Z39.50 was designed to enable communication and the exchange of data between computer systems such as those used to manage library catalogs. This communication could be between a cataloger's PC (or an OPAC terminal in public use) and the library catalog itself. It could be between two catalogs or a group of catalogs sharing data together. In any case, the purpose was to develop specifications allowing different computer systems to link and exchange data. It is a mature standard that represents the culmination of two decades of thinking and debate about how information retrieval functions can be modeled, standardized, and implemented in a distributed systems environment. The current version of Z39.50 is more properly known as "North American standard ANSI/NISO Z39.50-1995, Information Retrieval (Z39.50): Application Service Definition and Protocol Specification"; or, also known as the matching international standard "ISO 23950:1998, Information and documentation -- Information retrieval (Z39.50) -- Application service definition and protocol specification." This standard was processed and approved for submittal to ANSI by the National Information Standards Organization. It was balloted by the NISO Voting Members (of which AALL is a member) March 29, 2002 - May 13, 2002, and will next be reviewed in 2007.

Determining current versions of the standard can be a bit complicated. According to the introduction, Z39.50-1995 specifies versions 2 and 3 for the Z39.50 service and protocol. The standard Z39.50-200X also specifies versions 2 and 3, and additionally, incorporates many clarifications, amendments, defect corrections, and implementer agreements, all of which have been endorsed by the Z39.50 Implementers Group. Z39.50-1992 specifies version 2 only. Version 2 of Z39.50 is assumed identical to version 1 of Z39.50; thus implementations that support version 2 automatically support version 1. Implementations that support version 3 are required to support version 2 (and thus version 1 as well).

Clifford Lynch points out that, although the historical events leading to the development of Z39.50 are sometimes tracked back to the 1960s, momentum to standardize an information retrieval protocol began to sharpen in the early 1980s with the beginning of the Linked Systems Project, LSP, whose implementation began in 1982, and which became operational in 1985. The participants were the Library of Congress, RLG, and OCLC. The essence of LSP was the Authorities application: the establishment and maintenance of a nationwide database of name authority records. Two application level protocols were developed: Record Transfer and Information Retrieval. The primary function of the authorities application was the transfer of the authority records between systems supported by the Record Transfer protocol. A background function was the intersystem searching of authority records, supported by the Information Retrieval protocol.

Both the Record Transfer and information retrieval protocols were developed to support authority record exchange, but were intended to support record exchange and intersystem searching regardless of record type. In 1983 the LSP participants submitted both protocols, Record Transfer and Information Retrieval, for consideration as American National Standards. For Record Transfer, attempts to standardize were eventually abandoned (and ultimately, the Record Transfer protocol itself was replaced by FTP).

There was however substantial interest within the U.S. in standardizing an information retrieval protocol, and the LSP Information Retrieval protocol was submitted to ANSI/NISO, which formed a committee that prepared it for ballot, in 1984, when it was given the designation "Z39.50", as it is known today. It is a protocol that specifies data structures and interchange rules that allow a client machine (called an "origin" in the standard) to search databases on a server machine (called a "target" in the standard) and retrieve records that are identified as a result of the search.

The formal home of the standard is the Z39.50 Maintenance Agency, hosted by the Library of Congress. Continued development takes place within an informal group of implementers and developers known as the Z39.50 Implementers Group, or ZIG. The work of the ZIG is progressed on an active mailing list, and through two or three face-to-face meetings each year.

The Z39.50 Maintenance Agency maintains a list of implementers on their web site. It is a diverse group made up of digital library projects (such as the California Digital Library) traditional ILS vendors, the British Library and the European Space Agency.

According to the Z39.50 Maintenance Agency website, the name "Z39.50" comes from the fact that the National Information Standards Organization (NISO), the American National Standards Institute (ANSI)-accredited standards development organization serving libraries, publishing and information services, was once the Z39 committee of ANSI. NISO standards are numbered sequentially and Z39.50 is the fiftieth standard developed by NISO.

The future of Z39.50 centers on ZING, "Z39.50-International: Next Generation". It covers a number of initiatives by Z39.50 implementers to make the intellectual/semantic content of Z39.50 more broadly available and to make Z39.50 more attractive to information providers, developers, vendors, and users, by lowering the barriers to implementation while preserving the existing intellectual contributions of Z39.50 that have accumulated over nearly 20 years.

One of the current ZING initiatives is SRW, the "Search/Retrieve Web Service" protocol, which aims to integrate access to various networked resources, and to promote interoperability between distributed databases, by providing a common utilization framework. SRW is a web-service-based protocol whose underpinnings are formed by bringing together more than 20 years experience from the collective implementers of the Z39.50 Information Retrieval protocol with recent developments in the web technologies arena.

The SRW Initiative, building on Z39.50 along with web technologies, recognizes the importance of Z39.50 (as currently defined and deployed) for business communication, and focuses on getting information to the user. SRW provides semantics for searching databases containing metadata and objects, both text and non-text. Building on Z39.50 semantics enables the creation of gateways to existing Z39.50 systems while reducing the barriers to new information providers, to make their resources available via a standard search and retrieve service.

As a standard, Z39.50 has been with us for a long time. It has become almost ubiquitous in libraries as we regularly transfer data from OCLC to our local ILS, or move records between catalogs within our own consortia. Its applications, however, go well beyond libraries and library automation. With continued support, they may go further yet.

For further information:

• The Z39.50 Information Retrieval Standard: Part I: A Strategic View of Its Past, Present and Future / Clifford A. Lynch (D-Lib Magazine, April 1997) (http://www.dlib.org/dlib/april97/04lynch.html)
• The Z39.50 Maintenance Agency (http://lcweb.loc.gov/z3950/agency/)
• Z39.50 for All / Paul Miller (Ariadne, no. 21, April 1999) (http://www.ariadne.ac.uk/issue21/z3950/intro.html)