Interactive Information Services Using World-Wide Web Hypertext - (next, previous section)

Introduction

World-Wide Web (WWW) is a wide-area hypermedia information retrieval project started in 1989 by Tim Berners-Lee at the CERN European Laboratory for Particle Physics [Bern1]. WWW merges the techniques of information retrieval and hypertext to make an easy to use and powerful global information system. The WWW project is now being carried on cooperatively by several groups and individuals at sites throughout the world. The use of World-Wide Web browsers and servers for a wide variety of applications has increased dramatically over the last year [Hugh1].

WWW Hypertext is represented using Hypertext Markup Language (HTML), which is a simple SGML document type allowing structured text with links [Bern2, Ragg1]. In WWW, the activation of a link causes the retrieval of a new hypertext document as specified by a Uniform Resource Locator (URL). Each URL specifies the protocol, server, and a document identifier used to retrieve a particular document [Bern3].

A URL may specify any of a number of document retrieval protocols. The Hypertext Transfer Protocol (HTTP) was designed specifically for communication between WWW clients and servers [Bern4]. HTTP has the advantages of being fast, stateless, extensible, and easy to implement for both clients and servers.

[Diagram of WWW Client and Server]

HTML and HTTP work together to provide easy access to documents and information in a wide area distributed network such as the Internet. In a typical example, a client program communicates with a remote server program using the HTTP protocol, requesting some information using a URL. The server responds with the requested information encoded as an HTML document. The client displays the document to the user. Portions of the HTML document may be linked (via a URL) to other documents on the same or different server. The user can follow a link and retrieve the referenced document with a simple mouse click or key press.

Most HTTP servers operate as a hypertext file server, providing access to stored HTML and text documents. Commonly used HTTP server software, such as the CERN httpd, NCSA httpd, and Plexus all provide this capability [Bern5]. However the HTML hypertext format and the HTTP retrieval protocol can provide more than simple document browsing and retrieval. With additional server software, the same protocol and generic browsers can allow access to may kinds of interactive information sources while providing customized user interfaces. The document returned by an HTTP server need not be stored in a file. Instead of corresponding to a file name, a URL can cause the server to dynamically generate a new document based on information in the URL. In order to do this, the server must define a naming scheme to define what documents get created for different URLs [Bern6].

HTTP server implementors are now adopting a Common Gateway Interface (CGI) standard so that custom scripts can easily be invoked by a generic HTTP server program for appropriate URLs. The CGI allows a generic server program to process HTTP/1.0 headers, logging of requests, and access to static documents, while custom CGI scripts provide an interface to site-specific services [McCo1].

Two interactive HTTP services are described in this paper. The PARC Map Viewer uses a geographic database to create and display maps of any part of the world on demand. The Digital Tradition folk music server provides access to a large database of song lyrics and melodies. Both of these services are implemented using the perl scripting language [Wall1] running under the Plexus HTTP server on a Sun Unix workstation.

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