the Write Model<\/strong>: basically same thing with some differences. However, this is called an aggregate and annotated with @Aggregate annotation. This model interfaces with the data store through a repository that extends org.axonframework.modelling.command.Repository.<\/p>\nIn this demo, the write model is called Product as given below.<\/p>\n
@Aggregate<\/span>\r\npublic<\/span> class<\/span> Product<\/span> {<\/span>\r\n @AggregateIdentifier<\/span>\r\n private<\/span> String id;<\/span>\r\n private<\/span> Double price;<\/span>\r\n private<\/span> Integer stock;<\/span>\r\n private<\/span> String description;<\/span>\r\n\r\n public<\/span> Product<\/span>()<\/span> {<\/span>\r\n }<\/span>\r\n\r\n @CommandHandler<\/span>\r\n public<\/span> Product<\/span>(<\/span>AddProductCommand cmd){<\/span>\r\n apply(<\/span> new<\/span> ProductAddedEvent(<\/span>\r\n cmd.<\/span>getId<\/span>(),<\/span>\r\n cmd.<\/span>getPrice<\/span>(),<\/span>\r\n cmd.<\/span>getStock<\/span>(),<\/span>\r\n cmd.<\/span>getDescription<\/span>()<\/span>\r\n )<\/span>\r\n );<\/span>\r\n }<\/span>\r\n\r\n @CommandHandler<\/span>\r\n public<\/span> void<\/span> handle<\/span>(<\/span>UpdateStockCommand cmd){<\/span>\r\n if<\/span>(<\/span>this<\/span>.<\/span>stock<\/span> >=<\/span> cmd.<\/span>getStock<\/span>()){<\/span>\r\n apply(<\/span>new<\/span> StockUpdatedEvent(<\/span>\r\n cmd.<\/span>getId<\/span>(),<\/span> \r\n cmd.<\/span>getStock<\/span>())<\/span>\r\n );<\/span>\r\n }<\/span>\r\n else<\/span> {<\/span>\r\n throw<\/span> new<\/span> RuntimeException<\/span>(<\/span>\"Out of Stock!\"<\/span>);<\/span>\r\n }<\/span>\r\n }<\/span>\r\n\r\n @EventSourcingHandler<\/span>\r\n public<\/span> void<\/span> on<\/span>(<\/span>StockUpdatedEvent evt){<\/span>\r\n id =<\/span> evt.<\/span>getId<\/span>();<\/span>\r\n stock =<\/span> stock -<\/span> evt.<\/span>getStock<\/span>();<\/span>\r\n }<\/span>\r\n\r\n @EventSourcingHandler<\/span>\r\n public<\/span> void<\/span> on<\/span>(<\/span>ProductAddedEvent evt){<\/span>\r\n id =<\/span> evt.<\/span>getId<\/span>();<\/span>\r\n price =<\/span> evt.<\/span>getPrice<\/span>();<\/span>\r\n stock =<\/span> evt.<\/span>getStock<\/span>();<\/span>\r\n description =<\/span> evt.<\/span>getDescription<\/span>();<\/span>\r\n }<\/span>\r\n}<\/span>\r\n<\/pre>\n <\/p>\n
3. The Commands and Events<\/strong><\/h5>\nIn CQRS terms, a command is an expression of intent to perform certain operation while an event is a notification that an operation has occurred. So in our Ecommerce demo application, what commands can you think of? How about this:<\/p>\n
CreateOrder<\/strong>: that is to place a new order for a product<\/p>\nUpdateStock<\/strong>: deduct the number of product from the stock when an order is placed<\/p>\nAddProduct<\/strong>: add a new product to the stock<\/p>\nNote that, for each command, there is a corresponding event.\u00a0 For this demo, I have placed the commands and events inside the api package. The three commands are placed inside\u00a0 the commands.kt kotlin file while the events are placed inside the events.kt file. This is shown below:<\/p>\n
data class<\/span> CreateOrderCommand<\/span> (\r\n @TargetAggregateIdentifier\r\n val<\/span> orderId: String,\r\n val<\/span> price: Double,\r\n val<\/span> number:Int,\r\n val<\/span> productId:String\r\n)\r\n\r\ndata class<\/span> AddProductCommand<\/span> (\r\n @TargetAggregateIdentifier\r\n val<\/span> id: String,\r\n val<\/span> price: Double,\r\n val<\/span> stock:Int,\r\n val<\/span> description: String\r\n)\r\n\r\ndata class<\/span> UpdateStockCommand<\/span>(\r\n @TargetAggregateIdentifier\r\n val<\/span> id: String,\r\n val<\/span> stock: Int\r\n)\r\n<\/pre>\n <\/p>\n
The events are given below:<\/p>\n
data class<\/span> OrderCreatedEvent<\/span> (\r\n val<\/span> orderId: String,\r\n val<\/span> price: Double,\r\n val<\/span> number:Int,\r\n val<\/span> productId:String\r\n)\r\n\r\ndata class<\/span> ProductAddedEvent<\/span> (\r\n val<\/span> id: String,\r\n val<\/span> price: Double,\r\n val<\/span> stock:Int,\r\n val<\/span> description: String\r\n)\r\n\r\ndata class<\/span> StockUpdatedEvent<\/span>(\r\n val<\/span> id:String,\r\n val<\/span> stock: Int\r\n)\r\n<\/pre>\n <\/p>\n
What’s Left?<\/strong><\/p>\nSo next we would have to define the event handlers. Then we also create the controller file that receives request from the client. This is covered in the next part!<\/p>\n