public function __construct(DeploymentRepository $repository, MessageBus $eventBus) { /* ... */ }

public function handle(StartDeployment $command) { $deployment = $this->repository->find($command->getDeploymentUuid()); $deployment->start();

foreach ($deployment->raisedEvents() as $event) { $this->eventBus->handle($event); } }}

The plumbing<service id="app.deployment.auto_start.starts_when_created" class="App\Deployment\AutoStart\StartsWhenCreated"> <argument type="service" id="command_bus" />

<tag name="event_subscriber" subscribes_to="App\Event\DeploymentCreated" /></service>

<service id="app.deployment.handler.start_deployment" class="App\Deployment\Handler\StartDeploymentHandler"> <argument type="service" id="app.deployment_repository" /> <argument type="service" id="event_bus" />

<tag name="command_handler" handles="App\Command\StartDeployment" /></service>

What happened?[...]

4. A dispatched DeploymentCreated event5. A listener created a StartDeployment

command6. The command handler called the start

method on the Deployment7. The domain validated and raised a

DeploymentStarted event8. The DeploymentStarted was dispatched on

You'll go further...

final class Deployment{ // ...

public function finishedBuild(Build $build) { if ($build->isFailure()) { return $this->raise(new DeploymentFailed($this->uuid)); }

$this->builtImages[] = $build->getImage(); if (count($this->builtImages) == count($this->images)) { $this->raise(new DeploymentSuccessful($this->uuid)); } }}

Dependencies... the wrong wayfinal class Deployment{ private $notifier;

public function __construct(NotifierInterface $notifier) { /* .. */ }

public function notify() { $this->notifier->notify($this); }}

Dependencies... the right wayfinal class Deployment{ public function notify(NotifierInterface $notifier) { $notifier->notify($this); }}

Testing! (layers)1. Use your domain objects

2. Create commands and read your event store3. Uses your API and projections

What we just achieved1. Incoming HTTP requests

2. Commands to the command bus3. Handlers talk to your domain

4. Domain produces events5. Events are stored and dispatched

6. Projections built for fast query

Thank you!@samuelroze

continuouspipe.iohttps://joind.in/talk/03af6