Event-Driven Clean Architecture
Integrate event-driven patterns into Clean Architecture, using domain events to enhance decoupling and scalability.
Event-Driven Clean Architecture is a free Clean Architecture & Design Patterns in Practice lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Clean Architecture & Design Patterns in Practice learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Intro to Event-Driven Clean Arch
Welcome to Event-Driven Clean Architecture! In complex systems, parts often need to react to things happening elsewhere without being tightly coupled.
This lesson explores how to integrate event-driven patterns into your Clean Architecture, using domain events to enhance decoupling and scalability, while strictly adhering to the Dependency Rule.
What are Domain Events?
A Domain Event is something that happened in the domain that domain experts care about. It's an immutable fact, a record of an occurrence, like 'Order Placed' or 'User Registered'.
- They represent a change in the state of the domain.
- They are typically named in the past tense (e.g.,
OrderPlaced). - They should be simple data structures, containing only relevant information about the event.
Events and the Dependency Rule
In Clean Architecture, the Dependency Rule states that dependencies must flow inwards. Domain events fit perfectly:
- Entities can raise events, but don't know who handles them.
- Use Cases orchestrate entities and can publish events after an operation.
- Interface Adapters (e.g., presenters, external service gateways) can subscribe to and handle events from inner layers.
This maintains strict separation, as inner layers remain unaware of outer layer specifics.
Implementing a Domain Event
A domain event is essentially a data transfer object (DTO) that carries information about what happened. It's good practice to have a common interface for all domain events.
Here's a simple Java interface:
public interface DomainEvent {
long occurredOn();
}Code: Concrete Domain Event
Let's create a specific domain event: OrderPlacedEvent. It holds the orderId and the timestamp of when it occurred.
Try running this simple definition:
public class OrderPlacedEvent implements DomainEvent {
private final String orderId;
private final long occurredOn;
public OrderPlacedEvent(String orderId) {
this.orderId = orderId;
this.occurredOn = System.currentTimeMillis();
}
public String getOrderId() {
return orderId;
}
@Override
public long occurredOn() {
return occurredOn;
}
public static void main(String[] args) {
OrderPlacedEvent event = new OrderPlacedEvent("ORD-123");
System.out.println("Order event for: " + event.getOrderId());
}
}The Event Publisher
To publish events, we need an Event Publisher (also known as an Event Dispatcher or Event Bus). This component takes a domain event and dispatches it to all registered handlers.
It acts as a mediator, decoupling the event source from its consumers. The Use Case will depend on this publisher interface, not on specific handlers.
Code: Event Publisher Interface
Here's an interface for our EventPublisher. We'll also need a way for handlers to register themselves.
public interface EventPublisher {
void publish(DomainEvent event);
<T extends DomainEvent> void subscribe(Class<T> eventType, EventHandler<T> handler);
}Code: In-Memory Event Publisher
A simple in-memory implementation for demonstration purposes. In a real application, this might use a message queue (e.g., Kafka, RabbitMQ) for persistence and distribution.
Run this to see the basic publisher in action:
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public interface DomainEvent { long occurredOn(); }
public interface EventHandler<T extends DomainEvent> { void handle(T event); }
public class OrderPlacedEvent implements DomainEvent {
private final String orderId; private final long occurredOn;
public OrderPlacedEvent(String orderId) {
this.orderId = orderId; this.occurredOn = System.currentTimeMillis();
}
public String getOrderId() { return orderId; }
@Override public long occurredOn() { return occurredOn; }
}
public class SimpleEventPublisher implements EventPublisher {
private final Map<Class<? extends DomainEvent>, List<EventHandler<?>>> subscribers = new HashMap<>();
@Override
public void publish(DomainEvent event) {
List<EventHandler<?>> handlers = subscribers.get(event.getClass());
if (handlers != null) {
for (EventHandler handler : handlers) {
// Unchecked cast is safe due to type checking during subscription
((EventHandler<DomainEvent>) handler).handle(event);
}
}
}
@Override
public <T extends DomainEvent> void subscribe(Class<T> eventType, EventHandler<T> handler) {
subscribers.computeIfAbsent(eventType, k -> new ArrayList<>()).add(handler);
}
public static void main(String[] args) {
SimpleEventPublisher publisher = new SimpleEventPublisher();
publisher.subscribe(OrderPlacedEvent.class, event -> {
System.out.println("Handler 1: Order " + event.getOrderId() + " placed!");
});
publisher.subscribe(OrderPlacedEvent.class, event -> {
System.out.println("Handler 2: Notifying ops for order " + event.getOrderId());
});
OrderPlacedEvent event = new OrderPlacedEvent("DEMO-456");
publisher.publish(event);
}
}Integrating Use Cases & Handlers
Now, let's see how a Use Case publishes an event and how an Event Handler (residing in, for example, the Interface Adapters layer) reacts to it. The Use Case remains decoupled from the handler.
