Getting Started

In our little getting started example, we manage hotels. We keep the example small, so we can only create hotels and let guests check in and check out.

Define some events

First we define the events that happen in our system.

A hotel can be created with a name and an id:

use Patchlevel\EventSourcing\Aggregate\Uuid;
use Patchlevel\EventSourcing\Attribute\Event;

#[Event('hotel.created')]
final class HotelCreated
{
    public function __construct(
        public readonly Uuid $hotelId,
        public readonly string $hotelName,
    ) {
    }
}

A guest can check in by name:

use Patchlevel\EventSourcing\Aggregate\Uuid;
use Patchlevel\EventSourcing\Attribute\Event;

#[Event('hotel.guest_checked_in')]
final class GuestIsCheckedIn
{
    public function __construct(
        public readonly Uuid $hotelId,
        public readonly string $guestName,
    ) {
    }
}

And also check out again:

use Patchlevel\EventSourcing\Aggregate\Uuid;
use Patchlevel\EventSourcing\Attribute\Event;

#[Event('hotel.guest_checked_out')]
final class GuestIsCheckedOut
{
    public function __construct(
        public readonly Uuid $hotelId,
        public readonly string $guestName,
    ) {
    }
}

You can find out more about events.

Define aggregates

Next we need to define the hotel aggregate. How you can interact with it, which events happen and what the business rules are. For this we create the methods create, checkIn and checkOut. In these methods the business checks are made and the events are recorded. Last but not least, we need the associated apply methods to change the state.

use Patchlevel\EventSourcing\Aggregate\BasicAggregateRoot;
use Patchlevel\EventSourcing\Aggregate\Uuid;
use Patchlevel\EventSourcing\Attribute\Aggregate;
use Patchlevel\EventSourcing\Attribute\Apply;
use Patchlevel\EventSourcing\Attribute\Id;

#[Aggregate('hotel')]
final class Hotel extends BasicAggregateRoot
{
    #[Id]
    private Uuid $id;
    private string $name;

    /** @var list<string> */
    private array $guests;

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

    /** @return list<string> */
    public function guests(): array
    {
        return $this->guests;
    }

    public static function create(Uuid $id, string $hotelName): static
    {
        $self = new static();
        $self->recordThat(new HotelCreated($id, $hotelName));

        return $self;
    }

    public function checkIn(string $guestName): void
    {
        if (in_array($guestName, $this->guests, true)) {
            throw new GuestHasAlreadyCheckedIn($guestName);
        }

        $this->recordThat(new GuestIsCheckedIn($this->id, $guestName));
    }

    public function checkOut(string $guestName): void
    {
        if (!in_array($guestName, $this->guests, true)) {
            throw new IsNotAGuest($guestName);
        }

        $this->recordThat(new GuestIsCheckedOut($this->id, $guestName));
    }

    #[Apply]
    protected function applyHotelCreated(HotelCreated $event): void
    {
        $this->id = $event->hotelId;
        $this->name = $event->hotelName;
        $this->guests = [];
    }

    #[Apply]
    protected function applyGuestIsCheckedIn(GuestIsCheckedIn $event): void
    {
        $this->guests[] = $event->guestName;
    }

    #[Apply]
    protected function applyGuestIsCheckedOut(GuestIsCheckedOut $event): void
    {
        $this->guests = array_values(
            array_filter(
                $this->guests,
                static fn ($name) => $name !== $event->guestName,
            ),
        );
    }
}

You can find out more about aggregates.

Define projections

So that we can see all the hotels on our website and also see how many guests are currently visiting the hotels, we need a projection for it. To create a projection we need a projector. Each projector is then responsible for a specific projection.

use Doctrine\DBAL\Connection;
use Patchlevel\EventSourcing\Attribute\Projector;
use Patchlevel\EventSourcing\Attribute\Setup;
use Patchlevel\EventSourcing\Attribute\Subscribe;
use Patchlevel\EventSourcing\Attribute\Teardown;

#[Projector(self::TABLE)]
final class HotelProjector
{
    // use a const for easier access in the projector & to keep projector id and table name in sync
    private const TABLE = 'hotel';

    public function __construct(
        private readonly Connection $db,
    ) {
    }

    /** @return list<array{id: string, name: string, guests: int}> */
    public function getHotels(): array
    {
        return $this->db->fetchAllAssociative(sprintf('SELECT id, name, guests FROM %s;', self::TABLE));
    }

    #[Subscribe(HotelCreated::class)]
    public function handleHotelCreated(HotelCreated $event): void
    {
        $this->db->insert(
            self::TABLE,
            [
                'id' => $event->hotelId->toString(),
                'name' => $event->hotelName,
                'guests' => 0,
            ],
        );
    }

    #[Subscribe(GuestIsCheckedIn::class)]
    public function handleGuestIsCheckedIn(GuestIsCheckedIn $event): void
    {
        $this->db->executeStatement(
            sprintf('UPDATE %s SET guests = guests + 1 WHERE id = ?;', self::TABLE),
            [$event->hotelId->toString()],
        );
    }

    #[Subscribe(GuestIsCheckedOut::class)]
    public function handleGuestIsCheckedOut(GuestIsCheckedOut $event): void
    {
        $this->db->executeStatement(
            sprintf('UPDATE %s SET guests = guests - 1 WHERE id = ?;', self::TABLE),
            [$event->hotelId->toString()],
        );
    }

    #[Setup]
    public function create(): void
    {
        $this->db->executeStatement(sprintf('CREATE TABLE IF NOT EXISTS %s (id VARCHAR PRIMARY KEY, name VARCHAR, guests INTEGER);', self::TABLE));
    }

    #[Teardown]
    public function drop(): void
    {
        $this->db->executeStatement(sprintf('DROP TABLE IF EXISTS %s;', self::TABLE));
    }
}

You can find out more about projectors.

