Pattern: Messaging


You have applied the Microservice architecture pattern. Services must handle requests from the application’s clients. Furthermore, services often collaborate to handle those requests. Consequently, they must use an inter-process communication protocol.


  • Services often need to collaborate
  • Synchronous communicate results in tight runtime coupling, both the client and service must be available for the duration of the request


How do services in a microservice architecture communicate?


Use asynchronous messaging for inter-service communication. Services communicating by exchanging messages over messaging channels.

There are several different styles of asynchronous communication:

  • Request/response - a service sends a request message to a recipient and expects to receive a reply message promptly
  • Notifications - a sender sends a message a recipient but does not expect a reply. Nor is one sent.
  • Request/asynchronous response - a service sends a request message to a recipient and expects to receive a reply message eventually
  • Publish/subscribe - a service publishes a message to zero or more recipients
  • Publish/asynchronous response - a service publishes a request to one or recipients, some of whom send back a reply


There are numerous examples of asynchronous messaging technologies

OrderService from the FTGO Example application publishes an Order Created event when it creates an Order.

public class OrderService {


  public Order createOrder(long consumerId, long restaurantId,
                           List<MenuItemIdAndQuantity> lineItems) {
    Restaurant restaurant = restaurantRepository.findById(restaurantId)
            .orElseThrow(() -> new RestaurantNotFoundException(restaurantId));

    List<OrderLineItem> orderLineItems = makeOrderLineItems(lineItems, restaurant);

    ResultWithDomainEvents<Order, OrderDomainEvent> orderAndEvents =
            Order.createOrder(consumerId, restaurant, orderLineItems);

    Order order = orderAndEvents.result;;


    OrderDetails orderDetails = new OrderDetails(consumerId, restaurantId, orderLineItems, order.getOrderTotal());

    CreateOrderSagaState data = new CreateOrderSagaState(order.getId(), orderDetails);
    createOrderSagaManager.create(data, Order.class, order.getId());

    meterRegistry.ifPresent(mr -> mr.counter("placed_orders").increment());

    return order;

Resulting context

This pattern has the following benefits:

  • Loose runtime coupling since it decouples the message sender from the consumer
  • Improved availability since the message broker buffers messages until the consumer is able to process them
  • Supports a variety of communication patterns including request/reply, notifications, request/async response, publish/subscribe, publish/async response etc

This pattern has the following drawbacks:

  • Additional complexity of message broker, which must be highly available

This pattern has the following issues:

  • Request/reply-style communication is more complex

See also

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