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

Historical Access of Events uses an EventFilter. It is important to understand the differences between applying an EventFilter to current Event Notifications, and historical Event retrieval.

In real time monitoring Events are received via Notifications when subscribing to an EventNotifier. The EventFilter provides the filtering and content selection of Event Subscriptions. If an Event Notification conforms to the filter defined by the where parameter of the EventFilter, then the Notification is sent to the Client.

In historical Event retrieval the EventFilter represents the filtering and content selection used to describe what parameters of Events are 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 Event was generated may have been stored in history.

The HistoricalEventFilter may change over time so a Client may specify any field for any EventType in 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 selectClause or the whereClause.

A HistoricalEventNode that has Event history available will provide the Property. This Property is 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 Server to determine which HistoricalEventNode fields are available in history. It may also include a where clause that indicates the types of Events or restrictions on the Events that are available via the HistoricalEventNode. The HistoricalEventFilter Property can be used as a guideline for what Event fields the Historian is currently storing. But this field may have no bearing on what Event fields the Historian is capable of storing.

HistoricalEventNodes are Objects or Views in the AddressSpace that expose historical Events. These Nodes are identified via the EventNotifier Attribute, and provide some historical subset of the Events generated by the Server.

Each HistoricalEventNode is represented by an Object or View with a specific set of Attributes. The HistoricalEventFilter Property specifies the fields available in the history.

Not every Object or View in the AddressSpace may be a HistoricalEventNode. To qualify as HistoricalEventNodes , a Node has to contain historical Events. To see if historical Events are available, a Client will look for the HistoryRead/Write states in the EventNotifier Attribute. See OPC 10000-3 for details on the use of this Attribute.

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

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

Subclause 5.3.4 lists the Attributes of Objects or Views that have particular importance for historical Events. They are specified in detail in OPC 10000-3. The following Attributes are particularly important for HistoricalEventNodes .

• EventNotifier

The EventNotifier Attribute is used to indicate if the Node can be used to read and/or update historical Events.