Processor

In our example we also want to email the head office as soon as a guest is checked in.

use Patchlevel\EventSourcing\Attribute\Processor;
use Patchlevel\EventSourcing\Attribute\Subscribe;

#[Processor('admin_emails')]
final class SendCheckInEmailProcessor
{
    public function __construct(
        private readonly Mailer $mailer,
    ) {
    }

    #[Subscribe(GuestIsCheckedIn::class)]
    public function onGuestIsCheckedIn(GuestIsCheckedIn $event): void
    {
        $this->mailer->send(
            'hq@patchlevel.de',
            'Guest is checked in',
            sprintf('A new guest named "%s" is checked in', $event->guestName),
        );
    }
}

You can find out more about processors.

Configuration

After we have defined everything, we still have to plug the whole thing together:

If you use symfony, you can use our symfony bundle to skip this step.

use Doctrine\DBAL\DriverManager;
use Doctrine\DBAL\Tools\DsnParser;
use Patchlevel\EventSourcing\Metadata\AggregateRoot\AttributeAggregateRootRegistryFactory;
use Patchlevel\EventSourcing\Repository\DefaultRepositoryManager;
use Patchlevel\EventSourcing\Serializer\DefaultEventSerializer;
use Patchlevel\EventSourcing\Store\DoctrineDbalStore;
use Patchlevel\EventSourcing\Subscription\Engine\DefaultSubscriptionEngine;
use Patchlevel\EventSourcing\Subscription\Engine\StoreMessageLoader;
use Patchlevel\EventSourcing\Subscription\Repository\RunSubscriptionEngineRepositoryManager;
use Patchlevel\EventSourcing\Subscription\Store\DoctrineSubscriptionStore;
use Patchlevel\EventSourcing\Subscription\Subscriber\MetadataSubscriberAccessorRepository;

$connection = DriverManager::getConnection(
    (new DsnParser())->parse('pdo-pgsql://user:secret@localhost/app'),
);

$projectionConnection = DriverManager::getConnection(
    (new DsnParser())->parse('pdo-pgsql://user:secret@localhost/projection'),
);

/* your own mailer */
$mailer;

$serializer = DefaultEventSerializer::createFromPaths(['src/Domain/Hotel/Event']);
$aggregateRegistry = (new AttributeAggregateRootRegistryFactory())->create(['src/Domain/Hotel']);

$eventStore = new DoctrineDbalStore(
    $connection,
    $serializer,
);

$hotelProjector = new HotelProjector($projectionConnection);

$subscriberRepository = new MetadataSubscriberAccessorRepository([
    $hotelProjector,
    new SendCheckInEmailProcessor($mailer),
]);

$subscriptionStore = new DoctrineSubscriptionStore($connection);

$engine = new DefaultSubscriptionEngine(
    new StoreMessageLoader($eventStore),
    $subscriptionStore,
    $subscriberRepository,
);

$repositoryManager = new RunSubscriptionEngineRepositoryManager(
    new DefaultRepositoryManager(
        $aggregateRegistry,
        $eventStore,
    ),
    $engine,
);

$hotelRepository = $repositoryManager->get(Hotel::class);

You can find out more about stores.

The RunSubscriptionEngineRepositoryManager is a decorator that triggers the Subscription Engine when an Aggregate is saved. Normally, you'd use the DefaultRepositoryManager and a worker to run the Subscription Engine.

Learn more about the subscription engine.

Database setup

So that we can actually write the data to a database, we need the associated schema and databases.

use Doctrine\DBAL\Connection;
use Patchlevel\EventSourcing\Schema\ChainDoctrineSchemaConfigurator;
use Patchlevel\EventSourcing\Schema\DoctrineSchemaDirector;
use Patchlevel\EventSourcing\Store\Store;
use Patchlevel\EventSourcing\Subscription\Engine\SubscriptionEngine;
use Patchlevel\EventSourcing\Subscription\Store\SubscriptionStore;

/**
 * @var Connection $connection
 * @var Store $eventStore
 * @var SubscriptionStore $subscriptionStore
 */
$schemaDirector = new DoctrineSchemaDirector(
    $connection,
    new ChainDoctrineSchemaConfigurator([
        $eventStore,
        $subscriptionStore,
    ]),
);

$schemaDirector->create();

/** @var SubscriptionEngine $engine */
$engine->setup(skipBooting: true);

You can use the predefined cli commands for this.

Usage

We are now ready to use the Event Sourcing System. We can load, change and save aggregates.

use Patchlevel\EventSourcing\Aggregate\Uuid;
use Patchlevel\EventSourcing\Repository\Repository;

$hotel1 = Hotel::create(Uuid::generate(), 'HOTEL');
$hotel1->checkIn('David');
$hotel1->checkIn('Daniel');
$hotel1->checkOut('David');

/** @var Repository $hotelRepository */
$hotelRepository->save($hotel1);

$hotel2 = $hotelRepository->load(Uuid::fromString('d0d0d0d0-d0d0-d0d0-d0d0-d0d0d0d0d0d0'));
$hotel2->checkIn('David');
$hotelRepository->save($hotel2);

$hotels = $hotelProjector->getHotels();

You can also use other forms of IDs such as uuid version 6 or a custom format. You can find more about this in the aggregate id documentation.

Result

We have successfully implemented and used event sourcing.

Feel free to browse further in the documentation for more detailed information. If there are still open questions, create a ticket on Github and we will try to help you.