Una guía para el marco Axon

1. Resumen

En este artículo, analizaremos Axon y cómo nos ayuda a implementar aplicaciones con CQRS (Segregación de responsabilidad de consultas de comandos) y Event Sourcing en mente.

Durante esta guía, se utilizarán tanto Axon Framework como Axon Server. El primero contendrá nuestra implementación y el segundo será nuestra solución de enrutamiento de mensajes y tienda de eventos dedicada.

La aplicación de muestra que crearemos se centra en un dominio de pedidos . Para ello, aprovecharemos los componentes básicos de CQRS y Event Sourcing que nos proporciona Axon .

Tenga en cuenta que muchos de los conceptos compartidos provienen directamente de DDD, lo que está más allá del alcance de este artículo actual.

2. Dependencias de Maven

Crearemos una aplicación Axon / Spring Boot. Por lo tanto, necesitamos agregar la última dependencia axon-spring-boot-starter a nuestro pom.xml , así como la dependencia axon-test para las pruebas:

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

3. Servidor Axon

Usaremos Axon Server como nuestro almacén de eventos y nuestra solución dedicada de enrutamiento de comandos, eventos y consultas.

Como Tienda de Eventos nos brinda las características ideales que se requieren a la hora de almacenar eventos. Este artículo proporciona antecedentes por qué esto es deseable.

Como solución de enrutamiento de mensajes, nos brinda la opción de conectar varias instancias juntas sin enfocarnos en configurar cosas como RabbitMQ o un tema de Kafka para compartir y enviar mensajes.

Axon Server se puede descargar aquí. Como es un simple archivo JAR, la siguiente operación es suficiente para iniciarlo:

java -jar axonserver.jar

Esto iniciará una única instancia de Axon Server a la que se puede acceder a través de localhost: 8024 . El punto final proporciona una descripción general de las aplicaciones conectadas y los mensajes que pueden manejar, así como un mecanismo de consulta hacia el almacén de eventos contenido en Axon Server.

La configuración predeterminada de Axon Server junto con la dependencia axon-spring-boot-starter garantizará que nuestro servicio de pedidos se conecte automáticamente a él.

4. API de servicio de pedidos: comandos

Configuraremos nuestro servicio de pedidos con CQRS en mente. Por lo tanto, enfatizaremos los mensajes que fluyen a través de nuestra aplicación.

Primero, definiremos los Comandos, es decir, las expresiones de intención. El servicio de pedidos es capaz de gestionar tres tipos de acciones diferentes:

  1. Realizar un nuevo pedido
  2. Confirmando un pedido
  3. Envío de un pedido

Naturalmente, habrá tres mensajes de comando con los que nuestro dominio puede tratar: PlaceOrderCommand , ConfirmOrderCommand y 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 }

La anotación TargetAggregateIdentifier le dice a Axon que el campo anotado es una identificación de un agregado dado al que debe dirigirse el comando. Tocaremos brevemente los agregados más adelante en este artículo.

Además, tenga en cuenta que marcamos los campos en los comandos como finales. Esto es intencional, ya que es una práctica recomendada que la implementación de cualquier mensaje sea inmutable .

5. API de servicio de pedidos: eventos

Nuestro agregado manejará los comandos , ya que está a cargo de decidir si un Pedido se puede realizar, confirmar o enviar.

Notificará al resto de la aplicación de su decisión publicando un evento. Tendremos tres tipos de eventos: OrderPlacedEvent, OrderConfirmedEvent y 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. El modelo de comando: orden agregado

Ahora que hemos modelado nuestra API principal con respecto a los comandos y eventos, podemos comenzar a crear el Modelo de comandos.

Como nuestro dominio se enfoca en tratar con Órdenes, crearemos un OrderAggregate como el centro de nuestro Modelo de Orden.

6.1. Clase agregada

Por lo tanto, creemos nuestra clase agregada básica:

@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() { } }

La anotación agregada es una anotación específica de Axon Spring que marca esta clase como agregada. Notificará al marco que los bloques de construcción específicos CQRS y Event Sourcing necesarios deben instanciarse para este OrderAggregate .

Como un agregado manejará comandos que están destinados a una instancia agregada específica, necesitamos especificar el identificador con la anotación AggregateIdentifier .

Nuestro agregado comenzará su ciclo de vida al manejar PlaceOrderCommand en el 'constructor de manejo de comandos' de OrderAggregate . Para decirle al marco que la función dada es capaz de manejar comandos, agregaremos la anotación CommandHandler .

Al manejar PlaceOrderCommand , notificará al resto de la aplicación que se realizó un pedido mediante la publicación de OrderPlacedEvent. Para publicar un evento desde dentro de un agregado, usaremos AggregateLifecycle # apply (Object…) .

A partir de este punto, podemos comenzar a incorporar Event Sourcing como la fuerza impulsora para recrear una instancia agregada de su flujo de eventos.

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.