Hi,
the problem is that open62541 doesn't handle asynchronous messages, as admitted in
https://github.com/acplt/open62541/issues/417.
The UaExpert, or better the underlying C++ based OPC UA Client SDK, which handles publish notifications, sends multiple Publish Requests to the server. This feature is by design, this is not a bug.
OPC UA is an asynchronous protocol, not a ping-pong protocol like HTTP, where one waits for the response before sending the next request.
In OPC UA a server can only send "events" (datachange or event notifications) by first getting a publish request. To avoid latencies when many subscriptions have data ready or a lot of alarms need to be sent it would be inapplicable if the server needs to wait until getting the next publish request.
For this reasons a client sends multiple publish requests. These get stored in the server's publish queue. So whenever the server wants to send notifications it dequeues a publish request context and sends the according response without any delay.
This seems to be completely missing in open62541 yet.
This behaviour is described in OPC UA Spec Part 4, Section 5.13.1 - Subscription Model.
I hope this helps understanding the issue.
>> UAExpert does not respect the publishinterval it set. Instead, it floods the server with publishRq.
This is wrong. The publishing interval has nothing to do with the client. The server should not send publish responses for a subscription faster than this interval.
There is no time constraint for publish requests, also one publish request is used for polling multiple subscriptions. It is a mass operation. So this would not make sense anyway.
Regards,
Unified Automation Support Team
