Create Dead Letter Queue

Dead Letter Queue Generator

Creates Dead Letter Queue pattern infrastructure for capturing and retrying failed messages.

When to Use

Component Characteristics

DeadLetterMessage

• Entity capturing failed message details
• Original message body, headers, routing key
• Error message and stack trace
• Attempt count and timestamps
• Failure classification (Transient, Permanent, Unknown)

DeadLetterStoreInterface

• Application layer port
• store(DeadLetterMessage): void
• findRetryable(int limit): array — find messages eligible for retry
• markRetried(string id): void — increment attempt count
• markResolved(string id): void — mark as successfully reprocessed
• purge(DateTimeImmutable before): int — remove old entries

DeadLetterHandler

• Catches exceptions from message handlers
• Classifies failure type (transient vs permanent)
• Stores to DLQ with full context
• Logs failure details

RetryStrategy

• Configurable max attempts
• Exponential backoff with jitter
• Failure type classification
• Skip permanent failures

FailureClassifier

• Classifies exceptions as Transient or Permanent
• Configurable exception mapping
• Default: ConnectionException=Transient, ValidationException=Permanent

DlqProcessor

• Processes retryable messages from DLQ
• Applies RetryStrategy for backoff
• Removes successfully processed messages
• Re-stores still-failing messages

Generation Process

Step 1: Analyze Request

Determine:

• Storage backend (Database)
• Max retry attempts (default: 5)
• Backoff strategy (exponential with jitter)

Step 2: Generate Core Components

1. Domain Layer (src/Domain/Shared/DeadLetter/)

   • FailureType.php — Enum (Transient, Permanent, Unknown)
   • DeadLetterMessage.php — Message entity

2. Application Layer (src/Application/Shared/DeadLetter/)

   • DeadLetterStoreInterface.php — Storage port
   • DeadLetterHandler.php — Exception handler
   • RetryStrategy.php — Retry configuration and backoff calculation
   • FailureClassifier.php — Exception classification
   • DlqProcessor.php — Retry processor

3. Infrastructure Layer (src/Infrastructure/DeadLetter/)

   • DatabaseDeadLetterStore.php — PDO implementation
   • Database migration

4. Tests

   • DeadLetterMessageTest.php
   • RetryStrategyTest.php
   • FailureClassifierTest.php
   • DlqProcessorTest.php

File Placement

Key Principles

Failure Classification

1. Transient: Connection errors, timeouts, rate limits — eligible for retry
2. Permanent: Validation errors, business rule violations — never retry
3. Unknown: Unclassified exceptions — retry with caution

Retry Strategy

1. Max attempts configurable (default: 5)
2. Exponential backoff: delay = baseDelay * 2^attempt
3. Jitter: random ±25% to prevent thundering herd
4. Skip messages exceeding max attempts

Message Lifecycle

Message → Handler fails → DeadLetterHandler → Store to DLQ
                                                    ↓
                                          DlqProcessor retries
                                                    ↓
                                          Success → Remove from DLQ
                                          Failure → Re-store with incremented attempts
                                          Max exceeded → Mark as permanently failed

Naming Conventions

Quick Template Reference

DeadLetterMessage

final class DeadLetterMessage
{
    public function __construct(
        public readonly string $id,
        public readonly string $originalBody,
        public readonly string $originalRoutingKey,
        public readonly array $originalHeaders,
        public readonly string $errorMessage,
        public readonly string $errorTrace,
        public readonly FailureType $failureType,
        public readonly int $attemptCount,
        public readonly \DateTimeImmutable $failedAt,
        public readonly ?\DateTimeImmutable $nextRetryAt,
        public readonly ?\DateTimeImmutable $resolvedAt = null,
    ) {}

    public function isRetryable(int $maxAttempts): bool;
    public function withIncrementedAttempt(\DateTimeImmutable $nextRetryAt): self;
}

DeadLetterStoreInterface

interface DeadLetterStoreInterface
{
    public function store(DeadLetterMessage $message): void;
    /** @return array<DeadLetterMessage> */
    public function findRetryable(int $limit = 100): array;
    public function markRetried(string $id, \DateTimeImmutable $nextRetryAt): void;
    public function markResolved(string $id): void;
    public function purge(\DateTimeImmutable $before): int;
    public function countByType(FailureType $type): int;
}

Database Schema

CREATE TABLE dead_letter_messages (
    id VARCHAR(255) PRIMARY KEY,
    original_body TEXT NOT NULL,
    original_routing_key VARCHAR(255) NOT NULL,
    original_headers JSONB NOT NULL DEFAULT '{}',
    error_message TEXT NOT NULL,
    error_trace TEXT,
    failure_type VARCHAR(50) NOT NULL,
    attempt_count INTEGER NOT NULL DEFAULT 1,
    failed_at TIMESTAMP(6) NOT NULL,
    next_retry_at TIMESTAMP(6),
    resolved_at TIMESTAMP(6)
);

CREATE INDEX idx_dlq_retryable ON dead_letter_messages (failure_type, next_retry_at)
    WHERE resolved_at IS NULL AND failure_type != 'permanent';
CREATE INDEX idx_dlq_failed_at ON dead_letter_messages (failed_at);

References

For complete PHP templates and test examples, see:

• references/templates.md — All component templates
• references/examples.md — Message broker integration example and unit tests

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