tagType and tagSet OID relationship

Issue raised by: Ronald van der Meer Databasix Information Systems B.V. Mon, 7 Jul 1997 21:33:07 +0200

Is there a relation between the tagType value and the last number of the tagSet OID? For example tagSet-M is 1.2.840.10003.14.1 and the tagType value is 1, tagSet-G is 1.2.840.10003.14.2 and the tagType value is 2. The problem starts with tagType value 3 which should correspond to tagSet-STAS but is defined as 'locally defined'.

Schemas definitions provide mappings of (scalar integer) tagType to tagSet. Tag types 1, 2, and 3 are well-known (see Appendix 12, TAG, introductory prose prior to TAG.1.). Beginning with 4, the tagSet referenced depends upon the schema that is in effect. For example, in a GRS retrieval record where tagType 4 occurs, if the Collections profile is in effect, it refers to the collections tagSet. If the GILS profile is in effect, tagType 4 refers to the GILS tagSet.

So the fact that the STAS tagSet is assigned the OID 1.2.840.10003.14.3 does not mean that tagType 3 is to be used for STAS. Note also that tagSet oid 1.2.840.10003.14.1 is assigned to WAIS and 1.2.840.10003.14.2 is assigned to GILS, and, as you note, tagTypes 1 and 2 are reserved for M and G.

The two tagSets M and G are always to be referenced by tagTypes 1 and 2. In contrast, tagType 4 doesn't globally identify any specific tagSet; it identifies a tagSet only in the context of a specific schema, and the schema definition provides this identification by mapping 4 to a specific oid, which does identify a tagSet globally.

GILS would use type 4 to express GILS elements, when operating under the GILS schema, and another schema could import one or more GILS elements, using a different tagType, say 5, because it is using 4 for its own defined tagSet -- for example, if the Collections profile ever needed to import a GILS element into its abstract record structure, it wouldn't use 4, because 4 is already assigned. It might use 5, and then the Collection schema mapping would be modified to reflect this assignment.

Note also, the wais schema doesn't even define a tagSet of its own, it defines an abstract record structure where all the elements come from tagSets 1, 2, and 3.

So this means that there is not a one-to-one relationship between schemas (or abstract record structures) and tagSets: some use a single (registered) tagSet, some use more than one, and some use none. That was the reasons that attempts were abandoned (early in the development of the retrieval architecture) to maintain a correspondence between tagType and, say, the last component of the oid.

Note: this was originally proposed as a "ZIG Comentary" but was changed to a "clarification" at the August 97 ZIG meeting.