IETF RFC 2056 PDF

Introduction

Z39.50 is an information retrieval protocol that does not fit neatly into a retrieval model designed primarily around the stateless fetch of data. Instead, it models a general user inquiry as a sessionoriented, multi-step task, any step of which may be suspended temporarily while the server requests additional parameters from the client before continuing. Some, none, or all of these client/server interactions may require participation of the client user, depending only on the client software (the protocol itself makes no such requirements).

On the other hand, retrieval of "well-known" data may be performed in a single step, that is, with a degenerate Z39.50 session consisting of exactly one protocol search request and response. Besides the basic search sub-service, there are several ancillary sub-services (e.g., Scan, Result Set Delete). Among the functions covered by combinations of the sub-services, two core functions emerge as appropriately handled by two separate URL schemes: the Session URL and the Retrieval URL.

Using two schemes instead of one makes a critical distinction between a Z39.50 Session URL, which opens a client session initialized for interactive use by the user, and a Z39.50 Retrieval URL, which opens and closes a client session to retrieve a specific information item. Making this distinction at the scheme level allows the user interface to reflect it on to the user, without requiring the user interface to parse otherwise opaque parts of the URL (consistent with current practice).