3. Build the Core\u00a0 API (an event-driven API)<\/strong><\/h5>\nIn this part, we are going to build the core api which is made up of commands, events and queries.<\/p>\n
Commands:<\/strong> Create a kotlin file named commands in the commands package. Write the data class for CreateFoodCartCommand<\/em>,\u00a0 SelectProductCommand<\/em>, DeselectProductCommand<\/em> and ConfirmOrderCommand<\/em><\/p>\nThe codes for these four commands is given below:
\n<\/p>\n
data class<\/span> CreateFoodCartCommand<\/span>(\r\n @TargetAggregateIdentifier\r\n val<\/span> foodCartId: UUID\r\n)\r\n\r\ndata class<\/span> SelectProductCommand<\/span>(\r\n @TargetAggregateIdentifier\r\n val<\/span> foodCartId: UUID,\r\n val<\/span> productId: UUID,\r\n val<\/span> quantity: Int\r\n)\r\n\r\ndata class<\/span> DeselectProductCommand<\/span>(\r\n @TargetAggregateIdentifier\r\n val<\/span> foodCartId: UUID,\r\n val<\/span> productId: UUID,\r\n val<\/span> quantity: Int\r\n)\r\n\r\ndata class<\/span> ConfirmOrderCommand<\/span>(\r\n @TargetAggregateIdentifier\r\n val<\/span> foodCartId: UUID\r\n)\r\n<\/pre>\nThe val<\/em> specification ensures that the fields are final fields.<\/p>\ndata class<\/em> provide equal and hash code and ToString() methods<\/p>\n <\/p>\n
Events:<\/strong> Create a kotlin file named events in the coreapi package.<\/p>\nWrite the data classes for the events corresponding to each command you wrote previously. This are shown in the code below:<\/p>\n
data class<\/span> FoodCardCreatedEvent<\/span>(\r\n val<\/span> foodCardId: UUID\r\n)\r\n\r\ndata class<\/span> ProductSelectedEvent<\/span>(\r\n val<\/span> foodCardId: UUID,\r\n val<\/span> productId: UUID,\r\n val<\/span> quantity: Int\r\n)\r\n\r\ndata class<\/span> ProductDeselectedEvent<\/span>(\r\n val<\/span> foodCardId: UUID,\r\n val<\/span> productId: UUID,\r\n val<\/span> quantity: Int\r\n)\r\n\r\ndata class<\/span> OrderConfirmedEvent<\/span>(\r\n val<\/span> foodCardId: UUID\r\n)\r\n<\/pre>\nNote that for the events, an id is specified for the foodCardCreatedEvent. This is because, an event indicated that the foodCart has been created and therefore exists.<\/p>\n
<\/p>\n
Queries:<\/strong> Create a kotlin class named queries. In it, write a data class for FindFoodCartQuery and another class for RetrieveProductOptions. This is shown below<\/p>\ndata class<\/span> FindFoodCartQuery<\/span>(\r\n val<\/span> foodCartId: UUID\r\n)\r\n\r\n\/\/Which products can be selected<\/span>\r\nclass<\/span> RetrieveProductOptionsQuery<\/span>\r\n<\/pre>\n <\/p>\n
4. Develop the Command Model (Build the Aggregate)<\/strong><\/h5>\nBuilding the Aggregate:<\/strong> You need to create the FoodCart aggregate class in the commands package, create an empty constructor. Then annotate this class with the @Aggregate annotation.<\/p>\nThe @Aggregate annotation serves the following purpose:<\/p>\n
\n- makes the framework creates an aggregate factory for us<\/li>\n
- make the framework know that this class would contain commandhandler that would be subscribed to<\/li>\n
- that this class would publish events and therefore provides the repository<\/li>\n<\/ul>\n
This aggregate should have two fields: the foodCardId and the selectedProducts<\/p>\n
Handling Commands<\/strong>: Then you need to create a command handler for the constructor for FoodCart. Next, create two more command handlers for SelectProduct and DeselectProduct commands<\/p>\nThe three commandhandlers are shown below with an empty constructor as well<\/p>\n
