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1. Overview

In this article, we'll be looking at Axon and how it helps us implement applications with CQRS (Command Query Responsibility Segregation) and Event Sourcing in mind.

During this guide both Axon Framework and Axon Server will be utilized. The former will contain our implementation and the latter will be our dedicated Event Store and Message Routing solution.

The sample application we'll be building focuses on an Order domain. For this, we'll be leveraging the CQRS and Event Sourcing building blocks Axon provides us.

Note that a lot of the shared concepts come right out of DDD, which is beyond the scope of this current article.

2. Maven Dependencies

We'll create an Axon / Spring Boot application. Hence, we need to add the latest axon-spring-boot-starter dependency to our pom.xml, as well as the axon-test dependency for testing:

<dependency>
    <groupId>org.axonframework</groupId>
    <artifactId>axon-spring-boot-starter</artifactId>
    <version>4.1.2</version>
</dependency>

<dependency>
    <groupId>org.axonframework</groupId>
    <artifactId>axon-test</artifactId>
    <version>4.1.2</version>
    <scope>test</scope>
</dependency>

3. Axon Server

We will use Axon Server to be our Event Store and our dedicated command, event and query routing solution.

As an Event Store it gives us the ideal characteristics required when storing events. This article provides background why this is desirable.

As a Message Routing solution it gives us the option to connect several instances together without focusing on configuring things like a RabbitMQ or a Kafka topic to share and dispatch messages.

Axon Server can be downloaded here. As it is a simple JAR file, the following operation suffices to start it up:

java -jar axonserver.jar

This will start a single Axon Server instance which is accessible through localhost:8024. The endpoint provides an overview of the connected applications and the messages they can handle, as well as a querying mechanism towards the Event Store contained within Axon Server.

The default configuration of Axon Server together with the axon-spring-boot-starter dependency will ensure our Order service will automatically connect to it.

4. Order Service API – Commands

We'll set up our Order service with CQRS in mind. Therefore we'll emphasize the messages that flow through our application.

First, we'll define the Commands, meaning the expressions of intent. The Order service is capable of handling three different types of actions:

  1. Placing a new order
  2. Confirming an order
  3. Shipping an order

Naturally, there will be three command messages that our domain can deal with — PlaceOrderCommand, ConfirmOrderCommand, and ShipOrderCommand:

public class PlaceOrderCommand {
 
    @TargetAggregateIdentifier
    private final String orderId;
    private final String product;
 
    // constructor, getters, equals/hashCode and toString 
}
public class ConfirmOrderCommand {
 
    @TargetAggregateIdentifier
    private final String orderId;
    
    // constructor, getters, equals/hashCode and toString
}
public class ShipOrderCommand {
 
    @TargetAggregateIdentifier
    private final String orderId;
 
    // constructor, getters, equals/hashCode and toString
}

The TargetAggregateIdentifier annotation tells Axon that the annotated field is an id of a given aggregate to which the command should be targeted. We'll briefly touch on aggregates later in this article.

Also, note that we marked the fields in the commands as final. This is intentional, as it's a best practice for any message implementation to be immutable.

5. Order Service API – Events

Our aggregate will handle the commands, as it's in charge of deciding if an Order can be placed, confirmed, or shipped.

It will notify the rest of the application of its decision by publishing an event. We'll have three types of events — OrderPlacedEvent, OrderConfirmedEvent, and OrderShippedEvent:

public class OrderPlacedEvent {
 
    private final String orderId;
    private final String product;
 
    // default constructor, getters, equals/hashCode and toString
}
public class OrderConfirmedEvent {
 
    private final String orderId;
 
    // default constructor, getters, equals/hashCode and toString
}
public class OrderShippedEvent { 

    private final String orderId; 

    // default constructor, getters, equals/hashCode and toString 
}

6. The Command Model – Order Aggregate

Now that we've modeled our core API with respect to the commands and events, we can start creating the Command Model.

As our domain focuses on dealing with Orders, we'll create an OrderAggregate as the center of our Command Model.

6.1. Aggregate Class

Thus, let's create our basic aggregate class:

@Aggregate
public class OrderAggregate {

    @AggregateIdentifier
    private String orderId;
    private boolean orderConfirmed;

    @CommandHandler
    public OrderAggregate(PlaceOrderCommand command) {
        AggregateLifecycle.apply(new OrderPlacedEvent(command.getOrderId(), command.getProduct()));
    }

    @EventSourcingHandler
    public void on(OrderPlacedEvent event) {
        this.orderId = event.getOrderId();
        orderConfirmed = false;
    }

    protected OrderAggregate() { }
}

The Aggregate annotation is an Axon Spring specific annotation marking this class as an aggregate. It will notify the framework that the required CQRS and Event Sourcing specific building blocks need to be instantiated for this OrderAggregate.

