I'm new to OPC UA and I wonder, if it is possible to call a published method (part 4, chapter 5.11) or modify an attribute of a node stored in a server (publisher) from a client (subscriber) in a PubSub infrastructure using AMQP transport? I've read part 14 of the standard, but did not find the related information.
I assume all services (defined in part 4) are possible, because a subscriber of a topic may have read or write access to the underlying data. But possibly, I've missed something.
Thank you for your help.
Any service that requires a request-response message pattern is not suited for PubSub.
That said, we did explore defining a client-server transport over AMQP/MQTT but our testing established that brokers need to be carefully tuned to make them useful for applications using a request-response pattern.
In theory, sessionless service calls over AMQP/MQTT could work without broker tuning but we are waiting for users to tell the committee that they really need such capabilities.
Carefully tuned means the queues have to be configured to minimize delay and support large messages. What creates problems is brokers tuned in this way no longer scale as well for huge numbers of clients which negates one of the reasons for using a broker in the first place.
There is a prototype that has not been maintained here: