The Event Model defines a general purpose eventing system that can be used in many diverse vertical markets.
Events represent specific transient occurrences. System configuration changes and system errors are examples of Events. Event Notificationsreport the occurrence of an Event. Eventsdefined in this document are not directly visible in the OPC UA AddressSpace. Objects andViews can be used to subscribe to Events. The EventNotifier Attributeof those Nodesidentifies if the Nodeallows subscribing to Events. Clientssubscribe to such Nodesto receive Notificationsof Eventoccurrences.
Any OPC UA Serverthat supports eventing shall expose at least one Nodeas EventNotifier. The Server Objectdefined in OPC 10000-5is used for this purpose. Eventsgenerated by the Serverare available via this Server Object. A Serveris not expected to produce Eventsif the connection to the event source is down for some reason (i.e. the system is offline).
Eventsmay also be exposed through other Nodesanywhere in the AddressSpace. These Nodes(identified via the EventNotifier Attribute) provide some subset of the Eventsgenerated by the Server. The position in the AddressSpace dictates what this subset will be. For example, a process area Objectrepresenting a functional area of the process would provide Eventsoriginating from that area of the process only. It should be noted that this is only an example and it is fully up to the Serverto determine what Eventsshould be provided by which Node.