From the root of any sub tree in the address space of the UA A&C Server, the A&E COM Client may request the list of areas and/or sources contained within that level. The resultant list of area names or source names will consist of the set of browse names belonging to those Nodes which meet the criteria for area or source designation as described above. These names are “short” names meaning that they are not fully qualified. The A&E COM Client may request the fully qualified representation of any of the short area or source names. In the case of sources, the fully qualified source name returned to the A&E COM Client will be the string encoded value of the NodeId as defined in OPC 10000-6 (e.g., “ns=10;i=859“). In the case of areas, the fully qualified area name returned to the COM Client will be the relative path to the notifier Node as defined in OPC 10000-4 (e.g., “/6:Boiler1/6:Pipe100X/1:Input/2:Measurement“). Relative path indices refer to the namespace table described below.
UA Server Namespace table indices may vary over time. This represents a problem for those A&E COM Clients which cache and reuse fully qualified area names. One solution to this problem would be to use a qualified name syntax which includes the complete URIs for all referenced table indices. This however would result in fully qualified area names which are unwieldy and impractical for use by A&E COM Clients. As an alternative, the A&E COM UA Proxy will maintain an internal copy of the UA A&C Server’s namespace table together with the locally cached endpoint description. The A&E COM UA Proxy will evaluate the UA A&C Server’s namespace table at connect time against the cached copy and automatically handle any re-mapping of indices if required. The A&E COM Client can continue to present cached fully qualified area names for filter purposes and the A&E COM UA Proxy will ensure these names continue to reference the same notifier Node even if the Server’s namespace table changes over time.
To implement the relative path, the A&E COM UA Proxy maintains a stack of INode interfaces of all the Nodes browsed leading to the current level. When the A&E COM Client calls GetQualifiedAreaName, the A&E COM UA Proxy first validates that the area name provided is a valid area at the current level. Then looping through the stack, the A&E COM UA Proxy builds the relative path. Using the browse name of each Node, the A&E COM UA Proxy constructs the translated name as follows:
QualifiedName translatedName = new QualifiedName(Name,(ushort) ServerMappingTable[NamespaceIndex]) where
Name – the unqualified browse name of the Node
NamespaceIndex – the Server index
the ServerMappingTable provides the Client namespace index that corresponds to the Server index.
A ‘/’ is appended to the translated name and the A&E COM UA Proxy continues to loop through the stack until the relative path is fully constructed.