Introduction --- What is it for?
What is it purpose?
How was it developed?
How does it works?
References?
Books and index cards are what used to be found in a library, the later are to search for the former. With the use of large capacity mainframe computers in the past decade and progress in telecom networking technology a few years ago, huge network of networks of many libraries' books in different geographical locations are searchable in seconds. A few years ago, you will have to ask the librarian in your college to ask the librarian in, e.g., University of California to see if they have a book you want. Now you can do it yourself.
Why is it important? --- An example, MELVYL. MELVYL is the name of an online catalog. Online meaning it is from a mainframe computer which you who have a password, = users, can dial in to access the information. Catalog meaning it is an index, in fact many indexes. Index of names, of book titles, of subjects, .... index of indexes (what index is available, and specialized subject database indexes). In this case index of all the University of California 19 campuses' and California State University campuses' libraries' books (i.e. collection).
Before having this electronic, computerized, online catalog, one can rely on a book-form catalog, or the index card cabinet in a library to find out what book or subject they would like to read in one particular library. Since subject is subjectively determined by librarians of "different denominations", "false-drops" result (i.e, retrieved imfo are not what you wanted). A combination of searches of an author name, subject, publishing years .... and other attributes may require cumbersome methods and days to complete, or in practise just not possible. With the use of computer system, a new revolution of "keyword search" appear. Any word can be searched (except the common ones: a, an, the, .... etc., which result in humogous amount of false drops) and every book record with the particular word appear on screen in seconds. Combination of searches with names, any words, subjects, ... etc became possible.
Better yet, being online and networked, searching other libraries in other geographical location is also possible. A user from Hawaii can find out what is available in U. C. libraries without flying there.
It replaces not only the index card cabinets, it even replaces the journal index, journal abstracts, and in many cases, the journal itself; since all of these are being input into specialized subject computer database for sale in CD ROM format or by monthly subscription networked to your library catalog network (such as MELVYL).
Hopefully it is a matter of (a short) time that, with a lower annual subscription fee, libraries can simply order journals to be read and downloaded on line, saving everybody's time and energy in getting, shelving, xeroxing, and re-shelving.
As it is a network of many many different networks, all the machines have to speak a common language / protocol. The Z39.50 protocol evolved out of the Linked Systems Project, which involved the Library of Congress, the Online Computer Library Center (OCLC), the Research Libraries Information Network (RLIN) and the Western Library Network (WLN). This project built a prototype network to transfer records among these groups (of major "book index catalogs"), which became a national standard.
The Z39.50 protocol provides a mechanism for transmitting and managing queries (search questions) and result sets using the client-server model. The searches that the client (computer) performs against their server create result sets. Records from these result sets can be presented to the client and / or combined with other searches to produce new result sets. The organization and storage of the databases on the server is not limited by this protocol, what is prescribed is the mechanism for communication of searches and the presentation fo results between the client and server.
The MELVYL software runs on an IBM mainframe computer. Most of the MELVYL user interface software is written in PL/1. The database management software used by MELVYL system, as well as the communications and networking software supporting it are written in IBM assembler language.
As long as you have access to any college of public libraries that is networked to MELVYL you can follow the instructions on screen and arrive at this catalog. Simple command such as "find", "display" will tell the computer to search for your wishes.
Unfortunately the "bibliographical entries" (i.e., librarian-classified subject categories) are not searchable. And many subject specific database are proprietary, accessible only to the privileged. Although search-ability is revolutionarily improved, accessibility awaits leaping advancement, getting to the real book / article is not feasible at presence. (Although getting to the article is within easier reach with services such as full text delivery).
Hopefully in the near future, multi-media computer technology (such as electronic books) will enhance the accessibility of full text content inside. .
Lynch, Clifford A. and West, Richard (1992). Special section: Happy birthday to Melvyl. Part 3. Information Technology and Libraries. 11 (4) 405-419.
Besser, Howard (1990) Visual access to visual image: the UC Berkeley image database project. Libray Trends 38 (4) 787-798.
Lynch, Clifford A (1992). Beyond the ordinary card catalog: Melvyl learn from years of experience. EUDCOM Review 27 (6) 20-23.
Lynch, Clifford A (1989) From Telecommunications to networking: the Melvyl online union catalog and the development of intercampus networks and the University of California Library Hi. Tech. 7 (2) 61-83.