public<\/span> FoodCart<\/span>(){<\/span>\r\n\r\n}<\/span>\r\n\r\n@CommandHandler<\/span>\r\npublic<\/span> FoodCart<\/span>(<\/span>CreateFoodCartCommand cmd){<\/span>\r\n apply(<\/span>new<\/span> FoodCartCreatedEvent(<\/span>cmd.<\/span>getFoodCartId<\/span>()));<\/span>\r\n}<\/span>\r\n\r\n@CommandHandler<\/span>\r\npublic<\/span> void<\/span> handle<\/span>(<\/span>SelectProductCommand cmd){<\/span>\r\n apply(<\/span>new<\/span> ProductSelectedEvent(<\/span>\r\n foodCartId,<\/span>\r\n cmd.<\/span>getProductId<\/span>(),<\/span>\r\n cmd.<\/span>getQuantity<\/span>()<\/span>\r\n ));<\/span>\r\n}<\/span>\r\n\r\n@CommandHandler<\/span>\r\npublic<\/span> void<\/span> handle<\/span>(<\/span>DeselectProductCommand cmd)<\/span>\r\n throws<\/span> ProductDeselectedException {<\/span>\r\n UUID productId =<\/span> cmd.<\/span>getProductId<\/span>();<\/span>\r\n if<\/span>(!<\/span>selectedProducts.<\/span>containsKey<\/span>(<\/span>productId)){<\/span>\r\n throw<\/span> new<\/span> ProductDeselectedException<\/span>();<\/span>\r\n }<\/span>\r\n apply(<\/span>new<\/span> ProductDeselectedEvent(<\/span>\r\n foodCartId,<\/span>\r\n cmd.<\/span>getProductId<\/span>(),<\/span>\r\n cmd.<\/span>getQuantity<\/span>()<\/span>\r\n ));<\/span>\r\n}<\/span>\r\n<\/pre>\nThe @CommandHandler annotation tells the framework that this is a commandhandling function. So the method is registered to the command bus as capable of handling a particular command.<\/p>\n
<\/p>\n
Event Sourcing the Aggregate:<\/strong> Here, you need to write the event sourcing handlers for FoodcartCreated, ProductSelected and ProductDeselected events.<\/p>\nThe methods are shown below<\/p>\n
@EventSourcingHandler\r\npublic<\/span> void<\/span> on<\/span>(FoodCardCreatedEvent evt){\r\n foodCardId = evt.getFoodCardId();\r\n \/\/Instantiate the selected product<\/span>\r\n \/\/once the foodcart is created<\/span>\r\n selectedProducts = new<\/span> HashMap<>();\r\n}\r\n\r\n@EventSourcingHandler\r\npublic<\/span> void<\/span> on<\/span>(ProductSelectedEvent evt){\r\n selectedProducts.merge(evt.getProductId(), evt.getQuantity(), Integer::sum);\r\n}\r\n\/\/The merge() function is used to combine<\/span>\r\n\/\/ multiple mapped values for a key<\/span>\r\n\/\/ using the mapping function<\/span>\r\n<\/pre>\n <\/p>\n
Validate the DeselectProduct command<\/strong><\/p>\nYou need to modify the DeleteProduct command to make sure the product being deselected actually exists. To do this, you need to check if the hashmap actually contains the given key.<\/p>\n
I also choose to create an exceptions kotlin file to place the ProductDeselectionException<\/p>\n
Now the modified DeselectProductCommand is shown below:<\/p>\n
@CommandHandler\r\npublic<\/span> void<\/span> handle<\/span>(DeselectProductCommand cmd)\r\n throws ProductDeselectedException {\r\n UUID productId = cmd.getProductId();\r\n if<\/span>(!selectedProducts.containsKey(productId)){\r\n throw<\/span> new<\/span> ProductDeselectedException<\/span>(); \r\n }\r\n\/\/ apply() omitted<\/span>\r\n}\r\n<\/pre>\nNote<\/strong>: You need to create the ProductDeletedException() class in the coreapi package. It should inherit Exception (see video)<\/p>\n <\/p>\n
5. Develop the Query Model<\/strong><\/h5>\nWe would now build the query model, to handle events and queries.<\/p>\n
Create a file in the query package and name it FoodCartView.<\/p>\n
Create class with same name and annotate this class with @Entity annotation. This class should contain two fields:<\/p>\n
foodCartId and products<\/p>\n
Inside this class, create an interface FoodCartViewRepository that inherits JpaRepository. The code is given below:<\/p>\n
@Entity\r\ndata class<\/span> FoodCartView<\/span>(\r\n @Id\r\n val<\/span> foodCartId: UUID,\r\n @ElementCollection\r\n val<\/span> products: Map<UUID, Int>\r\n)\r\n\r\ninterface<\/span> FoodCartViewRepository:JpaRepository<FoodCartView, UUID>\r\n<\/pre>\n <\/p>\n
We need a component to handle the events and queries.<\/p>\n
So create a java class in the query package and call it FoodCartProjector. This would be in charge of projecting the FoodCartView as well as updating the FoodCardView.<\/p>\n
