DefaultEventPublicationRegistry markCompleted method's REQUIRES_NEW transaction propagation means events are marked completed before handler transactions are committed.

https://github.com/spring-projects/spring-modulith/blob/c5f73e675d74c7a40d68957ca56be35d4d2c8ea3/spring-modulith-events/spring-modulith-events-core/src/main/java/org/springframework/modulith/events/core/DefaultEventPublicationRegistry.java#L104-L105

The markCompleted method of DefaultEventPublicationRegistry is invoked in a MethodInterceptor defined in CompletionRegisteringAdvisor, which intercepts the event handler method, invokes it, then calls the EventPublicationRegistry's markCompleted method to mark the eventPublication as complete.

Because of the REQUIRES_NEW transaction propagation type, markCompleted runs with a completely distinct database connection, and independent transaction from the event handler's transaction. This means there's a chance the event handler's transaction can commit, but the markCompleted transaction does not, which leaves the eventPublication appearing incomplete in the database even though it was completely successfully handled.

I propose that markCompleted should have the REQUIRED transaction propagation type. I believe this should cause markCompleted to commit the eventPublication as completed in the same transaction as the event handler.

Let me know what you think, I can submit a PR.