Messaging

Context

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.

Forces

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

Problem

How do services in a microservice architecture communicate?

Solution

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

Examples

There are numerous examples of asynchronous messaging technologies

Apache Kafka
RabbitMQ

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;
    orderRepository.save(order);

    orderAggregateEventPublisher.publish(order, orderAndEvents.events);

    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

The [[Saga]] pattern and [[CQRS]] pattern use messaging
The [[Transactional Outbox]] pattern enables messages to be sent as part of a database transaction
The [[Externalized configuration]] pattern supplies the (logical) message channel names and the location of the message broker
The [[Domain-specific protocol]] pattern is an alternative pattern
The [[RPI]] pattern is an alternative pattern