About data subscription

[TangDynasty06]

I'm learning about data subscription in milo (0.6.16) and noticed two different implementations in the examples. Can you tell me the difference between them?

[kevinherron]

The "ManagedSubscription" API is a little higher level and easier to use than the lower level Subscription API it's built on.

[TangDynasty06]

Okay, got it. Thanks!

[kevinherron]

Neither API exists as is in 1.0, but the "managed" API is closer to the API in 1.0.

[TangDynasty06]

Hello,
I'm using Milo version 0.6.16 and plan to use ManagedSubscription for data subscription. Could you help clarify these questions:

1. What's the maximum number of data items a single ManagedSubscription can handle?
2. Do I need to manually close each subscribed data item when shutting down my application?
3. After a disconnect/reconnect event, must I re-add all subscription items?
4. In which thread is the data-update callback logic executed?

[kevinherron]

What's the maximum number of data items a single ManagedSubscription can handle?

There's no limit on the Client, but you will often find the Server has some limit. Usually when you've added too many MonitoredItems you start getting Bad_TooManyMonitoredItems StatusCode when attempting to create them.

Do I need to manually close each subscribed data item when shutting down my application?

If you call OpcUaClient::disconnect it will close the Session and delete all of the Subscriptions, which includes all of the MonitoredItems.

After a disconnect/reconnect event, must I re-add all subscription items?

If you receive onSubscriptionTransferFailed callback for a Subscription it must be re-created, and all items re-created and added.

milo/opc-ua-sdk/sdk-client/src/main/java/org/eclipse/milo/opcua/sdk/client/subscriptions/ManagedSubscription.java

Lines 1196 to 1210 in bd15067

	/**
	* A subscription transfer has failed.
	* <p>
	* Upon reconnecting after a connection interruption an {@link OpcUaClient} will attempt to resume its previous
	* Session. If this fails a new Session is created and the Subscriptions from the previous Session are
	* transferred to the new one.
	* <p>
	* This callback is notified when the transfer to the new session has failed and indicates that the
	* subscription and all of its {@link ManagedDataItem}s and {@link ManagedEventItem}s will need to be created
	* again.
	*
	* @param subscription the {@link ManagedSubscription} for which transfer has failed.
	* @param statusCode the {@link StatusCode} for the transfer failure.
	*/
	default void onSubscriptionTransferFailed(ManagedSubscription subscription, StatusCode statusCode) {}

In which thread is the data-update callback logic executed?

Queued and delivered on a thread from the Client's configured Executor.

milo/opc-ua-sdk/sdk-client/src/main/java/org/eclipse/milo/opcua/sdk/client/subscriptions/ManagedSubscription.java

Lines 1233 to 1263 in bd15067

	private class ManagedSubscriptionNotificationListener implements UaSubscription.NotificationListener {
	private final ExecutionQueue executionQueue = new ExecutionQueue(client.getConfig().getExecutor());
	//region UaSubscription.NotificationListener
	@Override
	public void onDataChangeNotification(
	UaSubscription subscription,
	List<UaMonitoredItem> monitoredItems,
	List<DataValue> dataValues,
	DateTime publishTime
	) {
	List<ManagedDataItem> itemsToNotify = new ArrayList<>(monitoredItems.size());
	List<DataValue> valuesToNotify = new ArrayList<>(monitoredItems.size());
	for (int i = 0; i < monitoredItems.size(); i++) {
	UInteger key = monitoredItems.get(i).getClientHandle();
	ManagedDataItem dataItem = dataItems.get(key);
	if (dataItem != null) {
	itemsToNotify.add(dataItem);
	valuesToNotify.add(dataValues.get(i));
	}
	}
	changeListeners.forEach(
	changeListener ->
	changeListener.onDataReceived(itemsToNotify, valuesToNotify)
	);
	}

[TangDynasty06]

Got it - really appreciate it!