{"id":77,"date":"2019-11-14T21:27:09","date_gmt":"2019-11-14T21:27:09","guid":{"rendered":"https:\/\/www.kindsonthegenius.com\/microservices\/?p=77"},"modified":"2019-11-14T21:27:09","modified_gmt":"2019-11-14T21:27:09","slug":"cqrs-with-axon-tutorial-part-3-setup-the-commands-events-and-aggregate","status":"publish","type":"post","link":"https:\/\/www.kindsonthegenius.com\/microservices\/cqrs-with-axon-tutorial-part-3-setup-the-commands-events-and-aggregate\/","title":{"rendered":"CQRS With Axon Tutorial: Part 3 – Setup the Commands, Events and Aggregate"},"content":{"rendered":"

In this part, we are going to be setting up the Commands, Events and Aggregate. We also would set up the command and event handlers.We would cover the following<\/p>\n

    \n
  1. Setup the Commands and Events Classes<\/a><\/li>\n
  2. Setup the Aggregate<\/a><\/li>\n
  3. Add the CommandHandlers<\/a><\/li>\n
  4. Add the EventSourcingHandlers<\/a><\/li>\n<\/ol>\n

     <\/p>\n

    1. Write the Command and Events<\/strong><\/h5>\n

    Remember that we chose Kotlin when we setup our project. The reason is that Kotlin provides a simple way of writing classes in just one line. Besides, several of such classes can be placed in a single kotlin file. Such classes are called data classes<\/p>\n

    Follow the steps below:<\/p>\n

    Step 1:<\/strong> Add a kotlin file in the command package. Name it api.kt. This would contain our data classes<\/p>\n

    Step 2:<\/strong> Write the classes for IssueCommand, IssuedEvent, RedeemCommand and RedeemedEvent. The Content of the file will be as shown below:<\/p>\n

    <\/p>\n

    import org.axonframework.modelling.command.TargetAggregateIdentifier\r\n\r\ndata class<\/span> IssueCommand<\/span>(@TargetAggregateIdentifier val<\/span> id: String, val<\/span> amount: Int);\r\ndata class<\/span> IssuedEvent<\/span>(val<\/span> id: String, val<\/span> amount: Int);\r\ndata class<\/span> RedeemCommand<\/span>(@TargetAggregateIdentifier val<\/span> id: String, val<\/span> amount: Int);\r\ndata class<\/span> RedeemedEvent<\/span>(val<\/span> id: String, val<\/span> amount: Int);\r\n<\/pre>\n
    Note the @TargetAggregateIdentifier annotation. This enables targeting the particular aggregate when a command is issued<\/p>\n
     <\/p>\n
    2. Setup the Aggregate<\/strong><\/h5>\n
    An aggregate is “a logical group of entities that is treated as a single unit”. In this case, our aggregate would be a gift card.<\/p>\n
    To set up the aggregate:<\/p>\n
    Step 1<\/strong>: Create a java file in the command package. Name it GiftCard<\/p>\n
    Step 2:<\/strong> Annotate the class with the @Aggregate annotation<\/p>\n
    Step 3:<\/strong> Create a private member variable id. It would be a String.<\/p>\n
    Step 4:<\/strong> Annotate this variable with @AggregateIdentifier annotation<\/p>\n
     <\/p>\n
    3. Add the CommandHandlers<\/strong><\/h5>\n
    CommandHandler is a function that specifies what happen when a command is executed. CommandHandlers are normally void functions with the name handle() and the take as parameter the command they are going to respond to.<\/p>\n
    Now we would add two command handlers for IssueCommand and RedeemCommand.<\/p>\n
    Step 1:<\/strong> Simply copy and paste the two event handlers after private variable id. Also note that I have annotated the code to explain what each part means.<\/p>\n
       @CommandHandler<\/span>\r\n    public<\/span> GiftCard<\/span>(<\/span>IssueCommand cmd)<\/span> {<\/span>\r\n        if<\/span>(<\/span>cmd.<\/span>getAmount<\/span>()<=<\/span>0<\/span>)<\/span>\r\n            throw<\/span> new<\/span> IllegalArgumentException<\/span>(<\/span>\"amount <= 0\"<\/span>);<\/span>\r\n        \r\n        AggregateLifecycle.<\/span>apply<\/span>(<\/span>new<\/span> IssuedEvent(<\/span>cmd.<\/span>getId<\/span>(),<\/span> cmd.<\/span>getAmount<\/span>()));<\/span>\r\n    }<\/span>\r\n\r\n    @CommandHandler<\/span>\r\n    public<\/span> void<\/span> handle<\/span>(<\/span>RedeemCommand cmd)<\/span> {<\/span>\r\n        if<\/span>(<\/span>cmd.<\/span>getAmount<\/span>()<=<\/span>0<\/span>)<\/span>\r\n            throw<\/span> new<\/span> IllegalArgumentException<\/span>(<\/span>\"amount <= 0\"<\/span>);<\/span>\r\n        \r\n        if<\/span>(<\/span>cmd.<\/span>getAmount<\/span>()<\/span> ><\/span> balance)<\/span>\r\n            throw<\/span> new<\/span> IllegalArgumentException<\/span>(<\/span>\"amount > balance\"<\/span>);<\/span>\r\n        \r\n        AggregateLifecycle.<\/span>apply<\/span>(<\/span>new<\/span> RedeemedEvent(<\/span>cmd.<\/span>getId<\/span>(),<\/span> cmd.<\/span>getAmount<\/span>()));<\/span>\r\n    }<\/span>\r\n<\/pre>\n
     <\/p>\n
    Step 2:<\/strong> Create a private Integer field, balance in the GiftCard<\/p>\n
    Step 3:<\/strong> Create also an empty private constructor.<\/p>\n
     <\/p>\n
    4. Add the EventSourcingHandlers<\/strong><\/h5>\n
    An EventSourcingHandler is an event executed when an event occurs. They are normally function with name on(). In this case we have two event sourcing handlers.<\/p>\n
    Step 1:<\/strong> Copy and Paste the code after the command handlers<\/p>\n
        @EventSourcingHandler<\/span>\r\n    public<\/span> void<\/span> on<\/span>(<\/span>IssuedEvent evt){<\/span>\r\n        \/\/for a new card, the amount is the balance<\/span>\r\n        id =<\/span> evt.<\/span>getId<\/span>();<\/span>\r\n        balance =<\/span> evt.<\/span>getAmount<\/span>();<\/span>\r\n    }<\/span>\r\n\r\n    @EventSourcingHandler<\/span>\r\n    public<\/span> void<\/span> on<\/span>(<\/span>RedeemedEvent evt)<\/span> {<\/span>\r\n        \/\/update the balance when a card is redeemed<\/span>\r\n        balance =<\/span> balance -<\/span> evt.<\/span>getAmount<\/span>();<\/span>\r\n    }<\/span>\r\n<\/pre>\n
     <\/p>\n
    So we’ve completed writing the handlers. I would recommend you build the project. Just to make sure everything is fine.<\/p>\n
     <\/p>\n","protected":false},"excerpt":{"rendered":"
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