Using Domain Events in Microservices

Understanding Domain Events

1. Decoupled Microservices (Bounded Contexts)

2. Helps us to deeply understand the domain model

3. Providing data sources for audits and reports

4. Towards Event Sourcing , CQRS, and More

[[196315]]

public abstract class Event {
    private final UUID id;
    private final DateTime createdTime;

    public Event() {
        this.id = UUID.randomUUID();
        this.createdTime = new DateTime();
    }
}

public final class CustomerArrivedEvent extends Event {
    private final int customerNumber;

    public CustomerArrivedEvent(int customerNumber) {
        super();
        this.customerNumber = customerNumber;
    }
}

public class Order {
    public Order(EventPublisher eventPublisher) {
        //create order        
        //…        
        eventPublisher.publish(new OrderPlacedEvent());    
        }
}

public class Order {
    public Order() {
        //create order
        //...
        EventPublisher.publish(new OrderPlacedEvent());
    }
}

public class Order {

    private List<Event> events;

    public Order() {
        //create order
        //...
        events.add(new OrderPlacedEvent());
    }

    public List<Event> getEvents() {
        return events;
    }

    public void clearEvents() {
        events.clear();

    }
}

@Test
public void shouldPublishEventWhenCreateOrder() {
    Order order = new Order();
    List<Event> events = order.getEvents();
    assertEquals(1, events.size());
    Event event = events.get(0);
    assertTrue(event instanceof OrderPlacedEvent);
}

public class OrderRepository {
    private EventPublisher eventPublisher;

    public void save(Order order) {
        //save the order
        //...
        List<Event> events = order.getEvents();
        events.forEach(event -> eventPublisher.publish(event));
        order.clearEvents();
    }
}