Z39.50

Z39.50 is an international standard client–server, application layer for searching and retrieving information from a database over a TCP/IP computer network. It is covered by ANSI/NISO standard Z39.50, and ISO standard 23950. The standard's maintenance agency is the Library of Congress.

Z39.50 is widely used in library environments and is often incorporated into integrated library systems and personal bibliographic reference software. Interlibrary catalogue searches for interlibrary loan are often implemented with Z39.50 queries.

Work on the Z39.50 protocol began in the 1970s, and led to successive versions in 1988, 1992, 1995 and 2003. The Contextual Query Language (formerly called the Common Query Language) is based on Z39.50 semantics.

It supports a number of actions, including search, retrieval, sort, and browse. Searches are expressed using attributes, typically from the bib-1 attribute set, which defines six attributes to be used in searches of information on the server computer: use, relation, position, structure, truncation, completeness. The syntax of the Z39.50 protocol allows for very complex queries.

In practice, however, the functional complexity is limited by the uneven implementations by developers and commercial vendors. The syntax of Z39.50 is abstracted from the underlying database structure; for example, if the client specifies an author search (Use attribute 1003), it is up to the server to determine how to map that search to the indexes it has at hand. This allows Z39.50 queries to be formulated without having to know anything about the target database; but it also means that results for the same query can vary widely among different servers. One server may have an author index; another may use its index of personal names, whether they are authors or not; another may have no name index and fall back on its keyword index; and another may have no suitable index and return an error.

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