Event-Driven Architecture Knowledge Base

Quick reference for Event-Driven Architecture (EDA) patterns and PHP implementation guidelines.

Core Principles

Event-Driven Architecture Overview

┌─────────────────────────────────────────────────────────────────────────┐
│                     EVENT-DRIVEN ARCHITECTURE                            │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│   ┌──────────┐     ┌──────────────────┐     ┌──────────────────┐        │
│   │ Producer │────▶│  Message Broker  │────▶│    Consumer      │        │
│   │          │     │  (RabbitMQ/Kafka)│     │                  │        │
│   └──────────┘     └──────────────────┘     └──────────────────┘        │
│        │                    │                       │                    │
│        │                    │                       │                    │
│   Publishes            Routes/Stores            Subscribes               │
│   Events               Messages                 to Events                │
│                                                                          │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│   Event Types:                                                           │
│   • Domain Events    - Business facts (OrderPlaced, PaymentReceived)     │
│   • Integration Events - Cross-boundary communication                    │
│   • System Events    - Infrastructure notifications                      │
│                                                                          │
│   Patterns:                                                              │
│   • Pub/Sub          - Many subscribers per event                        │
│   • Message Queue    - One consumer per message                          │
│   • Event Sourcing   - Store all events as source of truth              │
│   • Saga/Process Manager - Coordinate distributed transactions          │
│                                                                          │
└─────────────────────────────────────────────────────────────────────────┘

Key Concepts

Concept	Description
Event	Immutable fact that something happened
Producer	Publishes events to broker
Consumer	Subscribes to and processes events
Message Broker	Routes events between producers and consumers
Topic/Exchange	Event routing mechanism
Queue	Stores messages for consumers
Eventual Consistency	System state converges over time

Quick Checklists

Event Design Checklist

• Events are immutable
• Events use past tense (OrderPlaced, not PlaceOrder)
• Events contain necessary data (no fetching needed)
• Events have unique IDs and timestamps
• Events include correlation/causation IDs
• Events are versioned for schema evolution

Producer Checklist

• Publishes events after state change
• Uses transactional outbox for reliability
• Events are serializable (JSON/Protobuf)
• Handles publish failures gracefully
• No business logic dependent on consumers

Consumer Checklist

• Idempotent processing
• Handles out-of-order events
• Implements retry with backoff
• Dead letter queue for failures
• No side effects until processing complete

PHP 8.4 Event-Driven Patterns

Domain Event

<?php

declare(strict_types=1);

namespace Domain\Order\Event;

use Domain\Shared\Event\DomainEvent;

final readonly class OrderPlaced implements DomainEvent
{
    public function __construct(
        public string $eventId,
        public string $orderId,
        public string $customerId,
        public int $totalCents,
        public \DateTimeImmutable $occurredAt,
        public ?string $correlationId = null,
        public ?string $causationId = null
    ) {}

    public function eventName(): string
    {
        return 'order.placed';
    }

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

    public function toArray(): array
    {
        return [
            'event_id' => $this->eventId,
            'order_id' => $this->orderId,
            'customer_id' => $this->customerId,
            'total_cents' => $this->totalCents,
            'occurred_at' => $this->occurredAt->format('c'),
            'correlation_id' => $this->correlationId,
            'causation_id' => $this->causationId,
        ];
    }
}

Event Publisher Interface

<?php

declare(strict_types=1);

namespace Application\Shared\Port\Output;

use Domain\Shared\Event\DomainEvent;

interface EventPublisherInterface
{
    public function publish(DomainEvent $event): void;

    /** @param array<DomainEvent> $events */
    public function publishAll(array $events): void;
}

Event Consumer/Handler

<?php

declare(strict_types=1);

namespace Application\Order\EventHandler;

use Application\Shared\Port\Output\EventHandlerInterface;
use Domain\Order\Event\OrderPlaced;

final readonly class SendOrderConfirmationEmail implements EventHandlerInterface
{
    public function __construct(
        private EmailServiceInterface $emailService,
        private CustomerRepositoryInterface $customers
    ) {}

    public function __invoke(OrderPlaced $event): void
    {
        $customer = $this->customers->findById($event->customerId);

        $this->emailService->send(
            to: $customer->email(),
            template: 'order-confirmation',
            data: [
                'order_id' => $event->orderId,
                'total' => $event->totalCents / 100,
            ]
        );
    }

    public static function subscribedTo(): string
    {
        return OrderPlaced::class;
    }
}

RabbitMQ Publisher (Infrastructure)

<?php

declare(strict_types=1);

namespace Infrastructure\Messaging\RabbitMQ;

use Application\Shared\Port\Output\EventPublisherInterface;
use Domain\Shared\Event\DomainEvent;
use PhpAmqpLib\Channel\AMQPChannel;
use PhpAmqpLib\Message\AMQPMessage;

final readonly class RabbitMQEventPublisher implements EventPublisherInterface
{
    public function __construct(
        private AMQPChannel $channel,
        private string $exchangeName
    ) {}

    public function publish(DomainEvent $event): void
    {
        $message = new AMQPMessage(
            json_encode($event->toArray()),
            [
                'content_type' => 'application/json',
                'delivery_mode' => AMQPMessage::DELIVERY_MODE_PERSISTENT,
                'message_id' => $event->eventId,
                'timestamp' => $event->occurredAt->getTimestamp(),
            ]
        );

        $this->channel->basic_publish(
            $message,
            $this->exchangeName,
            $event->eventName()
        );
    }

    public function publishAll(array $events): void
    {
        foreach ($events as $event) {
            $this->publish($event);
        }
    }
}

Common Violations Quick Reference

Violation	Where to Look	Severity
Synchronous calls disguised as events	Consumer calls HTTP API	Critical
Missing idempotency	No deduplication check	Critical
Tight coupling via events	Event contains entity references	Warning
Fire and forget	No error handling on publish	Warning
Blocking consumer	Slow processing without async	Warning

Detection Patterns

# Find event classes
Glob: **/Event/**/*Event.php
Glob: **/Events/**/*.php

# Check for event publishers
Grep: "EventPublisher|EventDispatcher|publish\(" --glob "**/*.php"

# Find event handlers/consumers
Grep: "implements.*EventHandler|implements.*Consumer" --glob "**/*.php"

# Check for message broker usage
Grep: "AMQPChannel|RabbitMQ|Kafka" --glob "**/Infrastructure/**/*.php"

# Find potential issues
Grep: "new.*Event\(" --glob "**/Controller/**/*.php"  # Events in controllers
Grep: "->findById|->save" --glob "**/EventHandler/**/*.php"  # Sync calls in handlers

References

For detailed information, load these reference files:

• references/event-patterns.md — Event types, structure, publishing patterns
• references/messaging-patterns.md — Message broker patterns, queues, topics
• references/saga-patterns.md — Distributed transactions, choreography, orchestration
• references/antipatterns.md — Common violations with detection patterns