Domain Event Generator

Domain Event Generator

Generate DDD-compliant Domain Events with metadata and tests.

Domain Event Characteristics

• Immutable: final readonly class
• Past tense: Describes something that happened
• Self-contained: All data needed to understand what happened
• Metadata: Event ID, timestamp, causation/​correlation IDs
• No behavior: Pure data, no logic

Template

<?php

declare(strict_types=1);

namespace Domain\{BoundedContext}\Event;

use Domain\Shared\Event\DomainEvent;

final readonly class {Name}Event implements DomainEvent
{
    public function __construct(
        {properties}
        public EventMetadata $metadata
    ) {}

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

    public function occurredAt(): DateTimeImmutable
    {
        return $this->metadata->occurredAt;
    }
}

Event Metadata

<?php

declare(strict_types=1);

namespace Domain\Shared\Event;

final readonly class EventMetadata
{
    public function __construct(
        public string $eventId,
        public DateTimeImmutable $occurredAt,
        public ?string $causationId = null,
        public ?string $correlationId = null,
        public int $version = 1,
        public ?string $userId = null
    ) {}

    public static function create(): self
    {
        return new self(
            eventId: self::generateId(),
            occurredAt: new DateTimeImmutable()
        );
    }

    public static function withCausation(string $causationId, ?string $correlationId = null): self
    {
        return new self(
            eventId: self::generateId(),
            occurredAt: new DateTimeImmutable(),
            causationId: $causationId,
            correlationId: $correlationId ?? $causationId
        );
    }

    private static function generateId(): string
    {
        $data = random_bytes(16);
        $data[6] = chr(ord($data[6]) & 0x0f | 0x40);
        $data[8] = chr(ord($data[8]) & 0x3f | 0x80);
        return vsprintf('%s%s-%s-%s-%s-%s%s%s', str_split(bin2hex($data), 4));
    }
}

Domain Event Interface

<?php

declare(strict_types=1);

namespace Domain\Shared\Event;

interface DomainEvent
{
    public function aggregateId(): string;

    public function occurredAt(): DateTimeImmutable;
}

Common Domain Events

Order Events

<?php

declare(strict_types=1);

namespace Domain\Order\Event;

use Domain\Shared\Event\DomainEvent;
use Domain\Shared\Event\EventMetadata;

final readonly class OrderCreatedEvent implements DomainEvent
{
    public function __construct(
        public string $orderId,
        public string $customerId,
        public DateTimeImmutable $createdAt,
        public EventMetadata $metadata
    ) {}

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

    public function occurredAt(): DateTimeImmutable
    {
        return $this->metadata->occurredAt;
    }
}

final readonly class OrderLineAddedEvent implements DomainEvent
{
    public function __construct(
        public string $orderId,
        public string $productId,
        public string $productName,
        public int $quantity,
        public int $unitPriceCents,
        public string $currency,
        public EventMetadata $metadata
    ) {}

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

    public function occurredAt(): DateTimeImmutable
    {
        return $this->metadata->occurredAt;
    }
}

final readonly class OrderConfirmedEvent implements DomainEvent
{
    public function __construct(
        public string $orderId,
        public int $totalCents,
        public string $currency,
        public DateTimeImmutable $confirmedAt,
        public EventMetadata $metadata
    ) {}

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

    public function occurredAt(): DateTimeImmutable
    {
        return $this->metadata->occurredAt;
    }
}

final readonly class OrderCancelledEvent implements DomainEvent
{
    public function __construct(
        public string $orderId,
        public string $reason,
        public DateTimeImmutable $cancelledAt,
        public EventMetadata $metadata
    ) {}

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

    public function occurredAt(): DateTimeImmutable
    {
        return $this->metadata->occurredAt;
    }
}

final readonly class OrderShippedEvent implements DomainEvent
{
    public function __construct(
        public string $orderId,
        public string $trackingNumber,
        public string $carrier,
        public DateTimeImmutable $shippedAt,
        public EventMetadata $metadata
    ) {}

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

    public function occurredAt(): DateTimeImmutable
    {
        return $this->metadata->occurredAt;
    }
}

User Events

<?php

declare(strict_types=1);

namespace Domain\User\Event;

final readonly class UserRegisteredEvent implements DomainEvent
{
    public function __construct(
        public string $userId,
        public string $email,
        public string $name,
        public DateTimeImmutable $registeredAt,
        public EventMetadata $metadata
    ) {}

