NotifyHub is a callback facility. NotifyHub is used by the informer to notify clients about arbitrary events in the informer. NotifyHub contains notification sets (NotifySet). NotifySet is identified by set ID and it can include one or many notifications (Notify). Client defines the action performed at notification (callback). Informer activates the notification when notification events occur. Notifications can be enabled/disabled on different levels (NotifyHub, NotifySet, and Notify).