Question:
Why use tags? What is the purpose of TagsetInfo in Explain?
Response:
A tag may be thought of informally as a field name -- the name of a field (or "element" as it is called in Z39.50) in a database record. For example, in a personnel record you might have the field "years of service", an integer. Without a field name, you may be able to discern that the datatype of the field is an integer (that would be conveyed via ASN.1) but the value isn't of much use unless you know what it represents, that is, unless the field name is also conveyed. Typically, for Z39.50, tagSets are developed for specific applications, that list field names and description, and assign each field a numeric tag (an integer). So, for example, say "years of service" is assigned the numeric tag 10, when it is encoded in a GRS-1 record, the value 10 is understood to mean "years of service" because it is accompanied by a "tagType" which identifies the tagSet.TagSetInfo is included as an Explain category to allow a client to discover information about a tagSet supported on the target. The target can convey, for a supported tagSet, the identifier of the tagSet (tagSets are registered; they are assigned a global Object Identifier), and for each supported element, its name, tag, datatype, and description.
How (and if) this Explain category will be put to practical use remains to be seen. One scenario involves collaboration between the end-user and client, where the client retrieves the TagSetInfo information and conveys the names and descriptions to the end-user, who then decides which elements are of interest and requests the client to retrieve those specific elements, which the client requests via their tags. In this scenario, the semantics of an individual element are conveyed transparently to the end-user, so the client never needs to understand their meaning, only their datatype and tag; and the end-user never knows the tag or datatype.
Status: Approved (8/97)
Library
of Congress(10/23/97)