Is it legal to produce events of the same persistent entity to more than one topic at the same time

Lagom
1

Suppose I want to split events of some persistent entity (e.g. FooEntity) to produce messages to different topics. Some events should go to topic A and others should go to B. Also consumers of both of these topics may be “interested” in the same event but the message format may differ. Is it legal to do it such a way in the service implementation?

@Override
public Topic<A> topicA() {
    return TopicProducer.taggedStreamWithOffset(FooEntity.SHARDED_TAG.allTags(),
        (tag, offset) -> registry
            .eventStream(tag, offset)
            .filter(this::filterEventsOnlyForTopicA) // may include events for B
            .mapAsync(1, pair -> completedFuture(Pair.create(convertToA(pair.first()), pair.second()))));
}

@Override
public Topic<B> topicB() {
    return TopicProducer.taggedStreamWithOffset(FooEntity.SHARDED_TAG.allTags(),
        (tag, offset) -> registry
            .eventStream(tag, offset)
            .filter(this::filterEventsOnlyForTopicB) // may include events for A
            .mapAsync(1, pair -> completedFuture(Pair.create(convertToB(pair.first()), pair.second()))));
}
2

This is perfectly fine.
If this would be ilegal then it would NOT be possible to declare more then one topics for publishing. Taking in mind that is recommended to use one persistant entity per service.

3

Thank you Alan! I was confused with the “possible concurrent” producing only the message a(X) for the event X by the only one topic A implementation istead of producing messages a(X) and b(x) for the event X