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

    public function occurredAt(): DateTimeImmutable
    {
        return $this->metadata->occurredAt;
    }
}

final readonly class UserEmailChangedEvent implements DomainEvent
{
    public function __construct(
        public string $userId,
        public string $previousEmail,
        public string $newEmail,
        public EventMetadata $metadata
    ) {}

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

    public function occurredAt(): DateTimeImmutable
    {
        return $this->metadata->occurredAt;
    }
}

final readonly class UserDeactivatedEvent implements DomainEvent
{
    public function __construct(
        public string $userId,
        public string $reason,
        public DateTimeImmutable $deactivatedAt,
        public EventMetadata $metadata
    ) {}

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

    public function occurredAt(): DateTimeImmutable
    {
        return $this->metadata->occurredAt;
    }
}

Test Template

<?php

declare(strict_types=1);

namespace Tests\Unit\Domain\Order\Event;

use Domain\Order\Event\OrderCreatedEvent;
use Domain\Shared\Event\EventMetadata;
use PHPUnit\Framework\Attributes\CoversClass;
use PHPUnit\Framework\Attributes\Group;
use PHPUnit\Framework\TestCase;

#[Group('unit')]
#[CoversClass(OrderCreatedEvent::class)]
final class OrderCreatedEventTest extends TestCase
{
    public function testCreatesEventWithAllData(): void
    {
        $metadata = EventMetadata::create();
        $createdAt = new DateTimeImmutable();

        $event = new OrderCreatedEvent(
            orderId: 'order-123',
            customerId: 'customer-456',
            createdAt: $createdAt,
            metadata: $metadata
        );

        self::assertSame('order-123', $event->orderId);
        self::assertSame('customer-456', $event->customerId);
        self::assertSame($createdAt, $event->createdAt);
        self::assertSame($metadata, $event->metadata);
    }

    public function testReturnsAggregateId(): void
    {
        $event = $this->createEvent();

        self::assertSame('order-123', $event->aggregateId());
    }

    public function testReturnsOccurredAt(): void
    {
        $metadata = EventMetadata::create();
        $event = new OrderCreatedEvent(
            orderId: 'order-123',
            customerId: 'customer-456',
            createdAt: new DateTimeImmutable(),
            metadata: $metadata
        );

        self::assertEquals($metadata->occurredAt, $event->occurredAt());
    }

    private function createEvent(): OrderCreatedEvent
    {
        return new OrderCreatedEvent(
            orderId: 'order-123',
            customerId: 'customer-456',
            createdAt: new DateTimeImmutable(),
            metadata: EventMetadata::create()
        );
    }
}

Event Naming Conventions

Past Tense Naming

Bad Names (Avoid)

Event Data Guidelines

Include All Relevant Data

// GOOD: All data needed to understand what happened
final readonly class OrderConfirmedEvent
{
    public function __construct(
        public string $orderId,
        public int $totalCents,       // Include computed values
        public string $currency,
        public int $lineCount,         // Summary data
        public DateTimeImmutable $confirmedAt,
        public EventMetadata $metadata
    ) {}
}

// BAD: Missing important data
final readonly class OrderConfirmedEvent
{
    public function __construct(
        public string $orderId,        // Only ID, no details
        public EventMetadata $metadata
    ) {}
}

Use Primitive Types

// GOOD: Primitive types for serialization
final readonly class OrderCreatedEvent
{
    public function __construct(
        public string $orderId,        // Not OrderId
        public string $customerId,     // Not CustomerId
        public int $totalCents,        // Not Money
        public string $currency,
        public EventMetadata $metadata
    ) {}
}

// BAD: Value Objects in events (serialization issues)
final readonly class OrderCreatedEvent
{
    public function __construct(
        public OrderId $orderId,       // Serialization complex
        public Money $total,           // Serialization complex
        public EventMetadata $metadata
    ) {}
}

Generation Instructions

When asked to create a Domain Event:

1. Name in past tense (what happened)
2. Include aggregate ID for identification
3. Add all relevant data needed to understand the event
4. Use primitive types for easy serialization
5. Include metadata (event ID, timestamp)
6. Generate tests for structure and interface

Usage

To generate a Domain Event, provide:

• Name (e.g., "OrderConfirmed", "UserRegistered")
• Bounded Context (e.g., "Order", "User")
• Aggregate ID field
• Data fields needed
• Any computed/​summary data to include