Domain events for local consumption in DDD

Question

I'm familiarizing myself DDD lately and trying to get hold of the key concepts and I have a query incase of publishing domain events for the local subscribers, so can I assume that the event publisher, takes care of both publishing to remote subscribers thro AQMP while also leveraging observable to publish it towards the local subscribers, is this a scalable solution? or is there a familiar pattern to handle this?(Also advise if there is a reactive solution to the prblm, may be RxJava or the likes)

Answer 1

The locality of subscribers, local or remote, should be completely transparent to the domain entity raising an publishing the event.

Your thoughts are very similar to design I use. I use a quite primitive version of a local event publishers, which communicate via messaging middleware (AMQP to propagate events to all other local publishers).

I'm not doing that in Java, but in fact, for the local event "bus" I use Rx.Net (reactive extensions have an almost identical API in all languages, so RxJava would work)

a simplified version with transaction support ripped out would look like this:

public class EventHub 
{
    private readonly ISubject<object, object> _messages;
    public EventHub()
    {
        var _subject = new Subject<object>();
        m_messages = Subject.Synchronize(_subject);
    }

    public void Publish<T>(T message)
    {
        m_messages.OnNext(message);
    }

    public IObservable<T> AsObservable<T>()
    {
        return m_messages
            .ObserveOn(ThreadPoolScheduler.Instance)
            .OfType<T>()
            .Synchronize()
            .AsObservable();
    }
}

Adding remote capability would be adding another subscriber, which would subscribe to all events and route them to other local EventHubs, and also listen to other eventhubs and publish their events on the local hub. It would not change the EventHub component.