The Historical Eventmodel defines additional Properties. These descriptions also include required use cases for HistoricalEventNodes.

Historical Access of Eventsuses an EventFilter. It is important to understand the differences between applying an EventFilterto current Event Notifications, and historical Eventretrieval.

In real time monitoring Eventsare received via Notifications when subscribing to an EventNotifier. The EventFilterprovides the filtering and content selection of Event Subscriptions. If an Event Notificationconforms to the filter defined by the whereparameter of the EventFilter, then the Notificationis sent to the Client.

In historical Eventretrieval the EventFilterrepresents the filtering and content selection used to describe what parameters of Eventsare available in history. These may or may not include all of the parameters of the real-time Event, i.e. not all fields available when the Eventwas generated may have been stored in history.

The HistoricalEventFiltermay change over time so a Clientmay specify any field for any EventTypein the EventFilter. If a field is not stored in the historical collection then the field is set to null when it is referenced in the selectClauseor the whereClause.

A HistoricalEventNodethat has Eventhistory available will provide the Property. This Propertyis formally defined in Table 7.

Table 7– Historical Events Properties

Name

Use

Data Type

Description

Standard Properties

HistoricalEventFilter

M

EventFilter

A filter used by the Serverto determine which HistoricalEventNodefields are available in history. It may also include a where clause that indicates the types of Eventsor restrictions on the Eventsthat are available via the HistoricalEventNode.

The HistoricalEventFilter Propertycan be used as a guideline for what Eventfields the Historian is currently storing. But this field may have no bearing on what Eventfields the Historian is capable of storing.

HistoricalEventNodesare Objectsor Viewsin the AddressSpacethat expose historical Events. These Nodesare identified via the EventNotifier Attribute, and provide some historical subset of the Eventsgenerated by the Server.

Each HistoricalEventNodeis represented by an Objector View with a specific set of Attributes. The HistoricalEventFilter Propertyspecifies the fields available in the history.

Not every Objector Viewin the AddressSpacemay be a HistoricalEventNode. To qualify as HistoricalEventNodes, a Nodehas to contain historical Events. To see if historical Eventsare available, a Clientwill look for the HistoryRead/Write states in the EventNotifier Attribute. See OPC 10000-3for details on the use of this Attribute.

Figure 4illustrates the basic AddressSpaceModel of an Event that includes History.

image007.png

Figure 4– Representation of an Event with History in the AddressSpace

Subclause 5.3.4 lists the Attributesof Objectsor Viewsthat have particular importance for historical Events. They are specified in detail in OPC 10000-3. The following Attributesare particularly important for HistoricalEventNodes.

  • EventNotifier

The EventNotifier Attributeis used to indicate if the Nodecan be used to read and/or update historical Events.