Un guide du framework Axon

1. Aperçu

Dans cet article, nous examinerons Axon et comment il nous aide à implémenter des applications avec CQRS (Command Query Responsibility Segregation) et Event Sourcing à l'esprit.

Au cours de ce guide, Axon Framework et Axon Server seront utilisés. Le premier contiendra notre implémentation et le second sera notre solution dédiée de magasin d'événements et de routage de messages.

L'exemple d'application que nous allons créer se concentre sur un domaine Order . Pour cela, nous utiliserons les blocs de construction CQRS et Event Sourcing que Axon nous fournit .

Notez que de nombreux concepts partagés proviennent directement de DDD, ce qui dépasse le cadre de cet article actuel.

2. Dépendances de Maven

Nous allons créer une application Axon / Spring Boot. Par conséquent, nous devons ajouter la dernière dépendance axon-spring-boot-starter à notre pom.xml , ainsi que la dépendance axon-test pour les tests:

 org.axonframework axon-spring-boot-starter 4.1.2   org.axonframework axon-test 4.1.2 test 

3. Serveur Axon

Nous utiliserons Axon Server comme notre magasin d'événements et notre solution dédiée de routage de commandes, d'événements et de requêtes.

En tant que magasin d'événements, il nous donne les caractéristiques idéales requises lors du stockage d'événements. Cet article explique pourquoi cela est souhaitable.

En tant que solution de routage de messages, elle nous donne la possibilité de connecter plusieurs instances ensemble sans nous concentrer sur la configuration d'éléments tels qu'un RabbitMQ ou un sujet Kafka pour partager et envoyer des messages.

Axon Server peut être téléchargé ici. S'agissant d'un simple fichier JAR, l'opération suivante suffit pour le démarrer:

java -jar axonserver.jar

Cela démarrera une seule instance Axon Server accessible via localhost: 8024 . Le point de terminaison fournit une vue d'ensemble des applications connectées et des messages qu'elles peuvent traiter, ainsi qu'un mécanisme d'interrogation vers le magasin d'événements contenu dans Axon Server.

La configuration par défaut d'Axon Server ainsi que la dépendance axon-spring-boot-starter garantira que notre service de commande s'y connectera automatiquement.

4. API Order Service - Commandes

Nous mettrons en place notre service de commande en pensant au CQRS. Par conséquent, nous mettrons l'accent sur les messages qui circulent dans notre application.

Tout d'abord, nous allons définir les commandes, c'est-à-dire les expressions d'intention. Le service de commande est capable de gérer trois types d'actions différents:

  1. Passer une nouvelle commande
  2. Confirmer une commande
  3. Expédition d'une commande

Naturellement, il y aura trois messages de commande que notre domaine peut traiter - PlaceOrderCommand , ConfirmOrderCommand et 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 }

L' annotation TargetAggregateIdentifier indique à Axon que le champ annoté est un identifiant d'un agrégat donné vers lequel la commande doit être ciblée. Nous aborderons brièvement les agrégats plus loin dans cet article.

Notez également que nous avons marqué les champs dans les commandes comme définitifs. Ceci est intentionnel, car il est recommandé que toute implémentation de message soit immuable .

5. API Order Service - Événements

Notre agrégat gérera les commandes , car il est chargé de décider si une commande peut être passée, confirmée ou expédiée.

Il notifiera au reste de l'application sa décision en publiant un événement. Nous aurons trois types d'événements: OrderPlacedEvent, OrderConfirmedEvent et 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. Le modèle de commande - Agrégat de commande

Maintenant que nous avons modélisé notre API principale en ce qui concerne les commandes et les événements, nous pouvons commencer à créer le modèle de commande.

Comme notre domaine se concentre sur le traitement des commandes, nous créerons un OrderAggregate au centre de notre modèle de commande.

6.1. Classe agrégée

Ainsi, créons notre classe d'agrégat de base:

@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() { } }

L' annotation Aggregate est une annotation spécifique à Axon Spring marquant cette classe comme un agrégat. Il notifiera au framework que les blocs de construction spécifiques CQRS et Event Sourcing doivent être instanciés pour cet OrderAggregate .

Comme un agrégat gérera les commandes ciblées pour une instance d'agrégat spécifique, nous devons spécifier l'identifiant avec l' annotation AggregateIdentifier .

Notre agrégat commencera son cycle de vie lors de la gestion de PlaceOrderCommand dans le 'constructeur de gestion de commandes' OrderAggregate . Pour indiquer au framework que la fonction donnée est capable de gérer les commandes, nous ajouterons l' annotation CommandHandler .

Lors de la gestion de PlaceOrderCommand , il informera le reste de l'application qu'une commande a été passée en publiant le OrderPlacedEvent. Pour publier un événement à partir d'un agrégat, nous utiliserons AggregateLifecycle # apply (Object…) .

À partir de là, nous pouvons réellement commencer à incorporer Event Sourcing comme force motrice pour recréer une instance agrégée à partir de son flux d'événements.

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 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 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 handle(FindAllOrderedProductsQuery query) { return new ArrayList(orderedProducts.values()); }

The QueryHandler annotated function will handle the FindAllOrderedProductsQuery and is set to return a List 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:

 org.springframework.boot spring-boot-starter-data-jpa com.h2database h2 runtime 

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:

 org.springframework.boot spring-boot-starter-web 

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 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.