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Cover image for RabbitMQ: Fanout Exchange Pattern.
Daniel Azevedo
Daniel Azevedo

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RabbitMQ: Fanout Exchange Pattern.

#eventdriven #microservices #cleancode #csharp

Hi devs

The Fanout Exchange Pattern is one of the most commonly used messaging patterns in RabbitMQ. It's perfect for scenarios where a message needs to be sent to multiple consumers simultaneously. In this post, we'll dive into what the Fanout Exchange is, how it works, and how to implement it in a real-world example using RabbitMQ and .NET.

What is a Fanout Exchange?

A Fanout Exchange is a type of exchange in RabbitMQ that broadcasts messages to all queues bound to it, regardless of the routing key. This is useful for:

  • Broadcasting events like system-wide notifications or updates.
  • Workflows where multiple services must act upon the same event.
  • Pub/Sub architectures where multiple subscribers listen to a single publisher.

How Does It Work?

  1. A producer sends a message to a fanout exchange.
  2. The exchange routes the message to all bound queues.
  3. Each queue has one or more consumers that process the messages.

Key Features:

  • No routing key is required for message delivery.
  • Messages are delivered to all bound queues, ensuring parallel processing.

Example Use Case

Imagine a payment system where multiple services need to act on a PaymentCompleted event:

  1. Notification Service: Sends a payment receipt to the customer.
  2. Analytics Service: Updates financial dashboards.
  3. Inventory Service: Restocks sold items.

Each of these services will consume the same event from the fanout exchange.

Implementing the Fanout Exchange Pattern

Let’s build a simple example with a producer and two consumers using RabbitMQ in .NET.

1. Set Up RabbitMQ

Install RabbitMQ locally or use Docker:

docker run -d --name rabbitmq -p 5672:5672 -p 15672:15672 rabbitmq:management
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Access the RabbitMQ management UI at http://localhost:15672 (default credentials: guest/guest).

2. Producer: Publish Messages to the Fanout Exchange 

using RabbitMQ.Client; using System.Text; public class PaymentProducer { public void PublishPaymentCompletedEvent(string paymentId) { var factory = new ConnectionFactory() { HostName = "localhost" }; using var connection = factory.CreateConnection(); using var channel = connection.CreateModel(); // Declare a fanout exchange channel.ExchangeDeclare(exchange: "payments_exchange", type: ExchangeType.Fanout); var message = $"Payment completed: {paymentId}"; var body = Encoding.UTF8.GetBytes(message); // Publish message to the exchange channel.BasicPublish(exchange: "payments_exchange", routingKey: "", body: body); Console.WriteLine($"Published: {message}"); } } // Usage var producer = new PaymentProducer(); producer.PublishPaymentCompletedEvent("12345");
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This code declares a fanout exchange called payments_exchange and publishes a PaymentCompleted message to it.

3. Consumer 1: Notification Service 

using RabbitMQ.Client; using RabbitMQ.Client.Events; using System.Text; public class NotificationConsumer { public void Start() { var factory = new ConnectionFactory() { HostName = "localhost" }; using var connection = factory.CreateConnection(); using var channel = connection.CreateModel(); // Declare the exchange and a unique queue channel.ExchangeDeclare(exchange: "payments_exchange", type: ExchangeType.Fanout); var queueName = channel.QueueDeclare().QueueName; channel.QueueBind(queue: queueName, exchange: "payments_exchange", routingKey: ""); var consumer = new EventingBasicConsumer(channel); consumer.Received += (model, ea) => { var body = ea.Body.ToArray(); var message = Encoding.UTF8.GetString(body); Console.WriteLine($"[Notification Service] Received: {message}"); }; channel.BasicConsume(queue: queueName, autoAck: true, consumer: consumer); Console.WriteLine("Notification Service is running..."); } } // Usage var notificationConsumer = new NotificationConsumer(); notificationConsumer.Start();
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4. Consumer 2: Analytics Service 

using RabbitMQ.Client; using RabbitMQ.Client.Events; using System.Text; public class AnalyticsConsumer { public void Start() { var factory = new ConnectionFactory() { HostName = "localhost" }; using var connection = factory.CreateConnection(); using var channel = connection.CreateModel(); // Declare the exchange and a unique queue channel.ExchangeDeclare(exchange: "payments_exchange", type: ExchangeType.Fanout); var queueName = channel.QueueDeclare().QueueName; channel.QueueBind(queue: queueName, exchange: "payments_exchange", routingKey: ""); var consumer = new EventingBasicConsumer(channel); consumer.Received += (model, ea) => { var body = ea.Body.ToArray(); var message = Encoding.UTF8.GetString(body); Console.WriteLine($"[Analytics Service] Received: {message}"); }; channel.BasicConsume(queue: queueName, autoAck: true, consumer: consumer); Console.WriteLine("Analytics Service is running..."); } } // Usage var analyticsConsumer = new AnalyticsConsumer(); analyticsConsumer.Start();
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Testing the Setup

  1. Run NotificationConsumer and AnalyticsConsumer.
  2. Publish a message using PaymentProducer.
  3. Both consumers should receive the message and process it independently.

Key Advantages of the Fanout Exchange Pattern

  1. Scalable: Easily add more consumers without changing the producer.
  2. Decoupled Communication: Producers don’t need to know about consumers.
  3. Parallel Processing: Multiple services can act on the same message simultaneously.
  4. Flexibility: Supports broadcasting and real-time updates.

Conclusion

The Fanout Exchange Pattern is a powerful way to broadcast messages to multiple services in a microservices architecture. It’s simple to implement with RabbitMQ and provides scalability and decoupled communication.

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