PublishRequestCount, DataChangeNotificationsCount meaning

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ap0
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Joined: 19 Feb 2020, 09:36

PublishRequestCount, DataChangeNotificationsCount meaning

Post by ap0 »

Hello,

I am currently analysing a problem I have with a OPC-UA implementation in Python that I have created. I know this is not a library supported here but my questions is more general and concerning UaExperts Server Diagnostics View.

In the tab Subscription Diagnostic there are the parameters PublishRequestCount and DataChangeNotificationsCount. I am looking for the meaning of these parameter.
Is this the amount of requests the server receives by the client whether the value of the subscribed node has changed?
And is the DataChangeNotificationsCount the amount of answers by the server that "yes" the value did change. Would this counter not incease if the value did not change?
Does the server take care of checking if the value is still the same or does the server always return the current value and checking must be done by the client?

As a side node: my problem is that I receive data change notifications by a OPC-UA server (numeric values) although the value did not change. So in a high frequency I receives new values like 1, 1, 1, 1, 10, 10, 10, 10, etc.

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Support Team
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Joined: 18 Mar 2011, 15:09

Re: PublishRequestCount, DataChangeNotificationsCount meanin

Post by Support Team »

Hi,

unlike in the old OPC-DA you can subscribe for value change and timestamp change of status change, therefore in theory there could be the case where the value has not changed but the status code has changed, hence you would receive a notification, if you subscribe for exactly this.

However, typically you would subscribe to value changes only (which comes along with timestamp change). But in your case it seems that the server has NOT correctly implemented the OPC UA subscription mechanizm. The Server produces unnessecary high load on the network and is probably not certified for compliance by the OPC Foundation. For that reason we strongly recomend NOT to use this server in production environment.
Best regards
Unified Automation Support Team

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