Problems with the bib-1 attribute set began to surface at that time (1992). Within the bibliographic community, implementors had no published definitions of the bib-1 attribute semantics, thus vendors implemented the bib-1 attribute set with their own interpretations of the attribute usage. A document was produced to clarify this (Bib-1 Semantics Document), although it was never formally included as part of the standard.
As the Internet grew, more communities wanted to implement Z39.50 and, in turn, needed additional attributes (beyond those already in bib-1) to reflect the types of data they wanted to exchange. This proved difficult as Z39.50-1992 did not allow a query to include attributes from more than a single attribute set. Since bib-1 was the only publicly visible set, it was expanded to accommodate the needs of these communities. Thus, bib-1 grew without plan or rigor, evolving away from the bibliographic community where it had started, and "bib-1" became somewhat of a misnomer as it grew into a global set of attributes.
In 1994 and 1995, as Z39.50 version 3 was being finalized and as Z39.50 began to be widely implemented, additional concerns arose over the relationships among attribute sets that other groups were developing, notably the STAS and GILS attribute sets. The Z39.50 Implementors Group (ZIG) had many questions about the development and implementation of multiple attribute sets, including duplication of attributes across sets. In early 1996 a discussion paper by Cliff Lynch (Defining and Maintaining Attribute Sets for Use with the Z39.50 Protocol: A Discussion Paper ) detailed the issues:
The major conclusion of the group was that a new architecture for attribute sets should be developed; they went on to recommend an architecture based on classes of attribute sets, with expanded attribute types. Another major conclusion was that expert communities, rather than the ZIG, should be responsible for developing and maintaining attribute sets (following the example set by GILS and STAS). Notably, they recommended that the bibliographic community, rather than the ZIG, develop the next generation of bibliographic attributes. The ZIG should continue to be responsible for attributes that are general to Z39.50, that is, not specific to a given community.
The type-1 query consists of one or more search terms, each with a set of attributes, specifying, for example, the type of term (author, title, subject, etc.), whether the term is truncated, its structure, etc. The server is responsible for mapping attributes to the logical design of the database.
A term in a type-1 query, together with its accompanying collection of attributes, is called an operand. Operands may be combined in a type-1 query, linked by boolean operators (And, Or, And-not, and Proximity).
Each attribute is a pair: an attribute type and a value of that type. An Attribute set defines a set of attribute types, and for each type defines the set of possible values.
An attribute set definition is assigned an object identifier, referred to as its attribute set identifier.
Example: The bib-1 attribute set defines a number of attribute types; one of which is Use. For bib-1 Use attributes, many attribute values are defined, one of which is personal name. Each type is assigned a numeric value, and each value is assigned a numeric value: type Use is assigned the value 1, and Use attribute Personal Name is assigned the value 1. Thus bib-1 Use attribute Personal Name is represented as the pair (1,1). This pair is further qualified by the bib-1 attribute set identifier (1.2.840.10003.3.1) to distinguish it from the pair (1,1) that may be defined by another attribute set.
Version 2 of Z39.50 has two serious limitations inhibiting the development of attribute architecture, both corrected in version 3:
This architecture strongly recommends that an attribute set definition that conforms to a particular class but defines attribute types that are not defined for that class should carefully define the interactions between the new attribute types and existing types defined for that class.
The architecture provides the attribute-set-class approach to allow flexibility and future expansion within the existing architecture. It is believed that attribute set Class 1 meets all known needs for an attribute class at this time. There may be other approaches developed which partition the set of attributes into fundamentally different types. This might result in the definition of a new attribute class inconsistent with Class 1. However, no need for such a separate class has been identified and it is not known whether additional classes will be necessary.
An Attribute set may define the set of values for a particular attribute type as follows:
The purpose of enumerating all of the possible attribute types within this "universal" attribute class is to provide a template for developers of attribute sets, and to set up a framework for interoperability among independently defined attribute sets which are intended to serve various communities. In particular, it should be possible for groups of content experts to develop new Access Point attributes, ASN.1 datatypes, comparison operators, and perhaps Format/Structure attributes which fit comfortably within this framework. Based on the template defined here, server developers may recognize attribute types omitted in a query operand, as well as illegal repetitions or combinations of attributes of given types that would indicate a malformed query operand.
When an attribute set is intended to conform to Class 1, its definition should:
Any Class 1 attribute set follows the rules prescribed for Class 1 that apply to attribute types defined for that set. However, a Class 1 attribute set need not define nor populate every attribute type defined for Class 1. A Class 1 attribute set may define as few as one attribute type, or as many as all of the attribute types defined for Class 1.
Thus no specific attribute type is mandatory in the sense that it must be included in an attribute set definition. (This use of the term mandatory is different from the use of mandatory to mean that a particular attribute type may not be omitted in an operand, as used in the Occurrence column of the table in 3.3. For example, the Comparison attribute type is mandatory in an operand.)
However, a Class 1 attribute set must use the numeric values in the "Type Number" column in the table in section 3.3, to represent the types; if any of these types is omitted in the attribute set definition, the definition should skip the value for that type rather than renumber.
An attribute set might be developed for an application or profile and may refer to values of a particular attribute type that are defined by a different attribute set. If all of the values of that type that are required by the application are already defined by that other attribute set, then that attribute type need not be defined for the new set.
There may often be a close relationship between the development of a profile for a particular application, and the development of an attribute set definition to support the application. The profile might refer to several attribute sets in describing how to construct query operands (or entire queries). Thus the attribute set definition is not, itself, responsible for specifying all of the details of searching for the application when those details involve attributes from different attribute sets; however, the attribute set may offer as much commentary as it deems necessary and appropriate, for example, it may explain why a particular attribute has been omitted from its definition (for example, because another attribute set has defined it). It might explain how certain attributes that are defined in the set are to be combined with attributes from other sets. >