Add a reference to a node in a different server's address space

[FMeinicke]

Hi,

I would like to be able to add a reference to a node that is not in the address space of the local server but in the address space of another server that is available in the same network.

According to the OPC UA specification, this should be possible by adding the URI of the other server to the ServerArray property of the local server (https://reference.opcfoundation.org/Core/Part5/v105/docs/6.3.1). Then I should be able to use an ExpandedNodeId with the ServerIndex set to the index of that server in the ServerArray to identify any node in the address space of the other server.

Looking at the code, I think this is currently not yet possible. I also found FreeOpcUa/python-opcua#557 which mentions the same thing but for the sync-only version of the library. That issue wasn't resolved, so I believe there hasn't been done any work on this feature, yet.

Since I don't know, how this feature could be implemented, I'd like to ask if there is someone who would be willing to invest time in adding this feature.

Secondly, provided that this feature is implemented, what would I have to use as the URN of the other server in the ServerArray? Is that the URL of the other server (i.e. myotherserver.hostname:4840, maybe even with the opc.tcp:// protocol), or just its domain name (myotherserver.hostname)?
I guess the answer to this question depends on the implementation, so this can probably only be answered once the feature is implemented...

Thanks in advance!

[AndreasHeine]

https://reference.opcfoundation.org/Core/Part3/v105/docs/4.4.4

The TargetNode of a Reference may be in the same AddressSpace or in the AddressSpace of another OPC UA Server. TargetNodes located in other Servers are identified in OPC UA Services using a combination of the remote Server name and the identifier assigned to the Node by the remote Server.

OPC UA does not require that the TargetNode exists, thus References may point to a Node that does not exist.

So you can just add the server to the ServerArray ("urn:<your own urn here!>") and use the index of the added server in the expanded nodeid!

Index 0 is your own "urn:" (AppUri) and not the remote so you should have at least index 0 and index 1 after adding!

https://reference.opcfoundation.org/Core/Part4/v104/docs/7.11

Index that identifies the Server that contains the TargetNode. This Server may be the local Server or a remote Server.

This index is the index of that Server in the local Server’s Server table. The index of the local Server in the Server table is always 0. All remote Servers have indexes greater than 0. The Server table is contained in the Server Object in the AddressSpace (see OPC 10000-3 and OPC 10000-5).

The Client may read the Server table Variable to access the description of the target Server.

[FMeinicke]

If I understand correctly, then add_reference() would need to accept NodeIds that point to other server's address spaces.
Then a client (e.g. UaExpert) when it browses the address space of server1 sees that reference to the node in server2's address space and it (the client, so UaExpert) must figure out how to "resolve" the URN of server. Is that what the spec says?
(To be honest, I'm not that deep into the whole OPC UA spec. I saw that there is this feature where a server can reference nodes in other servers, which is something that I would need for the device I'm currently modeling in OPC UA.)

[FMeinicke]

if addref.TargetNodeId not in self._aspace: 
         return ua.StatusCode(ua.StatusCodes.BadTargetNodeIdInvalid) 

probably this is a false check according to spec.!

if addref.TargetNodeId not in self._aspace: 
         # only return Error if ServerIndex is 0 !!!
         return ua.StatusCode(ua.StatusCodes.BadTargetNodeIdInvalid) 

Yes, that's what I meant with add_reference() would need to accept NodeIds that point to other server's address spaces.

[AndreasHeine]

If I understand correctly, then add_reference() would need to accept NodeIds that point to other server's address spaces. Then a client (e.g. UaExpert) when it browses the address space of server1 sees that reference to the node in server2's address space and it (the client, so UaExpert) must figure out how to "resolve" the URN of server. Is that what the spec says? (To be honest, I'm not that deep into the whole OPC UA spec. I saw that there is this feature where a server can reference nodes in other servers, which is something that I would need for the device I'm currently modeling in OPC UA.)

thats what i know you just get informed with the ServerIndex that its not in this server and usually via opc ua global/local dicovery service you have to figure out with that "urn" is registered at the discovery server and maybe what the actual endpoint is -> from that you need to create a new client to do whatever you want to do (dont think spec defined the logic for this the number of usecases is infinite)

[FMeinicke]

Ok, I see. So I probably need a discovery server in the network alongside the two servers that reference each other, so that clients can ask the discovery server how they can connect to the server with "urn:my:opcua.other_server".

[AndreasHeine]

@FMeinicke yes