Good afternoon,

I was reading this topic regarding forcing a disconnect to the OPC server in a VB6 COM application:

1. The application writes data from the OPC server every 30 seconds and reads data every every 30 seconds.
2. The application communicates with an ABB AC800M OPC server
3. The application runs on a Windows2019 server.
4. The application uses client.ReadMultipleItems(arguments) and client.WriteMultipleItemValues(arguments)
5. The application runs without any problems for a week or two and after that it returns OPC errors for each item that that is read. It looks like that the items are not available anymore in the OPC server. Note that that a second application is running at the time the problem with the first application occurs. However the second application can still read all items from the same OPC server. However after some time this second application has the same problem.
6. The application does not recover from this problem until it is restarted (our client reboots the Windows 2019 server)
7. I did some tests with the Matrikon OPC server test application with general logging and found out that the QuickOPC client application never disconnects from the server
8. I like that force a disconnect from the OPC server because I think the problem is related to a connection that does not function correctly anymore
9. I configured the periods below but the the EasyDAClient does not disconnect.
10. The EasyDAclient object is defined defined in the VB module. Its not defined in the the function. I tried that before but then I get out of memory problems because the memory is not released.
11. I am using version QuickOPC-2022.1-Windows-
12. Please advise. Thank you

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These parameter settings do nothing regarding detaching to the OPC server
client.InstanceParameters.HoldPeriods.TopicWrite = 1
client.InstanceParameters.HoldPeriods.TopicRead = 1
client.InstanceParameters.HoldPeriods.ServerDetach = 1
client.InstanceParameters.HoldPeriods.ItemDetach = 1
client.InstanceParameters.HoldPeriods.ServerDetach = 1

Thank you. From testing looks like setting these two to zero before the EasyDAClient.UnsubscribeAllItems() call did the trick:
- EasyDAClient.InstanceParameters.HoldPeriods.ItemDetach = 0
- EasyDAClient.InstanceParameters.HoldPeriods.ServerDetach = 0

All working well so far testing my app with prerelease v5.59.0-rev23 with explicit subscriptions. I have follow up questions once more and this time related to disposing of all connections and resources. I do realize though that one of the main design principles of QuickOPC is to give the client a connectionless API.

Example:
My app subscribes to a handful of items on a given server. The app needs to be restarted for some reason or another. The app, before closing, disposes of an internal class which manages a reference to EasyDAClient. During dispose, the app calls EasyDAClient.UnsubscribeAllItems() and EasyDAClient.Dispose(). On the server side, I can see that the connection is maintained and the group of items previously subscribed are still maintained.

I imagine that the EasyDAClient instance will still wait some time before actually removing items from the group it created due to either "chunk size" settings and some other internal delays. My questions are around situations where I'd like to force a disconnect. I am open to other ways I should be thinking of or handling this type of situation.

Questions:

1. Is there a direct method of synchronously forcing an unsubscribe, removal of groups created, and disconnecting from a server?
2. Is there a better method of handling this situation?

Thanks for the great feedback, I'd rather head the right direction at the start than have to backtrack later. Depending on how soon the official release of QuickOPC 2020.3 is expected, I may just begin now with its prerelease. I expect my full production app to be needed in the next 30 days or so.

With our current OPC codebase which is not yet using the QuickOPC lib, we are chunking to limit the number of tags in each subscription request. There have been instances where the OPC server chokes if we send too many over at once. It appears that the chunk size is configurable in the QuickOPC 2020.3 release notes so that will be helpful.

Follow up question:
When subscribing to large set of tags, does QuickOPC make decisions on when to create another OPC group if the total tag set for a group is too large? I imagine certain OPC servers to have individual group size limitations but do not know if this might be handled automatically by QuickOPC or if it's something that could be configured.

In reference to these two posts:

• Disabling automatic subscriptions made with OPC Reads
• Subscribing and reading large number of tags

I actually like the idea of the EasyDAClient handling the creation of subscriptions for me in the background based on reads performed by my application. The last post above does suggest having a user's application handling the subscriptions directly. I would like to make sure that my use case below still leans toward the recommendation of handling the subscription directly.

Example use case:
My application has 10k tags which it needs an updated value for every 5 minutes. Roughly every 24 hours there may be tags added to or removed from the list to be updated. The application should only add/remove tags from subscription rather than incurring the cost of unsubscribing all and then resubscribing only the new tag set (not including getting disconnected from the OPC server here). Unlike the second post above, this application does not have the requirement of browsing recursively through the OPC server periodically, the application will either have a itemId that exists or does not in the OPC server.

Questions:

1. Should my application go straight to handling the subscription of all tags directly or ok to tune automatic subscription accordingly?
2. Does the EasyDAClient.ReadMultipleItems() method also perform an automatic subscription as described in the post Disabling automatic subscriptions made with OPC Reads ?
3. Is it wise to call EasyDAClient.SubscribeMultipleItems() with a list of 10k tags?
4. What happens if my application were to resubscribe to the same item twice with EasyDaClient.SubscribeMultipleItems()? Is this duplication handled automatically by the the QuickOPC lib?
5. I am new to the QuickOPC library, so what else have I not asked that may be helpful to know during my design phase here? I admit that I have not been able to make it through but 10% of the QuickOPC user guide.