Hello.
In OPC UA, there currently is no separate "request chunking" feature, but on the lower level, there are following features (referring to version 2024.1):

- QuickOPC reads the maximum nodes per Read request from the server, and splits the requests automatically based on this.
- QuickOPC reads the maximum nodes per Write request from the server, and splits the requests automatically based on this.

In version 2024.2 (should be released soon), in addition:

- QuickOPC reads the maximum number of monitored items per call from the server, and splits the requests automatically.
- QuickOPC reads the maximum number of monitored items that can be put into one subscription in total (even with multiple requests), and if necessary, creates multiple subscriptions for it.

Best regards

Good catch!
You would need to turn off the Subscription Blending for this workaround to work reliably.

Hello.

After checking deeper, currently there is no support for automatically chunking the subscription request this way. There are some internal reasons for that that have to do with peculiar difference of OPC XML from OPC DA. Specifically, OPC XML does not have an equivalent of "AddItems" in OPC DA. So, while it is possible to create a subscription (group) in OPC DA, and then add 100 items to it and then another 100 items and so on, in OPC XML in reality you can only create multiple subscriptions - you cannot modify (expand) the existing subscription. Of course we *could* chunk in this way, and create multiple subscriptions, but it is not currently implemented.

The workaround for you would be to do it manually, and use a trick where you would somehow force QuickOPC to create separate subscriptions. That can be done e.g. by having 100 items with the requested update rate, e.g. 1000 milliseconds. And then the next 100 items with update rate 999 milliseconds, and next 100 items with 998 milliseconds. This is ugly but should work.

(I am using the number "100 items" in this whole text as an example, in reality it will be much higher number that you know better).

We can enhance QuickOPC to create separate subscriptions in OPC XML with configurable maximum number of items, but I can only schedule it to version 2025.1, so we are looking multiple months before delivery. Let me know if it would make sense to do it this way.

Best regards

Hello. I understanhd.

I have contacted the developer of this part to check whether we have some configuration possibility available, and if not, how difficult it would be to add it.

Regards

Hello.
The request chunking is used for OPC XML as well as for OPC DCOM.

But now, reviewing the design, it appears that while it works as intended for synchronous operations (Read, Write), it may not work well with subscriptions. The reason for it is that the subscription requests are divided into chunks, but the underlying code can assemble them together again - which I think what is happening.

This is a design issue on our side.

Do I understand it correctly that your intent is to limit the number of item in subscription requests, for OPC XML? Can you explain the requirements - is it to limit the overall message size, or for some other reason, such as server limit? Based on your answers I will see what can be done.

Best regards