Z3950

Z39.50 is a standard protocol for information retrieval that enables a client application to search and retrieve bibliographic and other records from remote databases. It was developed by the Z39.50 committee of NISO and first published in the late 1980s; the standard has been revised over time and is formally recognized as ISO 23950 (and as ANSI/NISO Z39.50 in the United States). The goal of Z39.50 is to provide a common, interoperable method for querying diverse catalogs and databases across networked environments.

The protocol defines a client–server architecture in which a client sends search requests to a target database and a server returns result sets and, upon request, the actual records. Core operations include Initialize (to establish a session), Search (to submit a query and obtain a result set), Present (to retrieve one or more records from the result set), Explain (to disclose database capabilities and schema), and Close (to end the session). Queries and data exchanges are carried using an ASN.1 based encoding and are typically transported over TCP/IP.

Z39.50 supports a variety of record formats and metadata schemas, allowing servers to return records in formats such as MARC, Dublin Core, or other library metadata standards. It also provides mechanisms for index scanning and for retrieving large result sets through paged presentations. While influential in library networks and interlibrary loan workflows, Z39.50 has faced criticism for complexity and deployment challenges, leading many institutions to adopt web-based protocols like SRU/SRW (Search/Retrieve via URL) that build on similar concepts but use HTTP and XML or JSON. Nevertheless, Z39.50 remains a foundational protocol in the history of library information retrieval.