During most communication between the client and the server, OPC Unified Architecture uses namespace indices, which are integer numbers, to distinguish between namespaces. The namespaces indices reference a namespace table made available by the server, and only the namespace table actually contains the namespace URIs. The order of entries in the server’s namespace table can change between sessions. For this reason, namespace indices should not be persisted, as they alone are not generally sufficient to identify a namespace and thus a node.

There are, however, situations where you may want to include the namespace index in the expanded node Id string: 

• For optimization. If the namespace index is given, it may be slightly faster to reference the node.
• Sometimes the party you are working with does not fully understand the consequences of using a namespace index, and only gives you node Ids with the namespace indices, without the namespace URIs.

For situations like this, QuickOPC-UA allows the use of “ns=…” notation for a namespace index inside the expanded node ID strings, and has two additional provisions:

• When expanded node ID strings are provided out of QuickOPC-UA components, they contain BOTH the namespace URI, and the namespace index. For example, the browsing may return following expanded node ID string to you:

  nsu=http://test.org/UA/Data/;ns=2;i=10219 

• When QuickOPC-UA receives an expanded node ID string on input, it uses the namespace URI specified in the string by the “nsu=…” notation, if present. If the namespace URI is not present, a namespace index, specified by the “ns=…” notation (or defaulted to zero), is used. If both the namespace URI and the namespace index are present, semantically it always behaves the same as if only the namespace URI was specified, but the namespace index may be used for performance optimization. This means that the expanded node ID strings provided out of the component always refer to nodes through the namespace URI, even if they (in addition) contain the namespace index.

Note that namespace index 0 (zero) is reserved for the standard namespace maintained by the OPC Foundation ("http://opcfoundation.org/UA/"), and is not usually specified inside the expanded node ID strings. In addition (as an exception to the rule laid out above), it is safe to use nodes inside namespace index 0 without specifying their namespace URI, because the namespace index is "tied" to the URI by the provision in the standard, and the OPC servers are not allowed to change that.