A.5 Client use of an AliasName Server

AliasNames allow a Client to find TagVariables or Topics easily. Many industrial systems assign tags to specific measurement or sensors. These tags follow established nomenclatures. An example tag is TI101, which is a temperature indicator at a specific location in a plant. The example nomenclature is defined in ANSI/ISA-S5.1-1984 (R 1992), but this specification does not provide or even suggest any nomenclature. A Client could be configured to display or use the information provided by the sensor TI101, but the actual address/location of this sensor is often not known when the Client is configured. AliasNames can be used to resolve this tag to the actual sensor in the system.

The Client, on start-up or when it wants to access the tag, would call FindAlias on a local Server, aggregating Server or GDS depending on how a system is configured. A Client could select which AliasName source to call via a configuration setting. The FindAlias Method call returns a list of all ExpandedNodeIds that the AliasName has a Reference to. It is important to note that there could be more than one Node referenced by an AliasName and that the Client must be prepared for this. The first Node in the list of referenced Nodes returned by the FindAlias Method is the Node that the Server feels is the best match for the requested tag. The returned list could also return more than one instance of AliasNameType and each could have their own list of referenced Nodes. If the Method call was on a GDS or aggregating Server, the Client reads the ServerArray to resolve which Server the ExpandedNodeId was referencing. This is the last piece of information that a Client requires to be able to follow a normal connection pattern to obtain values from the Node.

AliasName could also be configured to provide other information, such as PubSub information, but again it is only the information that is required to initially subscribe to or configure an item.