Clients define MonitoredItems to subscribe to data and Events. Each MonitoredItem identifies the item to be monitored and the Subscription to use to send Notifications. The item to be monitored may be any Node Attribute.
Notifications are data structures that describe the occurrence of data changes and Events. They are packaged into NotificationMessages for transfer to the Client. The Subscription periodically sends NotificationMessages at a user-specified publishing interval, and the cycle during which these messages are sent is called a publishing cycle.
Four primary parameters are defined for MonitoredItems that tell the Server how the item is to be sampled, evaluated and reported. These parameters are the sampling interval, the monitoring mode, the filter and the queue parameter. Figure 15 illustrates these concepts.
Attributes, other than the Value Attribute, are only monitored for a change in value. The filter is not used for these Attributes. Any change in value for these Attributes causes a Notification to be generated.
The Value Attribute is used when monitoring Variables. Variable values are monitored for a change in value or a change in their status. The filters defined in this standard (see 7.17.2) and in OPC 10000-8 are used to determine if the value change is large enough to cause a Notification to be generated for the Variable.
Objects and views can be used to monitor Events. Events are only available from Nodes where the SubscribeToEvents bit of the EventNotifier Attribute is set. The filter defined in this standard (see 7.17.3) is used to determine if an Event received from the Node is sent to the Client. The filter also allows selecting fields of the EventType that will be contained in the Event such as EventId, EventType, SourceNode, Time and Description.