Event processing order

I have a doubt about the order in which the events will be handled/reduced.

Let’s say I send a command with CreateUser. That command will then register an event UserCreated that will be reduced by a User entity.

In my application, I want to be able to access that User that has just been created (via Booster.fetchEntitySnapshot), so I create a new event AfterUserCreation that will have User as a param. This new event will be fired by the UserCreated event handler:

@EventHandler(UserCreated)
export class HandleUserCreated {
    public static async handle(
        event: UserCreated,
        register: Register
    ): Promise<void> {

       //Here is where I'd perform Booster.fetchEntitySnapshot to get User

        register.events(new AfterUserCreation(
           //Here I'd pass the User resulting from the fetch
        ))
    }
}

My main concern here is whether the event handler will be able to fetch the User entity. I mean, WHEN does the event handler comes into action? does it wait until the Userentity reduces the first UserCreated? or is it launched “in parallel”?

[NickSeagull]

In the case of the AWS provider, the event handlers are subscribed to the change stream from the event store, which is DynamoDB. This is the same case for the reducers, so in your example, you could say that they both are launched in parallel.

[javiertoledo]

Oh, I see. Yes, this is a limitation that we should solve, in future versions, we should allow entities to reduce events that are not bound to them. I’ll make sure we have an issue to address that.