As an aggregate will handle commands which are targeted for a specific aggregate instance, we need to specify the identifier with the AggregateIdentifier annotation.

Our aggregate will commence its life cycle upon handling the PlaceOrderCommand in the OrderAggregate ‘command handling constructor'. To tell the framework that the given function is able to handle commands, we'll add the CommandHandler annotation.

When handling the PlaceOrderCommand, it will notify the rest of the application that an order was placed by publishing the OrderPlacedEvent. To publish an event from within an aggregate, we'll use AggregateLifecycle#apply(Object…).

From this point, we can actually start to incorporate Event Sourcing as the driving force to recreate an aggregate instance from its stream of events.

We start this off with the ‘aggregate creation event', the OrderPlacedEvent, which is handled in an EventSourcingHandler annotated function to set the orderId and orderConfirmed state of the Order aggregate.

Also note that to be able to source an aggregate based on its events, Axon requires a default constructor.

6.2. Aggregate Command Handlers

Now that we have our basic aggregate, we can start implementing the remaining command handlers:

@CommandHandler 
public void handle(ConfirmOrderCommand command) { 
    apply(new OrderConfirmedEvent(orderId)); 
} 

@CommandHandler 
public void handle(ShipOrderCommand command) { 
    if (!orderConfirmed) { 
        throw new UnconfirmedOrderException(); 
    } 
    apply(new OrderShippedEvent(orderId)); 
} 

@EventSourcingHandler 
public void on(OrderConfirmedEvent event) { 
    orderConfirmed = true; 
}

The signature of our command and event sourcing handlers simply states handle({the-command}) and on({the-event}) to maintain a concise format.

Additionally, we've defined that an Order can only be shipped if it's been confirmed. Thus, we'll throw an UnconfirmedOrderException if this is not the case.

This exemplifies the need for the OrderConfirmedEvent sourcing handler to update the orderConfirmed state to true for the Order aggregate.

7. Testing the Command Model

First, we need to set up our test by creating a FixtureConfiguration for the OrderAggregate:

private FixtureConfiguration<OrderAggregate> fixture;

@Before
public void setUp() {
    fixture = new AggregateTestFixture<>(OrderAggregate.class);
}

The first test case should cover the simplest situation. When the aggregate handles the PlaceOrderCommand, it should produce an OrderPlacedEvent:

String orderId = UUID.randomUUID().toString();
String product = "Deluxe Chair";
fixture.givenNoPriorActivity()
  .when(new PlaceOrderCommand(orderId, product))
  .expectEvents(new OrderPlacedEvent(orderId, product));

Next, we can test the decision-making logic of only being able to ship an Order if it's been confirmed. Due to this, we have two scenarios — one where we expect an exception, and one where we expect an OrderShippedEvent.

Let's take a look at the first scenario, where we expect an exception:

String orderId = UUID.randomUUID().toString();
String product = "Deluxe Chair";
fixture.given(new OrderPlacedEvent(orderId, product))
  .when(new ShipOrderCommand(orderId))
  .expectException(IllegalStateException.class);

And now the second scenario, where we expect an OrderShippedEvent:

String orderId = UUID.randomUUID().toString();
String product = "Deluxe Chair";
fixture.given(new OrderPlacedEvent(orderId, product), new OrderConfirmedEvent(orderId))
  .when(new ShipOrderCommand(orderId))
  .expectEvents(new OrderShippedEvent(orderId));

8. The Query Model – Event Handlers

So far, we've established our core API with the commands and events, and we have the Command model of our CQRS Order service, the Order aggregate, in place.

Next, we can start thinking of one of the Query Models our application should service.

One of these models is the OrderedProducts:

public class OrderedProduct {

    private final String orderId;
    private final String product;
    private OrderStatus orderStatus;

    public OrderedProduct(String orderId, String product) {
        this.orderId = orderId;
        this.product = product;
        orderStatus = OrderStatus.PLACED;
    }

    public void setOrderConfirmed() {
        this.orderStatus = OrderStatus.CONFIRMED;
    }

    public void setOrderShipped() {
        this.orderStatus = OrderStatus.SHIPPED;
    }

    // getters, equals/hashCode and toString functions
}
public enum OrderStatus {
    PLACED, CONFIRMED, SHIPPED
}

We'll update this model based on the events propagating through our system. A Spring Service bean to update our model will do the trick:

@Service
public class OrderedProductsEventHandler {

    private final Map<String, OrderedProduct> orderedProducts = new HashMap<>();

    @EventHandler
    public void on(OrderPlacedEvent event) {
        String orderId = event.getOrderId();
        orderedProducts.put(orderId, new OrderedProduct(orderId, event.getProduct()));
    }

    // Event Handlers for OrderConfirmedEvent and OrderShippedEvent...
}

As we've used the axon-spring-boot-starter dependency to initiate our Axon application, the framework will automatically scan all the beans for existing message-handling functions.

