Outbox Pattern Knowledge Base

Quick reference for Transactional Outbox pattern and PHP implementation guidelines.

Core Principles

Transactional Outbox Overview

┌─────────────────────────────────────────────────────────────────────────┐
│                    TRANSACTIONAL OUTBOX PATTERN                          │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│   ┌──────────────────────────────────────────────────────────────────┐  │
│   │                    SINGLE TRANSACTION                             │  │
│   │  ┌──────────┐      ┌───────────────┐      ┌────────────────┐     │  │
│   │  │ Business │─────▶│ Domain Table  │      │ Outbox Table   │     │  │
│   │  │  Logic   │      │  (orders)     │      │ (outbox_msgs)  │     │  │
│   │  └──────────┘      └───────────────┘      └────────────────┘     │  │
│   │       │                   ▲                       ▲              │  │
│   │       └───────────────────┴───────────────────────┘              │  │
│   │                    COMMIT/ROLLBACK                               │  │
│   └──────────────────────────────────────────────────────────────────┘  │
│                                                                          │
│   ┌──────────────────┐                    ┌──────────────────────────┐  │
│   │ Message Relay    │───────────────────▶│ Message Broker           │  │
│   │ (Polling/CDC)    │   publish events   │ (RabbitMQ/Kafka)         │  │
│   └──────────────────┘                    └──────────────────────────┘  │
│           │                                                              │
│           ▼                                                              │
│   Marks messages as processed                                           │
│                                                                          │
├─────────────────────────────────────────────────────────────────────────┤
│                                                                          │
│   Publishing Strategies:                                                 │
│   • Polling Publisher  - Periodic poll for unprocessed messages          │
│   • Transaction Log Tailing (CDC) - Debezium, Maxwell                   │
│   • Event Sourcing + Projections - Events = Outbox                      │
│                                                                          │
│   Guarantees:                                                            │
│   • At-least-once delivery                                              │
│   • No message loss on service crash                                    │
│   • Transactional consistency between data and events                   │
│                                                                          │
└─────────────────────────────────────────────────────────────────────────┘

Key Concepts

Concept	Description
Outbox Table	Database table storing pending messages within same transaction
Message Relay	Background process that publishes messages from outbox
Polling Publisher	Periodically queries outbox for unpublished messages
CDC (Change Data Capture)	Streams database changes to message broker
Idempotency Key	Unique identifier for message deduplication
At-least-once	Messages delivered at least once (consumers must be idempotent)

Quick Checklists

Outbox Table Checklist

• Messages inserted in same transaction as domain changes
• Unique message ID for deduplication
• Event type/name for routing
• Payload serialized as JSON
• Created timestamp
• Processed/Published flag or timestamp
• Aggregate ID for correlation
• Retry count for failure tracking

Message Relay Checklist

• Runs as separate process/cron
• Polls with configurable interval
• Batch processing for efficiency
• Marks messages as processed after publish
• Handles publish failures with retry
• Dead letter handling for poison messages
• Ordering guarantees per aggregate (if needed)

Consumer Checklist

• Idempotent processing (check message ID)
• Handles duplicate messages gracefully
• Stores processed message IDs
• Acknowledges only after successful processing

PHP 8.4 Outbox Patterns

OutboxMessage Entity

<?php

declare(strict_types=1);

namespace Domain\Shared\Outbox;

final readonly class OutboxMessage
{
    public function __construct(
        public string $id,
        public string $aggregateType,
        public string $aggregateId,
        public string $eventType,
        public string $payload,
        public \DateTimeImmutable $createdAt,
        public ?string $correlationId = null,
        public ?\DateTimeImmutable $processedAt = null,
        public int $retryCount = 0
    ) {}

    public function isProcessed(): bool
    {
        return $this->processedAt !== null;
    }

    public function withProcessed(\DateTimeImmutable $at): self
    {
        return new self(
            $this->id,
            $this->aggregateType,
            $this->aggregateId,
            $this->eventType,
            $this->payload,
            $this->createdAt,
            $this->correlationId,
            $at,
            $this->retryCount
        );
    }

    public function withRetry(): self
    {
        return new self(
            $this->id,
            $this->aggregateType,
            $this->aggregateId,
            $this->eventType,
            $this->payload,
            $this->createdAt,
            $this->correlationId,
            $this->processedAt,
            $this->retryCount + 1
        );
    }
}

OutboxRepository Interface (Domain)

<?php

declare(strict_types=1);

namespace Domain\Shared\Outbox;

interface OutboxRepositoryInterface
{
    public function save(OutboxMessage $message): void;

    /** @param array<OutboxMessage> $messages */
    public function saveAll(array $messages): void;

