Difference between UaSubscription.NotificationListener and UaSubscriptionManager.SubscriptionListener

[dead-land]

What is the difference between the two listeners. At first glance they have the same methods. The only difference I see is that the UaSubscription.NotificationListener can be added to the Subscription and the UaSubscriptionManager.SubscriptionListener can be added to the SubscriptionManager - but is there any other relevant difference between the two listeners?

[dead-land]

... and when is the onStatusChangedNotification() called?

I've added a subscription and a NotificationListener, but I could not observer any call to the onStatusChangedNotification(). I've expected at least one call when the subscription is initialized and another one when e.g. shutting down the server

[kevinherron]

There's a bit of overlap. One listener will receive callbacks for all subscriptions, one for just the subscription it was added to.

StatusChangeNotifications are a specific type of notification sent by the server and AFAIK the spec only uses them in 2 scenarios:

1. the Subscription has timed out (Bad_Timeout)
2. the Subscription was transferred (Good_Transferred)

It's not as useful as it sounds in a general purpose sense.

[dead-land]

Thank you for clarification

[kevinherron]

FWIW these confusing Subscription and MonitoredItem APIs are being reworked for 1.0.

You can see how it's shaping up here if you're interested: #1223

milo/milo-examples/client-examples/src/main/java/org/eclipse/milo/examples/client/SubscriptionDataExample.java

Lines 37 to 93 in 00e3090

	client.connect();
	var subscription = new OpcUaSubscription(client);
	// Set a listener for data changes at the subscription level.
	subscription.setSubscriptionListener(new OpcUaSubscription.SubscriptionListener() {
	@Override
	public void onDataReceived(
	OpcUaSubscription subscription,
	List<OpcUaMonitoredItem> items, List<DataValue> values
	) {
	for (int i = 0; i < items.size(); i++) {
	logger.info(
	"subscription onDataReceived: nodeId={}, value={}",
	items.get(i).getReadValueId().getNodeId(), values.get(i).getValue()
	);
	}
	}
	});
	// Create the subscription on the server.
	subscription.create();
	var monitoredItem = OpcUaMonitoredItem.newDataItem(NodeIds.Server_ServerStatus_CurrentTime);
	// Set a listener for data changes at the monitored item level.
	monitoredItem.setDataValueListener(
	(item, value) ->
	logger.info(
	"monitoredItem onDataReceived: nodeId={}, value={}",
	item.getReadValueId().getNodeId(), value.getValue()
	)
	);
	// Add the MonitoredItem to the Subscription
	subscription.addMonitoredItem(monitoredItem);
	// Synchronize the MonitoredItems with the server.
	// This will create, modify, and delete items as necessary.
	try {
	subscription.synchronizeMonitoredItems();
	} catch (MonitoredItemSynchronizationException e) {
	e.getCreateResults().forEach(result ->
	logger.warn(
	"failed to create item: nodeId={}, serviceResult={}, operationResult={}",
	result.monitoredItem().getReadValueId().getNodeId(),
	result.serviceResult(), result.operationResult()
	)
	);
	}
	Thread.sleep(5000);
	subscription.delete();
	future.complete(client);