Publishing Events from Aggregate Roots

Entities managed by repositories are aggregate roots. In a Domain-Driven Design application, these aggregate roots usually publish domain events. Spring Data provides an annotation called @DomainEvents that you can use on a method of your aggregate root to make that publication as easy as possible, as shown in the following example:

Exposing domain events from an aggregate root

class AnAggregateRoot {

    @DomainEvents // (1)
    Collection<Object> domainEvents() {
        // … return events you want to get published here
    }

    @AfterDomainEventPublication // (2)
    void callbackMethod() {
       // … potentially clean up domain events list
    }
}

1. The method that uses `@DomainEvents` can return either a single event instance or a collection of events.
It must not take any arguments.
2. After all events have been published, we have a method annotated with `@AfterDomainEventPublication`.
You can use it to potentially clean the list of events to be published (among other uses).

The methods are called every time one of the following a Spring Data repository methods are called:

• save(…), saveAll(…)
• delete(…), deleteAll(…), deleteAllInBatch(…), deleteInBatch(…)

Note, that these methods take the aggregate root instances as arguments. This is why deleteById(…) is notably absent, as the implementations might choose to issue a query deleting the instance and thus we would never have access to the aggregate instance in the first place.