* EventSourceRegistry will request registered event source to publish notifications
* whenever EventSourceRegistry has been asked to publish a certain type of notifications.
* EventSourceRegistry will call method JoinTopic to request EventSource to publish notification.
* EventSourceRegistry will request registered event source to publish notifications
* whenever EventSourceRegistry has been asked to publish a certain type of notifications.
* EventSourceRegistry will call method JoinTopic to request EventSource to publish notification.