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Node RED OPC-UA Subscription to "Extension Object" fails

Node RED OPC-UA Subscription to "Extension Object" fails


Hello together, 

within Node-RED I try to setup a subscription to an OPC-UA parameter. The datatype of the parameter is offered as "Extension Object".
Within UA-Expert testtool I am able to verify the object and the content of this object. There are 3 parameters defined within the object
- ID
- Name
- Description 

During the test the values are filled with "ID=160" and "Name=Distanzhuelse" ... so they are not empty. 


If I try to access this object using a NODE-RED flow I either choose the "Data Layer Request" of the "Data Layer Subscription". 
Using the Subscription the operation fails with debug message "DL_FAILED" 


Since the "ActiveTool" is the last node I can choose within the data structure I have exactly choosen this option.


If I work with READ / BROWSE Request instead, I receive an empty array as result. 



But it's also not possible to access the parameters "Id", "Name" or "Description" as they are shown in UA Expert before.

Does anyone have an idea how I can access the parameters in this object or why the array is shown as "empty" even if there are currently parameters and values contained? 

Thanks for your support!






Community Moderator
Community Moderator

Just to make things clear. You are connected with the ctrlX CORE - OPC UA client to a server on the network. Could you show us how the data looks like in the data layer viewer?

Which path did you enter in the subscription in Node-RED?

A Browse does show you an array of all registered sub nodes. If there is "only" a complex data object, there are no sub nodes registered eventually.

I did some tests subscribing to data objects in the data layer and it works fine see e.g. path framework/bundles:

ctrlX CORE - Node-RED subscrition to data objectctrlX CORE - Node-RED subscrition to data object

There could be a problem with your data type.

Hi everyone!

I've just met with a similar issue. I use in Node-Red the data layer request node and also the data layer subscribe node for reading connected  OPC UA server parameters as an OPC UA Client. When I created the flows everything worked properly (Friday), today (the ctrlX Core was not restarted, was on ) with subscription nodes I get the subscription failed: DL_Failed error while with the data layer request node everything looks ok.  I tried to redeploy the flows and select the reading parameters again to make it work again, but I was unsuccessful. The only solution was to reboot the ctrlX Core, after that the data layer subscribe nodes were working again.  I got this error also for another ctrlX Core...

Can you help me with this? Restarting the ctrlX Core all the time when this error occurs is not a good solution for us, the Cores have to work all the time. 

Occasional Contributor

Hi @arsz

Can you tell me which version of the OPC UA Client app you are using and which version the runtime on the ctrlX CORE has? 

Best regards 


Hi Nick!

I am using the 1.18.1 version of OPC UA Client and  v1.18 as runtime on the ctrlX CORE.

Best regards


Occasional Contributor

Hi Aron, 

there was a known issue with ctrlX OPC UA Client App V1.18. Subscriptions to OPC UA Client nodes did fail after a reconnect to the OPC UA Server was done by the client. So I think you can reproduce this error, if you disconnect and connect the OPC UA Server via the web-ui. 

To solve this, you would need to update to V1.20 of the ctrlX OPC UA Client App. This issue is solved here. 



Hi Nick!

Thanks for the help, I will try it.



After further investigation it seems that an "extension object" is currently not supported by the CtrlX. That's why the values (ID, name, description) are even not shown in the CtrlX Data Layer. 



Data Layer: 





Occasional Contributor

Hi @leonberger-m,

you are using the ctrlX OPC UA Client App, right? (from the initial question I was not sure if you are using the Client or the Pub/Sub App)

I think, the problem is, that you are trying to access a complex data type with the OPC UA Client. I took a look into the documentation of the OPC UA Client App and found a list of the supported data types (link to the docu). I think this might not match with your complex data type. 

Best regards, 


Good morning @nickH,

yes that's right. I am using the CtrlX OPC UA Client App. Depending on the documentation it seems that the extension object is currently not on the list of supported data types. 

Maybe in the future there will be a possibility within CtrlX to access these kind of structure. 

Best regards