Adding variable on OPC-UA Server writeable from Clients

[gteti]

Hello everyone,
I'm trying to create a writeable variable on OPC-UA Server. I'm using the standard server example https://github.com/FreeOpcUa/opcua-asyncio/blob/master/examples/server-example.py where I first added a variable like:

myvar2 = await myobj.add_variable(idx, "myvar", 0.0)
await myvar2.set_writable()

And after not being able to change it, I tried adding a new object and set that object to writable as:

myobj2 = await server.nodes.objects.add_object(idx, "MyObject2")
await myobj2.set_writable()
myvar2 = await myobj2.add_variable(idx, "myvar", 0.0)
await myvar2.set_writable()

But I'm still not able to write any value to it from a client. In this last configuration I even get an error:

Traceback (most recent call last):
  File "C:\InLav\Gianluca\Python\Server_opcua\test.py", line 153, in <module>
    asyncio.run(main())
  File "C:\Anaconda3\lib\asyncio\runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "C:\Anaconda3\lib\asyncio\base_events.py", line 647, in run_until_complete
    return future.result()
  File "C:\InLav\Gianluca\Python\Server_opcua\test.py", line 106, in main
    await myobj2.set_writable()
  File "C:\Anaconda3\lib\site-packages\asyncua\common\node.py", line 244, in set_writable
    await self.set_attr_bit(ua.AttributeIds.AccessLevel, ua.AccessLevel.CurrentWrite)
  File "C:\Anaconda3\lib\site-packages\asyncua\common\node.py", line 251, in set_attr_bit
    dv = await self.read_attribute(attr)
  File "C:\Anaconda3\lib\site-packages\asyncua\common\node.py", line 304, in read_attribute
    result[0].StatusCode.check()
  File "C:\Anaconda3\lib\site-packages\asyncua\ua\uatypes.py", line 328, in check
    raise UaStatusCodeError(self.value)
asyncua.ua.uaerrors._auto.BadAttributeIdInvalid: "The attribute is not supported for the specified Node."(BadAttributeIdInvalid)

The error happens when I use the await myobj2.set_writable().

Can you help me ? Am I doing something wrong ?

Thank you

[AndreasHeine]

what does the client return? an ErrorStatusCode like BadNotWritable?
how dose the AttributesTab with UAExpert look like?

[gteti]

Sorry to have bothered you. I've used UaExpert instead of a python opcua-client and was able to write the value to "myvar" correctly. Then I searched what was wrong with the other client and all seems to work.

I get this error when I use the await myobj2.set_writable():

Traceback (most recent call last):
  File "C:\InLav\Gianluca\Python\Server_opcua\test.py", line 153, in <module>
    asyncio.run(main())
  File "C:\Anaconda3\lib\asyncio\runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "C:\Anaconda3\lib\asyncio\base_events.py", line 647, in run_until_complete
    return future.result()
  File "C:\InLav\Gianluca\Python\Server_opcua\test.py", line 106, in main
    await myobj2.set_writable()
  File "C:\Anaconda3\lib\site-packages\asyncua\common\node.py", line 244, in set_writable
    await self.set_attr_bit(ua.AttributeIds.AccessLevel, ua.AccessLevel.CurrentWrite)
  File "C:\Anaconda3\lib\site-packages\asyncua\common\node.py", line 251, in set_attr_bit
    dv = await self.read_attribute(attr)
  File "C:\Anaconda3\lib\site-packages\asyncua\common\node.py", line 304, in read_attribute
    result[0].StatusCode.check()
  File "C:\Anaconda3\lib\site-packages\asyncua\ua\uatypes.py", line 328, in check
    raise UaStatusCodeError(self.value)
asyncua.ua.uaerrors._auto.BadAttributeIdInvalid: "The attribute is not supported for the specified Node."(BadAttributeIdInvalid)

[AndreasHeine]

objects aren't writable at all only variables an properties with the attribute value!

[gteti]

Answer explained by AndreasHeine in the comments!

Is there a way to allocate a variable at a specific address ?

I have a project which uses the old version of opcua server. So I'd like for something like

ns=2;i=30 for my variable (with add_variable)

[AndreasHeine]

you need to understand the namespaceconcept of opcua
ns=0 -> OPC UA Core Namespace
ns=1 -> OPC UA Server own namespace
ns=2 -> custom registered namespace or companionspec. namespace (xml imported namespace)
ns=3 -> ...
ns=4 -> "http://mynamespace"
ns=5 -> ...

Example:

"nsu=http://mynamespace;i=30/"

1. read namespace array -> [... , ... , ... , ... , "http://mynamespace", ...]
2. get index of my nsu -> idx=4 (identifier=30)
3. create a nodeid f"ns={idx};i={identifier}"
4. use class NodeId.from_string(f"ns={idx};i={identifier}") to get your nodeid instance
5. create a node using the nodeid

[gteti]

Thank you for the explaination.
I am trying to do exactly that:

opcua_idx = opcua_server.register_namespace(uri)

myobjtest_nodeid = ua.NodeId.from_string('ns=%d;i=40' % opcua_idx)  
        
myobjtest = objects.add_object(opcua_idx, "Myobj", myobjtest_nodeid)

myvartest_str = f"myvartest"
myvartest = myobjtest.add_variable(opcua_idx , myvartest_str, 0)

This created object is not in the i=30 position and allocated variable under it get also a different _.

In your example using a ns=4 will create the object at ns=4;i=1 (even if I add i=40) and the variable added to it will be ns=4;i=2.

[AndreasHeine]

    opcua_idx = await server.register_namespace("http://test")
    myobjtest_nodeid = ua.NodeId.from_string('ns=%d;i=40' % opcua_idx)  
    myobjtest = await server.nodes.objects.add_object(myobjtest_nodeid, "Myobj")
    myvartest_str = f"myvartest"
    myvartest = await myobjtest.add_variable(
        ua.NodeId.from_string('ns=%d;i=30' % opcua_idx), 
        myvartest_str,
        0.0,
        ua.VariantType.Float
    )

[AndreasHeine]

if you pass the index as first arg like you did it will generate a id -> if you pass the nodeid-class it use that instead ;)

[gteti]

Thank you, it works!
I was following the examples and the creation of the object was not starting with the nodeid and the variable too.

🥇