How to determine the namespace of an given identifier

[eclipse]

How to determine the namespace of an given identifier

[kevinherron]

I'm not sure what you mean by "only the associated identifier" - are you talking about the client handle you assign a monitored item when creating it, or something else?

Side note: the mailing list may be a better place for general questions like this.

[tosuns]

If I want to read the value of a node I have to provide a NodeId first. One of the constructors is NodeId(int index, String identifier).
In my client code I don’t want to mess with namespace indices. I only want to provide the identifier and the opc ua client should somehow figure out to which namespace the identifier belongs to.

I want the opc ua client to be one of possible client implementations, that could be used. That is why I try to create a client facade/service to possible servers, like opcua or beckhoff ads, bachmann and so on. Not all of them have a concept of namespaces, but alot of them have the concept of identifiers/symbols.

[kevinherron]

You always need the NodeId.

I think the only alternative would be using the TranslateBrowsePathsToNodeIds service, but you still need to be storing the whatever it is in a format that you could turn into a BrowsePath for the translate service call.

[ctron]

I actually stumbled over the same issue. In order to access a variable you also need the NodeId, and therefore the index. However to me it seems (as far as I look into the server side code) that the index is not static as the URI is. So there is some sort of mapping on the server side from URI -> Index.

So assuming in the client that the index never changes seems to be wrong to me, and going with the URI is the better approach. What I did is that I do make a lookup on the server table:

CompletableFuture<DataValue> future = this.client.readValue(0, TimestampsToReturn.Neither,
   Identifiers.Server_NamespaceArray);

   return future.thenApply(value -> {
      final Object rawValue = value.getValue().getValue();

      if (rawValue instanceof String[]) {
         final String[] namespaces = (String[]) rawValue;
         for (int i = 0; i < namespaces.length; i++) {
         if (namespaces[i].equals(namespaceUri)) { return Unsigned.ushort(i); }
      }
   }
return null;
});

I am not sure if this is the proper way and I also could imagine that the lookup result could be cached on the client side.

[kevinherron]

So what @ctron is suggesting is the most correct approach if you're going to be storing NodeIds. In practice I have never actually seen the need for this, as all the servers we deal with tend to have fixed namespaces 0, 1, and 2. (0 will never change. 1 is the namespace URI of the server; it shouldn't change. 2+ may change depending how the server uses namespaces).

[ctron]

So what is the correct way to extend the #1 namespace?

[kevinherron]

Right now the SDK registers a pretty sparse Namespace implementation called VendorNamespace as namespace 1. You could easily register your own Namespace at slot 1 instead if you wish to provide different/additional nodes.

In my experience with OPC-UA servers, such as Kepware etc..., all the nodes for configured devices and whatever else they expose are in namespace 2.