Heartbeat subscriptions

Example embodiments provide for "heartbeat" subscriptions which can be thought of as pseudo-events that can occur periodically for any subscription. Notification of a heartbeat event can be sent in lieu of, or in addition to, regular events to inform the subscriber application of such things like the subscription is still active and/or that connectivity exists between the subscriber application and the publishing service application. Alternatively, if the heartbeat notification does not arrive at the subscriber application, then the client knows that the connectivity is bad or that the subscription has expired and it may take corrective action (e.g., attempt to re-subscribe to the appropriate subscription). Note that similar embodiments also apply to pull requests received by the publishing service from the subscriber.