How create a Aggregate with eventBus?

Architectural Concepts
1

Hello,

I have a microservice A. When I create my aggregate A, I would like to create another aggregate B in microservice B. It’s possible with the eventBus ?

Microservice A

public class AggregateA {

    @AggregateIdentifier
    private UUID id;
    private UUID bId
    private String titre;
    private String description;
    private Date debut;
    private Date fin;

    @CommandHandler
    public AggregateA(CreateACommand command){
        apply(new ACreatedEvent(
                command.getId(),
                command.getTitre(),
        ));
    }

    @EventSourcingHandler
    public void on(ACreatedEvent event){
        this.id = event.getId();
        this.titre = event.getTitre();
      
    }

    @EventSourcingHandler
    public void on(BCreatedEvent event){
        this.idB= event.getB();
    }

Microservice B

public class BAggregate {

    @AggregateIdentifier
    private UUID id;
    private UUID aId;
    private String titre;
    private Date debut;
    private Date fin;
   
    @EventSourcingHandler
    public void on(ACreatedEvent event) {
        this.operationId = event.getId();
        this.id = UUID.randomUUID();
        this.titre = event.getTitre();
        apply(new BCreatedEvent(operationId, id));
    }


}

Thanks

2

Hi @Vincent62,

An Event can have multiple handlers each working performing their domain-related updates logic. An EventBus should be able to able to route events.

3

Hi, thank you for the answer

The example above, don’t work
The ACreatedEvent in the microservice B is never called
Do you have an idea knowing that each microservice must have a reference with the other ?

4

Hey @Vincent62,

Does this one not work for you?

Maybe what you can do is perform an @EventHandler (not @EventSourcingHandler) and then fire a command based on the desired event. If synchronicity is a concern I would not recommend this approach

Handling the event source directly on BAggregate will not work since they are distinct aggregates.

5

Hi,

Does this one not work for you?

I think it works when it’s in the same microservice

If i use @EventHandler to fire a command, a new aggregate is created when i restart the microservice. So I have several aggregate B

B service

    @EventHandler
    public void on(ACreatedEvent event) {
        commandGateway.send(new CreateBCommand(
                UUID.randomUUID(),
                event.getId(),
                event.getTitre()
        ));
    }

BAggregate

    @CommandHandler
    public BAggregate(CreateBCommand command) {
        apply(new BCreatedEvent(
                command.getId(),
                command.getOperationId(),
                command.getTitre()
        ));

    }

    @EventSourcingHandler
    public void on( BCreatedEvent event) {
        this.id = event.getId();
        this.AId = event.getAId();
        this.titre = event.getTitre();
        log.info("A is created {}",id);

    }

The synchronicity is not a concern. I would just create an other aggragate and link the two with id

6

Is this question somehow related to that one?

7

No. The other question, is to link two Aggregate in the same Microservice

Now, i would like create two aggregates ( in different microservices) and exchange the aggregate id
It’s possible to send two http request, but I thought it’s more attractive using the eventBus when the first aggregate is created

8

The Aggregate provided by Axon Framework is for a Command Model. The aggregate is essentially a component, to begin with. You can read more about it here.

In an event sourcing model, the @EventSourcingHandler in the aggregate handles the events originating from it. In your use case, add the @EventHandler annotation in a @Component but not an @Aggregate.

Hope this helps!

9

Hi,
For my use case, I just used the SAGA pattern and it’s work perfectly
Thank you very much for your help

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