This design allows new handlers to be added without modifying the Use Case.
Code: Full Event-Driven Flow
This example demonstrates a PlaceOrderUseCase publishing an OrderPlacedEvent, which is then handled by an OrderEmailNotifier.
Notice how PlaceOrderUseCase only depends on EventPublisher, not the specific notifier.
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
// Domain Layer Interfaces & Classes
interface DomainEvent { long occurredOn(); }
class OrderPlacedEvent implements DomainEvent {
private final String orderId; private final long occurredOn;
public OrderPlacedEvent(String orderId) {
this.orderId = orderId; this.occurredOn = System.currentTimeMillis();
}
public String getOrderId() { return orderId; }
@Override public long occurredOn() { return occurredOn; }
}
// Application Layer Interfaces & Classes
interface EventPublisher {
void publish(DomainEvent event);
<T extends DomainEvent> void subscribe(Class<T> eventType, EventHandler<T> handler);
}
interface EventHandler<T extends DomainEvent> { void handle(T event); }
// Use Case (Application Layer)
class PlaceOrderUseCase {
private final EventPublisher eventPublisher;
public PlaceOrderUseCase(EventPublisher eventPublisher) {
this.eventPublisher = eventPublisher;
}
public void execute(String orderDetails) {
// Simulate order creation logic
String newOrderId = "ORD-" + System.nanoTime();
System.out.println("Order " + newOrderId + " created with details: " + orderDetails);
// Publish domain event
eventPublisher.publish(new OrderPlacedEvent(newOrderId));
}
}
// Infrastructure/Interface Adapters Layer Implementation
class SimpleEventPublisher implements EventPublisher {
private final Map<Class<? extends DomainEvent>, List<EventHandler<?>>> subscribers = new HashMap<>();
@Override
public void publish(DomainEvent event) {
List<EventHandler<?>> handlers = subscribers.get(event.getClass());
if (handlers != null) {
for (EventHandler handler : handlers) {
((EventHandler<DomainEvent>) handler).handle(event);
}
}
}
@Override
public <T extends DomainEvent> void subscribe(Class<T> eventType, EventHandler<T> handler) {
subscribers.computeIfAbsent(eventType, k -> new ArrayList<>()).add(handler);
}
}
class OrderEmailNotifier implements EventHandler<OrderPlacedEvent> {
@Override
public void handle(OrderPlacedEvent event) {
System.out.println("Email Notifier: Sending email for order " + event.getOrderId());
}
}
class InventoryUpdater implements EventHandler<OrderPlacedEvent> {
@Override
public void handle(OrderPlacedEvent event) {
System.out.println("Inventory Updater: Updating inventory for order " + event.getOrderId());
}
}
public class Main {
public static void main(String[] args) {
SimpleEventPublisher publisher = new SimpleEventPublisher();
// Register handlers
publisher.subscribe(OrderPlacedEvent.class, new OrderEmailNotifier());
publisher.subscribe(OrderPlacedEvent.class, new InventoryUpdater());
// Create use case with publisher dependency
PlaceOrderUseCase placeOrderUseCase = new PlaceOrderUseCase(publisher);
// Execute use case, which publishes the event
placeOrderUseCase.execute("Laptop, Quantity: 1");
placeOrderUseCase.execute("Keyboard, Quantity: 2");
}
}Quick Check: Domain Events
When integrating domain events into Clean Architecture, what is the primary benefit of having Use Cases publish events rather than directly calling other services?
Recap: Event-Driven Clean Arch
In this lesson, we explored Event-Driven Clean Architecture. You learned:
- Domain Events are immutable facts representing significant occurrences.
- They enable decoupling, allowing Use Cases to publish events without knowing their subscribers.
- Event Publishers mediate event dispatch to Event Handlers.
- This pattern supports scalability and extensibility, making systems easier to evolve while adhering to the Dependency Rule.
By leveraging domain events, your Clean Architecture can become even more robust and adaptable.
Frequently asked questions
Is the “Event-Driven Clean Architecture” lesson free?
Yes — the full text of “Event-Driven Clean Architecture” is free to read here on the web, and the Clean Architecture & Design Patterns in Practice course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Clean Architecture & Design Patterns in Practice course, upgrade to CoddyKit PRO.
What will I learn in “Event-Driven Clean Architecture”?
Integrate event-driven patterns into Clean Architecture, using domain events to enhance decoupling and scalability. You practise Clean Architecture & Design Patterns in Practice with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Clean Architecture & Design Patterns in Practice?
No prior experience is required. Clean Architecture & Design Patterns in Practice on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Event-Driven Clean Architecture” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Clean Architecture & Design Patterns in Practice lesson?
Yes. Every Clean Architecture & Design Patterns in Practice lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Handling Cross-Cutting Concerns
- Event-Driven Clean Architecture
- Clean Architecture in Microservices
- CQRS within Clean Architecture