Annotate this class with @Component annotation<\/p>\n
In this class, create the repository variable and then the constructor.<\/p>\n
Create an EventHandler for FoodCartCreated event. This would simply create a new foodCart and persist it using the repository save() method.<\/p>\n
Create a QueryHandler to return find a FoodCart by Id<\/p>\n
The content of the FoodCartProjector is given below<\/p>\n
private<\/span> final FoodCartViewRepository repository;\r\n\r\npublic<\/span> FoodCartProjector<\/span>(FoodCartViewRepository repository) {\r\n this<\/span>.repository = repository;\r\n}\r\n\r\n@EventHandler\r\npublic<\/span> void<\/span> on<\/span>(FoodCartCreatedEvent evt){\r\n FoodCartView foodCartView = new<\/span> FoodCartView(\r\n evt.getFoodCartId(), \r\n Collections.emptyMap()\r\n );\r\n}\r\n\r\n@QueryHandler\r\npublic<\/span> FoodCartView handle<\/span>(FindFoodCartQuery query) {\r\n return<\/span> repository.findById(\r\n query.getFoodCartId())\r\n .orElse(null<\/span>);\r\n}\r\n<\/pre>\n <\/p>\n
6. Connection to the UI<\/strong><\/h5>\nHere we would introduce a component that we can invoke with regular REST operation. This UI component would us CommandGateway to dispatch command and query messages to the Framework<\/p>\n
Create a new package called the gui.<\/p>\n
In this\u00a0 FoodOrderingController in the gui package and annotate with with @RestController<\/p>\n
The content of the FoodOrderingController is given below:<\/p>\n
private<\/span> final CommandGateway commandGateway;\r\nprivate<\/span> final QueryGateway queryGateway;\r\n\r\npublic<\/span> FoodOrderingController<\/span>(CommandGateway commandGateway, QueryGateway queryGateway) {\r\n this<\/span>.commandGateway = commandGateway;\r\n this<\/span>.queryGateway = queryGateway;\r\n}\r\n\r\n@PostMapping(\"\/create\"<\/span>)\r\npublic<\/span> void<\/span> handle<\/span>(){\r\n commandGateway.send(new<\/span> CreateFoodCartCommand());\r\n}\r\n\r\n@GetMapping(\"\/footcart\/{foodCartId\"<\/span>)\r\npublic<\/span> CompletableFuture<FoodCartView> handle(@PathVariable(\"foodCartId\"<\/span>) String foodCartId){\r\n return<\/span> queryGateway.query(new<\/span> FindFoodCartQuery(UUID.fromString(foodCartId)),\r\n ResponseTypes.instanceOf(FoodCartView.class));\r\n}\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"This is a complete tutorial on CQRS and Event Sourcing based application using Axon Framework(Food Cart). For best learning experience, this step by step guide … <\/p>\n","protected":false},"author":1,"featured_media":179,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[12,27],"tags":[26,55,6],"class_list":["post-174","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-axon","category-axonframework","tag-axon-framework","tag-coding","tag-cqrs"],"_links":{"self":[{"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/posts\/174","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/comments?post=174"}],"version-history":[{"count":9,"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/posts\/174\/revisions"}],"predecessor-version":[{"id":240,"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/posts\/174\/revisions\/240"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/media\/179"}],"wp:attachment":[{"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/media?parent=174"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/categories?post=174"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.kindsonthegenius.com\/microservices\/wp-json\/wp\/v2\/tags?post=174"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
![\"Structure](\"https:\/\/www.kindsonthegenius.com\/microservices\/wp-content\/uploads\/sites\/18\/2020\/02\/Structure-of-a-CQRS-based-application-with-Axon-Framework.jpg\")
<\/a>