13.2 Message Queue Fundamentals and RabbitMQ Integration

A Message Queue (MQ) is middleware enabling asynchronous communication between services. Instead of Service A waiting for Service B's response, it places a message in the queue and moves on — Service B consumes it at its own pace. This dramatically reduces inter-service coupling.

1. AMQP and Exchange-Queue Routing Concepts

RabbitMQ operates on the AMQP protocol. Messages always flow in this order: Producer → Exchange → Binding → Queue → Consumer.

• Exchange Types:
  • Direct: Routes to queues where the routing key matches exactly (1:1)
  • Topic: Routes to multiple queues using wildcard (*, #) pattern matching
  • Fanout: Broadcasts to all bound queues (1:N)

2. Quick Spring AMQP Setup

implementation 'org.springframework.boot:spring-boot-starter-amqp'

@Configuration
public class RabbitMqConfig {

    public static final String QUEUE_NAME = "order.queue";
    public static final String EXCHANGE_NAME = "order.exchange";
    public static final String ROUTING_KEY = "order.created";

    @Bean
    public Queue orderQueue() {
        return new Queue(QUEUE_NAME, true); // durable=true: survives broker restarts
    }

    @Bean
    public DirectExchange orderExchange() {
        return new DirectExchange(EXCHANGE_NAME);
    }

    @Bean
    public Binding binding(Queue orderQueue, DirectExchange orderExchange) {
        return BindingBuilder.bind(orderQueue).to(orderExchange).with(ROUTING_KEY);
    }

    @Bean
    public MessageConverter jsonMessageConverter() {
        return new Jackson2JsonMessageConverter(); // Serializes Java objects to JSON
    }
}

3. Publishing (Producer) and Consuming (Consumer)

// [Producer] Publishing a message
@Service
@RequiredArgsConstructor
public class OrderProducer {
    private final RabbitTemplate rabbitTemplate;

    public void sendOrderCreatedEvent(OrderCreatedEvent event) {
        rabbitTemplate.convertAndSend(
            RabbitMqConfig.EXCHANGE_NAME,
            RabbitMqConfig.ROUTING_KEY,
            event
        );
        log.info("Order event published: {}", event.getOrderId());
    }
}

// [Consumer] Consuming from the queue
@Service
public class OrderConsumer {

    @RabbitListener(queues = RabbitMqConfig.QUEUE_NAME)
    public void handleOrderCreated(OrderCreatedEvent event) {
        log.info("Received order event: {}", event.getOrderId());
        // Async post-processing: send email, deduct inventory, etc.
    }
}

warning

If an exception is thrown inside @RabbitListener, RabbitMQ will by default requeue the message — causing an infinite retry loop. Always configure a Dead Letter Queue (DLQ) or a RetryInterceptor with a maximum retry count to prevent this.