The Distributed Object will provide a method to create a monitor object for a Property. A callback will be connected to the event and will be called by the monitor object when an event occurs[RD01 - 13.1.4. Events]. The monitor class provides methods to suspend, resume and destroy the monitor itself.
Events can be generated on any change of value[RD01 - 4.1.3 Rate]. Other conditions, for example any write, value increase/decrease, value less or greater than setpoint could also be included at a later stage.
- Periodic, at a specific interval rate
- Periodic, at a specific interval rate, synchronized with an absolute time
All events will be timestamped
Events generated by timers allow to retrieve time stamped data at a specific array time [RD01 - 4.1.5 Values at given time]
Properties also publish their value on the Archiving Notification Channel (see the corresponding section), a publisher/subscriber model. This can be used as another sort of data retrieval by event not based on callbacks.
A particular case is a Distributed Object State Machine. A State Machine class is a Distributed Object and the current state is represented by a State Property. This State Property can fire events whenever the state changes to allow external objects to monitor it.
Hardware monitor points, like monitor points on the CAN bus do not support any notification on change. To support events of monitor points they need to be polled.
For performance optimization, the final implementation will not leave to the single Distributed Object Properties the responsibility of managing timers, but a local centralized manager will take care of that, transparently to client applications. More details will be given in the ACS design.