OPCPublisher cannot subscribe to the Alarms message that already exists on the Server

[wudiqiang2024]

After OPCPublisher is started, it is unable to subscribe to the Alarms message that already exists on the Server. After checking the documentation, I found that ConditionHandling can be configured. However, after the configuration, although the OPCPublisher log prints the call to ConditionRefresh

, but the Server does not receive the request. After tracing the source code, I found that if this code in the OpcUaSubscription class is replaced with "ConditionRefresh();", the Server can receive the request.

OPCPublisher version is 2.9.11
Config content is

My questions:

1. Is it necessary to configure the ConditionHandling monitoring type for it to be a Condition? However, I only want to subscribe to existing messages on the server when the OPC Publisher starts, not to subscribe to duplicate messages.
2. Is there a problem with this code?

[marcschier]

Looks like a bug in the OPC UA stack and thanks for finding it. @wudiqiang2024 could you try out 2.9.15 version to see if it also behaves this way? The OPC UA stack looks ok to me so ConditionRefresh should call correctly, but 2.9.15 contains the latest released stack, so one could validate.

[wudiqiang2024]

@marcschier Thanks for your reply. I will test it in version 2.9.15. I have another question. Do I have to configure ConditionHandling before calling ConditionRefresh? Because this will send duplicate data at the configured time interval, but I just want to subscribe to the messages that exist in the Server when starting.

[marcschier]

@wudiqiang2024 , ConditionHandling handles calling ConditionRefresh for you (periodically) (snapshot), but if you do not use ConditionHandling you can also call ConditionRefresh or ConditionRefresh2 using the Call REST method periodically or whenever you want to refresh the alarm condition. In your case - that would be a 1 time call. A feature could be added to call it first time the writer is created and never again. If 2.9.15 works, then this could be a follow up feature request.

[wudiqiang2024]

@marcschier I will test 2.9.15 today and will reply to you with the results later. If I call ConditionRefresh through call REST method, the parameter SubscriptionId is required. How can I get it?

To add, I am talking about calling the method MethodCall_V2 in the Direct method provided by OPCPublisher

[wudiqiang2024]

@marcschier In 2.9.15, this problem is fixed. If I want to use Rest API or Direct method to call ConditionRefresh, how do I call it? I don't know how to get the SubscriptionId. Thank you very much!

[marcschier]

@wudiqiang2024 you are right, getting the subscription id is cumbersome, you would need to first get the current subscriptions the server knows about (in the Server Diagnostics object) and select the one that "fits", which is not easy as there could be more, then pass the id to ConditionRefresh method via Call. It is possible, but I think it would be better for a feature that allows you to invoke ConditionRefresh "first time" the alarm is added, but forgo the condition handling logic/Snapshotting logic. Or do you think you would need to also do condition refresh on a Timer? But no snapshotting?

[wudiqiang2024]

@marcschier Our current goal is to call ConditionRefresh only once and then subscribe to new messages based on changes. We don't want to use a Timer because configuring a time interval would result in repeated messages. I'm not sure if this is a good idea, as when we use client applications like UAExpert and Pxosys OPC UA Browser to connect to the server and subscribe to messages, they first call ConditionRefresh on the server.

[wudiqiang2024]

@marcschier I just tried calling the GetConnectionDiagnostics method to get the subscriptionId. I can confirm that there is only one connection, but when I use this id to call ConditionRefresh, I get an error.

[wudiqiang2024]

I just looked at the documentation and source code.

1. ConditionHandling UpdateInterval - If data changes and the interval is reached, cached data will be sent (the entire cached data, not just the changed cached data).
2. ConditionHandling SnapshotInterval - If the interval is reached, cached data will be sent (the entire cached data).
3. If the SnapshotInterval is configured, the OpcUaMonitoredItem will be of type OpcUaMonitoredItem.Condition. After a successful subscription, ConditionRefresh will be called once.
4. Only data with Retain set to true will be sent.

Am I understanding this correctly?