Introduction to OPC UA Events

POST IS WORK IN PROGRESS

Events are one of three ways to provide data from an OPC UA server. Important to note, it is a completely different mechanism than reading or subscribing to OPC UA variables.

When to use OPC UA Events?

It is one possible way to provide several consistent values in one transaction.
Events allow certain kind of flexibility. Events can be extended with additional values in a compatible way via a new inherited type. And the client can choose which values it wants to get. This is done with a so called event filter.
Events are a good way to model and transport other information than raw data values. Events should be used to indicate that certain things “happen.” For example state changes like a process has started or a process is completed.
Events can be used to inform about changes in the OPC UA address space. For example there is the BaseModelChangeEvent.
Finally Events are the foundation for the alarms and conditions system.

How to subscribe to Events?

Event instances are not visible in the address space, but the EventType definitions are. The EventType defines the content of an event like a template.
An object node and an event filter is needed to subscribe to events. Using the event filter, the subscription can be limited to certain EventTypes and certain attributes.
Every OPC UA server has to provide all it’s events via the predefined Server-Object node. Additionally it can provide the possibility to register for events on other object nodes.
An object node can be used for event subscription, when the attribute “EventNotifier” (https://reference.opcfoundation.org/v104/Core/docs/Part3/5.5.1/) is set to 1. That attribute is defined in the object node class, so it is an attribute of every object.

Example event hierarchy:

Code example with Python:

In python there is a great high level api, which allows to program OPC UA stuff with very few lines of code. In python adding a filter is optional. If no filter is passed, it is auto created. The example uses the sampling interval 0 and the PubSubStatusEventType.

sub = await client.create_subscription(0, callback)
h = await sub.subscribe_events(client.nodes.server, pubSubStatusEventType)

Code example with .NET:

The .NET library from OPCFoundation is more low level and needs more code. In the examples a helper class “FilterDeclaration” is used to auto create the event filter.

// the filter to use.
m_filter = new FilterDeclaration(pubSubStatusEventType, null);
 
// create a monitored item based on the current filter settings.            
m_monitoredItem = new MonitoredItem();
m_monitoredItem.StartNodeId = Opc.Ua.ObjectIds.Server;
m_monitoredItem.AttributeId = Attributes.EventNotifier;
m_monitoredItem.SamplingInterval = 0;
m_monitoredItem.QueueSize = 1000;
m_monitoredItem.DiscardOldest = true;
m_monitoredItem.Filter = m_filter.GetFilter();
 
m_subscription.AddItem(m_monitoredItem);

How is the filter auto created?