Dead Letter Queue Generator

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

When to Use

Scenario	Example
Poison messages	Messages that always cause handler failures
Processing failures	Temporary infrastructure issues (DB down, timeout)
Audit trail	Track all failed message processing attempts
Retry mechanism	Automatic retry with exponential backoff
Monitoring	Alert on DLQ threshold exceeded

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

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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

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File Placement

Layer	Path
Domain Types	src/Domain/Shared/DeadLetter/
Application	src/Application/Shared/DeadLetter/
Infrastructure	src/Infrastructure/DeadLetter/
Unit Tests	tests/Unit/{Layer}/{Path}/

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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

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Naming Conventions

Component	Pattern	Example
Failure Enum	FailureType	FailureType
Message Entity	DeadLetterMessage	DeadLetterMessage
Store Interface	DeadLetterStoreInterface	DeadLetterStoreInterface
Handler	DeadLetterHandler	DeadLetterHandler
Strategy	RetryStrategy	RetryStrategy
Classifier	FailureClassifier	FailureClassifier
Processor	DlqProcessor	DlqProcessor
Test	{ClassName}Test	RetryStrategyTest

---

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);

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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