As the OrderedProductsEventHandler has EventHandler annotated functions to store an OrderedProduct and update it, this bean will be registered by the framework as a class that should receive events without requiring any configuration on our part.

9. The Query Model – Query Handlers

Next, to query this model, for example, to retrieve all the ordered products, we should first introduce a Query message to our core API:

public class FindAllOrderedProductsQuery { }

Second, we'll have to update the OrderedProductsEventHandler to be able to handle the FindAllOrderedProductsQuery:

@QueryHandler
public List<OrderedProduct> handle(FindAllOrderedProductsQuery query) {
    return new ArrayList<>(orderedProducts.values());
}

The QueryHandler annotated function will handle the FindAllOrderedProductsQuery and is set to return a List<OrderedProduct> regardless, similarly to any ‘find all' query.

10. Putting Everything Together

We've fleshed out our core API with commands, events, and queries, and set up our Command and Query model by having an OrderAggregate and OrderedProducts model.

Next is to tie up the loose ends of our infrastructure. As we're using the axon-spring-boot-starter, this sets a lot of the required configuration automatically.

First, as we want to leverage Event Sourcing for our Aggregate, we'll need an EventStore. Axon Server which we have started up in step three will fill this hole.

Secondly, we need a mechanism to store our OrderedProduct query model. For this example, we can add h2 as an in-memory database and spring-boot-starter-data-jpa for ease of use:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
</dependency>

10.1. Setting up a REST Endpoint

Next, we need to be able to access our application, for which we'll be leveraging a REST endpoint by adding the spring-boot-starter-web dependency:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

From our REST endpoint, we can start dispatching commands and queries:

@RestController
public class OrderRestEndpoint {

    private final CommandGateway commandGateway;
    private final QueryGateway queryGateway;

    // Autowiring constructor and POST/GET endpoints
}

The CommandGateway is used as the mechanism to send our command messages, and the QueryGateway, in turn, to send query messages. The gateways provide a simpler, more straightforward API, compared to the CommandBus and QueryBus that they connect with.

From here on, our OrderRestEndpoint should have a POST endpoint to place, confirm, and ship an order:

@PostMapping("/ship-order")
public void shipOrder() {
    String orderId = UUID.randomUUID().toString();
    commandGateway.send(new PlaceOrderCommand(orderId, "Deluxe Chair"));
    commandGateway.send(new ConfirmOrderCommand(orderId));
    commandGateway.send(new ShipOrderCommand(orderId));
}

This rounds up the Command side of our CQRS application.

Now, all that's left is a GET endpoint to query all the OrderedProducts:

@GetMapping("/all-orders")
public List<OrderedProduct> findAllOrderedProducts() {
    return queryGateway.query(new FindAllOrderedProductsQuery(), 
      ResponseTypes.multipleInstancesOf(OrderedProduct.class)).join();
}

In the GET endpoint, we leverage the QueryGateway to dispatch a point-to-point query. In doing so, we create a default FindAllOrderedProductsQuery, but we also need to specify the expected return type.

As we expect multiple OrderedProduct instances to be returned, we leverage the static ResponseTypes#multipleInstancesOf(Class) function. With this, we have provided a basic entrance into the Query side of our Order service.

We completed the setup, so now we can send some commands and queries through our REST Controller once we've started up the OrderApplication.

POST-ing to endpoint /ship-order will instantiate an OrderAggregate that'll publish events, which, in turn, will save/update our OrderedProducts. GET-ing from the /all-orders endpoint will publish a query message that'll be handled by the OrderedProductsEventHandler, which will return all the existing OrderedProducts.

11. Conclusion

In this article, we introduced the Axon Framework as a powerful base for building an application leveraging the benefits of CQRS and Event Sourcing.

We implemented a simple Order service using the framework to show how such an application should be structured in practice.

Lastly, Axon Server posed as our Event Store and the message routing mechanism.

The implementation of all these examples and code snippets can be found over on GitHub.

For any additional questions you may have, also check out the Axon Framework User Group.

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