The domain events contain the event specific data, a timestamp, and identifiers that reference the domain entity.

The following snippet shows an example of an abstract MealEvent and the OrderPlaced event:

public abstract class MealEvent {

    private final Instant instant;

    protected MealEvent() {
        instant = Instant.now();
    }

    protected MealEvent(Instant instant) {
        Objects.requireNonNull(instant);
        this.instant = instant;
    }

    ...
}

public class OrderPlaced extends MealEvent {

    private final OrderInfo orderInfo;

    public OrderPlaced(OrderInfo orderInfo) {
        this.orderInfo = orderInfo;
    }

    public OrderPlaced(OrderInfo orderInfo, Instant instant) {
        super(instant);
        this.orderInfo = orderInfo;
    }

    ...
}

Domain events like these are the core of the application. The domain entity representations are calculated from these events.

The integration into Kafka ensures that these events are fired via CDI. They are observed in the corresponding functionality that updates the state representations, or invokes subsequent commands, respectively.