Two types of ModelChangeEvents are defined: the BaseModelChangeEvent that does not contain any information about the changes and the GeneralModelChangeEvent that identifies the changed Nodes via an array. The precision used depends on both the capability of the OPC UA Server and the nature of the update. An OPC UA Server may use either ModelChangeEvent type depending on circumstances. It may also define subtypes of these EventTypes adding additional information.
To ensure interoperability, the following guidelines for Events should be observed.
- If the array of the GeneralModelChangeEvent is present, then it should identify every Node that has changed since the preceding ModelChangeEvent.
- The OPC UA Server should emit exactly one ModelChangeEvent for an update or series of updates. It should not issue multiple types of ModelChangeEvent for the same update.
- Any Client that responds to ModelChangeEvents should respond to any Event of the BaseModelChangeEventType including its subtypes like the GeneralModelChangeEventType.
If a Client is not capable of interpreting additional information of the subtypes of the BaseModelChangeEventType, it should treat Events of these types the same way as Events of the BaseModelChangeEventType.