    /** @return array<OutboxMessage> */
    public function findUnprocessed(int $limit = 100): array;

    public function markAsProcessed(string $id, \DateTimeImmutable $at): void;

    public function incrementRetry(string $id): void;

    public function delete(string $id): void;
}

Outbox Publisher Service

<?php

declare(strict_types=1);

namespace Application\Shared\Outbox;

use Domain\Shared\Outbox\OutboxMessage;
use Domain\Shared\Outbox\OutboxRepositoryInterface;

final readonly class OutboxPublisher
{
    public function __construct(
        private OutboxRepositoryInterface $outbox,
        private EventPublisherInterface $publisher,
        private int $maxRetries = 3
    ) {}

    public function processOutbox(int $batchSize = 100): int
    {
        $messages = $this->outbox->findUnprocessed($batchSize);
        $processed = 0;

        foreach ($messages as $message) {
            try {
                $this->publisher->publish(
                    $message->eventType,
                    $message->payload,
                    $message->correlationId
                );
                $this->outbox->markAsProcessed(
                    $message->id,
                    new \DateTimeImmutable()
                );
                $processed++;
            } catch (\Throwable $e) {
                $this->handleFailure($message, $e);
            }
        }

        return $processed;
    }

    private function handleFailure(OutboxMessage $message, \Throwable $e): void
    {
        if ($message->retryCount >= $this->maxRetries) {
            // Move to dead letter / log critical
            $this->outbox->delete($message->id);
            return;
        }
        $this->outbox->incrementRetry($message->id);
    }
}

Transactional Event Dispatch

<?php

declare(strict_types=1);

namespace Application\Order\UseCase;

use Domain\Order\OrderRepositoryInterface;
use Domain\Shared\Outbox\OutboxRepositoryInterface;
use Domain\Shared\Outbox\OutboxMessage;

final readonly class PlaceOrderUseCase
{
    public function __construct(
        private OrderRepositoryInterface $orders,
        private OutboxRepositoryInterface $outbox,
        private TransactionInterface $transaction
    ) {}

    public function execute(PlaceOrderCommand $command): OrderId
    {
        return $this->transaction->execute(function () use ($command): OrderId {
            $order = Order::place(
                OrderId::generate(),
                CustomerId::fromString($command->customerId),
                $command->items
            );

            $this->orders->save($order);

            // Store event in outbox within same transaction
            foreach ($order->releaseEvents() as $event) {
                $this->outbox->save(new OutboxMessage(
                    id: $event->eventId,
                    aggregateType: 'Order',
                    aggregateId: $order->id()->toString(),
                    eventType: $event->eventName(),
                    payload: json_encode($event->toArray()),
                    createdAt: $event->occurredAt,
                    correlationId: $command->correlationId
                ));
            }

            return $order->id();
        });
    }
}

Common Violations Quick Reference

Violation	Where to Look	Severity
Publish before commit	Event published without outbox	Critical
No idempotency key	OutboxMessage without unique ID	Critical
Two-phase commit	Distributed transaction attempt	Critical
Missing retry logic	No retry count in outbox	Warning
No dead letter handling	Failed messages lost	Warning
Unbounded polling	No limit on batch size	Warning
Synchronous publish in transaction	HTTP call in DB transaction	Critical

Detection Patterns

# Find outbox implementations
Glob: **/Outbox/**/*.php
Glob: **/outbox*.php
Grep: "outbox|OutboxMessage|OutboxRepository" --glob "**/*.php"

# Check for proper transactional outbox
Grep: "->save.*->outbox|outbox.*transaction" --glob "**/UseCase/**/*.php"

# Detect anti-patterns: publishing in transaction
Grep: "transaction.*publish|->publish\(.*\)->commit" --glob "**/*.php"

# Find message relay/processor
Grep: "findUnprocessed|processOutbox|OutboxProcessor" --glob "**/*.php"

# Check for idempotency handling
Grep: "messageId|eventId|idempotencyKey" --glob "**/Consumer/**/*.php"

# Find Doctrine outbox table
Grep: "outbox_messages|OutboxMessage.*Entity" --glob "**/Infrastructure/**/*.php"

Database Schema Example

CREATE TABLE outbox_messages (
    id UUID PRIMARY KEY,
    aggregate_type VARCHAR(255) NOT NULL,
    aggregate_id VARCHAR(255) NOT NULL,
    event_type VARCHAR(255) NOT NULL,
    payload JSONB NOT NULL,
    correlation_id VARCHAR(255),
    created_at TIMESTAMP NOT NULL DEFAULT NOW(),
    processed_at TIMESTAMP NULL,
    retry_count INT NOT NULL DEFAULT 0,
    INDEX idx_unprocessed (processed_at, created_at)
);

References

For detailed information, load these reference files:

• references/outbox-patterns.md — Implementation strategies and patterns
• references/antipatterns.md — Common violations with detection patterns
• references/php-specific.md — PHP 8.4 specific implementations

Assets

• assets/report-template.md — Structured audit report template