Hands-On Industrial Internet of Things

OPC UA notifications

As mentioned in the previous section, the client can define one or more subscriptions within an active session. For each subscription, it can create monitored items, which produce notifications that are stored in specific queues. According to the frequency specified by the publish interval, for every subscription, the current contents of all queues of the monitored items related to the subscription are merged into a NotificationMessage, to be delivered to the client. In OPC Classic, this information was sent through callbacks, which allowed the server to invoke methods on the client. In OPC UA, the communication mechanism is based on unidirectional connections that don't use the callback mechanism. This means it can easily ...