☠️ Dead Letter Queues (DLQs)

📖 What is a Dead Letter Queue?

A Dead Letter Queue (DLQ) is a special queue used to store messages that cannot be delivered or processed successfully. Instead of losing these messages, they are moved to a DLQ for later inspection, troubleshooting, or reprocessing.

🛠 Example in .NET Core (Azure Service Bus)

Azure Service Bus automatically provides a DLQ for each queue and subscription.

// Install-Package Azure.Messaging.ServiceBus

using Azure.Messaging.ServiceBus;
using System;
using System.Threading.Tasks;

public class DeadLetterExample
{
    private const string connectionString = "<Your-Service-Bus-Connection-String>";
    private const string queueName = "demo-queue";

    public static async Task Main()
    {
        await using var client = new ServiceBusClient(connectionString);

        // Receiver for Dead Letter Queue
        string deadLetterQueuePath = EntityNameFormatter.FormatDeadLetterPath(queueName);
        ServiceBusReceiver dlqReceiver = client.CreateReceiver(deadLetterQueuePath);

        Console.WriteLine("Reading messages from Dead Letter Queue...");
        var message = await dlqReceiver.ReceiveMessageAsync();

        if (message != null)
        {
            Console.WriteLine($"Dead-lettered message: {message.Body}");
            Console.WriteLine($"Reason: {message.DeadLetterReason}");
            Console.WriteLine($"Description: {message.DeadLetterErrorDescription}");
        }
    }
}
    

✅ Advantages

• Prevents message loss by capturing unprocessed messages.
• Helps diagnose issues (invalid data, processing errors, expired TTL).
• Supports reprocessing after fixing the root cause.
• Improves system reliability and resilience.

⚠️ Disadvantages

• Requires monitoring and manual intervention.
• DLQ can grow indefinitely if not managed.
• Reprocessing logic can be complex.
• May expose sensitive data if not secured properly.

🧭 Best Practices

• Monitor DLQ regularly with alerts and dashboards.
• Implement automated reprocessing pipelines where possible.
• Log detailed error reasons when dead-lettering messages.
• Use DLQ as a safety net, not as a normal workflow.

🔒 Precautions

• Secure DLQ access with proper authentication and encryption.
• Set message TTL (time-to-live) to avoid stale data.
• Handle poison messages carefully to prevent infinite retry loops.
• Ensure compliance with data retention policies.

🎯 Summary

Dead Letter Queues are essential for building resilient distributed systems. They capture failed messages for later analysis and reprocessing, ensuring no data is silently lost. In .NET Core, DLQs can be used with Azure Service Bus, RabbitMQ, or Kafka depending on your messaging infrastructure.