Use subscriptions for data changes

[justinozavala]

Does self.callback_queue = Queue() member of the client class work? Would it be possible to use it for automatic enqueuing of changing data of some group? Would it be possible to use use through the gateway?

Thank you!

[renzop]

Hi, this is a good idea. I have looked at it lately in another context, i think it should be possible. The group reading stuff would need some love (aka refactoring) since it is mostly untouched from since the old openopc library. Would you like to giv e it a try and create a pullrequest?

[pensur]

I am using group read successfully, reading 18000 items each 10 seconds without issues.
I'll give a try, though I don't master python.

Correct me if I'm wrong, the steps would be:

• install python 32 bits
• test the queue with a local opc server
• expose the queue to pyro through a function
• test remotely

Edit: I'm Justino, was logged with the company user in the phone.

[justinozavala]

It didn't work as i expected. I think the queue is just to wait for one new value inside a read request call. I don't understand what the update parameter means, as changing it does nothing in my tests. Also if I don't explicitly ask for device values the server always return cached values.

[renzop]

I think it is not implemented as you expect. But OPC DA supports event based update of groups via a subscription to a group object. Check out the OpcDa Specificaition of the OPC Foundation, and the fine graybox_opc_automation_wrapper.py helps you as well.

But if you don't want to invest that much time, i go quite good results with normal group reading and polling.

[miguetronic]

I wanted to follow up and ask if there have been any advancements or updates regarding the subscription model implementation.

[renzop]

We did not plan to work on this issue. But every contribution (Pull request) is welcome.
I wonder what the your goal is.

When you create a group and read this synchronously you can easily read more than 10 times a second... Most OPC DA applications (I know) don't have a higher update rate than 100ms. Then you can do the change detection in your application.

Please write down what you want to achieve and provide a code example that works with the matricon simulation server. Try to write it as precisely as possible.