Thanks for good questions.

1. OPC UA does not implement direct call backs in practice, because that would require another network connection to be opened from the server to the client, which complicates the security settings, for example, too much.

Instead, there is a “polling” mechanism that actually provides the same results. The client application is sending PublishRequest messages to the server, which the server can then use whenever it needs to send a NotificationMessage (with data change or event notifications) back to the client. At that point, the server will send a PublishResponse corresponding to one of the PublishRequests sent earlier by the client.

2. UaClientListener.validatePublishResponse enables your client application to verify the PublishResponse message, in case there is something that you wish to do. In normal circumstances, you can ignore this and simply use the SubscriptionNotificationListener or MonitoredDataItemListener (or MonitoredEventItemListener) to handle the data changes (or events). MonitoredDataItemListener is plugged into individual MonitoredDataItem instances (but you can use the same listener) and provides you even simpler way to handle the individual data changes than SubscriptionNotificationListener.

They are totally independent of each other. Except, of course, that if you return ‘false’ from the UaClientListener.validatePublishResponse()-method, you will not get any notifications to the other listeners.