DataType syntax

[om327]

Hello,

I've been working on integrating some asyncio servers with LabVIEW's SystemLink OPC UA plug-in.

Following a couple of service requests we've noticed that there is a slight difference how DataType is represented. In the older opcua-python library this is represended as, for example 'String' or 'Double'. The asyncio version seems to display DataType as an ENUM for example i=12 (string). Is there a reason for this change and is it possible to enforce a particular format for DataType as it seems to prevent some clients from connecting?

Many thanks!

[schroeder-]

There isn't any change, DataTypes are always NodeIds, there are no Enums behind it. Maybe you have a minimal example?

[om327]

Hi, thanks for getting back to me!

I've fired up the minimal example for python-opcua and connected using the OPC UA client in SystemLink. It shows the DataType as Double and allows me to create a monitored item.

Double is also returned when using the Open2541 library.

The same minimal example in asyncua and DataType appears as i=11 and can not be used by the client to create a monitored item?

National Instruments believe there is something slightly different between the two impliementations that is causing this?

Thanks again!

[om327]

I've tried to use the same functions and syntax for both implimentations but i'm still getting the same DataType difference when using the asyncio vs pyton-opcua (DataType i=11 rather than DataType=Double)

Interestingly in UaExpert it doesn't seem to make a difference and in both instances DataType is shown as Double.

python-opcua

import time
from opcua import ua, Server

if __name__ == "__main__":

    server = Server()
    server.set_endpoint("opc.tcp://0.0.0.0:4840/freeopcua/server/")

    uri = "http://examples.freeopcua.github.io"
    idx = server.register_namespace(uri)

    myobj = server.nodes.objects.add_object(idx, "MyObject")
    myvar = myobj.add_variable(idx, "MyVariable", 6.7)
    myvar.set_writable()

    server.start()
    
    try:
        count = 0
        while True:
            time.sleep(1)
            count += 0.1
            myvar.set_value(count)
    finally:
        server.stop()

opcua-ascyncio

import asyncio
from asyncua import ua, Server

async def main():
    server = Server()
    await server.init()
    server.set_endpoint('opc.tcp://0.0.0.0:4840/freeopcua/server/')

    uri = 'http://examples.freeopcua.github.io'
    idx = await server.register_namespace(uri)

    myobj = await server.nodes.objects.add_object(idx, 'MyObject')
    myvar = await myobj.add_variable(idx, 'MyVariable', 6.7)
    await myvar.set_writable()
    async with server:
        while True:
            await asyncio.sleep(1)
            new_val = await myvar.get_value() + 0.1
            await myvar.write_value(new_val)


if __name__ == '__main__':
    asyncio.run(main())

[oroulet]

what is you see in the UI does not mean anything i=11 and Double are the same datatype. But it is a bit strange that what is shown in UI is differnet between python-opcua and opcua-asyncio. Something is confusing them. Maybe if you monitor the network with datashark you can find a difference when readong a simple double variable? maybe we are not encoding the nodeid the same way...

[schroeder-]

If compared the differences between python-opcua and asyncua and found the following:

I will make a fix to address the difference.

[oroulet]

if that is true, there is something strange. DataTypeDefinition should only be set for ObjectTypes. No idea where the bug is but this must somewhere very deep

[schroeder-]

The default is an empty ExtensionObject. We must return a error AttributeIdInvalid if the ExtensionObject is empty.
This change triggers some errors in some other code that does expect empty ExtensionObjects instead of StatusCode errors...

[oroulet]

OK looks like your screenshot is for DataType under Types/DataTypes. Great you are looking at it. Did you check the spec for the correct behaviour?

[schroeder-]

There is nothing in the spec about empty DataTypeDefinitions.
Found the following:
node-ua same as asyncua
open62541 same as python-opcua

I think we should return an error to keep some clients happy like UAExpert and maybe NI.

[schroeder-]

@om327 can you test it with fix #937 ?
If it works we can merge it, if it doesn't help I will do some more research about the spec.

[oroulet]

@schroeder- Aren't the clients more happy with None instead of error? I do not remember if we can set an ExtensionObject to None, but we probably can. Maybe that is what python-opcua does?

[oroulet]

we shoul check what prosys does. They are usually very good at following spec.
OK I checked they do:

I may not have the latest server from them though...

[schroeder-]

Ok DataTypeDefinition is optional as of https://reference.opcfoundation.org/v104/Core/docs/Part3/5.8.3/.
So return BadAttributeIdInvalid is ok if it doesn't exists or is empty ExtensionObject.

[om327]

More than happy to test with NI SystemLink OPC UA - thanks for looking into this!

How do i download the latest changes before they are merged?