system.opcua.callMethod throws ClassCastException when invoking with arguments

Ignition
,
1

Hi, calling a OPCUA method with two InputArguments (ua:String, ua:Int32) does not work. I get a ClassCastException.

result = system.opcua.callMethod("RobotArm", "ns=4;i=532", "ns=4;i=650", ["myArticleId", 100])

java.lang.ClassCastException: java.lang.ClassCastException: class org.eclipse.milo.opcua.sdk.client.nodes.UaVariableNode cannot be cast to class

When calling a method without any InputArguments it works fine:
result = system.opcua.callMethod("RobotArm", "ns=4;i=532", "ns=4;i=653", )

I also turned on the system.opcua.callMethod DEBUG logger. No helpful hints.
Trying to json encode the arguments didn't help either.

The gateway runs on a M1 Mac.

Thanks a lot for help! Toni

2

Hmm, this hints at a perhaps a data modelling problem in the underlying server...

Any chance you can make this OPC UA connection without security and get a Wireshark capture while you call this method? What server is this?

3

It would also be useful if you could provide the full error and stack trace from the logs.

4

I'm using the the opcua-asyncio (GitHub - FreeOpcUa/opcua-asyncio: OPC UA library for python >= 3.7) server. I can invoke the method with the two input arguments with the Prosys OPC UA Browser successfully.

So far I connect to the server without security. So network tracing should be possible.

5

Oh... I don't expect this to work then. Both of those Python OPC UA implementations are broken and they don't seem to care or respond to my bug reports.

1 Like
6

At some point I will rewrite the logic in system.opcua.callMethod that triggers this bug in the Python servers, but it's not a priority right now with all the other work we have going on. It's not a super trivial change.

7

I see. Well, I guess I have to switch to another OPC UA SDK. Thanks anyway.

8

My last post in the thread I linked to has some suggestions.

9

The implementation of system.opcua.callMethod was refactored in Ignition 8.1.43 to work around this bug in FreeOpcUa-based servers.

10

Hi Kevin,

I wonder what I'm doing wrong.
I've copied the example form the manual for the copua.callMethod and I get the following response:


|cwdLogger|20Mar2025 20:59:10|MOBZ.DAL.Msbs.mcSample() output values from the callt: [].|
|cwdLogger|20Mar2025 20:59:10|MOBZ.DAL.Msbs.mcSample() list of StatusCodes, one for each input argument: [].|
|cwdLogger|20Mar2025 20:59:10|MOBZ.DAL.Msbs.mcSample() StatusCode for the call: (2150105088L, u'Bad_SubscriptionIdInvalid', u'The subscription id is not valid.').|

The code is below, executed on the gateway using a tag-change script:

def mcSample():
	
	cwdLogging.wrapLog(cwdLogging.logNaam, ("MOBZ.DAL.Msbs.mcSample()" %()), 2)
	
	# Call the Server object's GetMonitoredItems method.
	result = system.opcua.callMethod(
        "Ignition OPC UA Server",
        "ns=0;i=2253",
        "ns=0;i=11492",
        [1]
    )
 
	# Below we print the various elements in the results. The print statements could easily be replaced by something more useful.
	
	# Prints the StatusCode for the call.
	cwdLogging.wrapLog(cwdLogging.logNaam, ("MOBZ.DAL.Msbs.mcSample()  StatusCode for the call: %s." %( str(result[0])) ), 2)
	
	# Prints the list of StatusCodes, one for each input argument passed to system.opcua.callMethod.
	cwdLogging.wrapLog(cwdLogging.logNaam, ("MOBZ.DAL.Msbs.mcSample()  list of StatusCodes, one for each input argument: %s." %( result[1]) ), 2)
	 
	# Prints the output values from the call.
	cwdLogging.wrapLog(cwdLogging.logNaam, ("MOBZ.DAL.Msbs.mcSample()  output values from the callt: %s." %( result[2])), 2)
11

You're calling a standard method in the OPC UA server called GetMonitoredItems and passing a subscription id that doesn't exist, according to the results.

What are you expecting to happen? Are you just trying to get this working before you call a method on some other server?