Request handling priority in OPC UA

I'm building an OPCUA Server in Visual C# to be put on a client's machines.

The client wants to make sure that Methods called on the server that have physical consequences have priority on all other requests, for example if a server has 10 active client sessions, and 9 of those clients send a read/write request and one of them sends a move command for a robotic arm, this command should be executed first and the read/write operations should be completed later.

This makes sense for safety reasons: I don't want to wait on 9 read operations before the server gets to the conveyor_belt_stop() command, which was issued to prevent a person from getting injured.

Requests management is handled internally by the Server SDK, so there is no way of implementing custom requests handling policies.

I studied the code of my SDK of choice, and I saw that it adds requests to a Queue<T> object and uses worker threads to satisfy the requests. Given that Queue<T> is a FIFO queue makes it impossible to somehow rearrange the request to favor those that I want.

For now I plan to use the sample client to benchmark the latency of this queue mechanism and trust the OPCUA devs that this is the right approach, but I would like to have a better answer for my client.

Does OPCUA provide a way to define priorities for the request handling from either the Client or Server side?

> Does OPCUA provide a way to define priorities for the request handling from either the Client or Server side?

No, it doesn't.

But I think you've misunderstood the implementation of the SDK you linked to. It does not queue requests and process them one-by-one. The object you link to is a Dictionary, not a Queue, and it's used for other bookkeeping purposes.

That said, you could consider keeping 2 sessions open, and using one of those sessions as your "high priority" session where you only issue method calls or other operations your application considers high priority.