Extensible Markup Language

XML describes structured data packages that move around the Web as easily as HTML.

Jon Udell

Applications based on simple, structured ASCII text -- e-mail, Usenet news, the Web -- make the Internet hum. These core applications all rely on fixed data-exchange formats. Extensible Markup Language (XML) enables extensible data-exchange formats. XML is Web-style Electronic Data Interchange (EDI).

Here's how XML might enable a work-flow application at BYTE. A vendor schedules a demonstration. The appointment data, entered on a Web form, lands in an XML file that then acquires annotations as it moves through the system ( see the figure ).

Today, Web developers routinely invent such application-specific formats. But supporting them means a lot of custom work to read, parse, annotate, and store the data, plus format it for display. With XML and supporting tools, much procedural work should go away. Data definition moves to a declarative mode, using SGML-style (Standard Generalized Markup Language) Document Type Definitions (DTDs) or the newly proposed XML-Data schemata. Parsing and validation of data also don't need procedural logic. Even formatting for display can in theory be declarative. It's a mapping between an XML object model and a browser's object model.

Because SGML DTDs aren't written in SGML, Microsoft is proposing XML-Data schemata. The idea is to write XML metadata using XML, speed development of XML applications, and simplify validation of both structure and (eventually) content. Extensible Style Language (XSL), proposed by Microsoft, ArborText, and In so, addresses rendering XML data in browsers. XSL wants to "embrace and extend" the still-nascent cascading style sheets (CSS) model, which depends on script interaction with the browser's document object model. XSL aims to move toward a declarative model that relies on the advanced formatting of Document Style and Semantics Specification Language (DSSSL).

Early applications of XML include Channel Definition Format (CDF), which defines the packages of data that govern how browsers interact with Webcasting services, and Open Software Distribution (OSD), which aims to describe the resources and dependencies of installable software. But the field is wide open. In 1998, we'll see all kinds of line-of-business applications using XML to move structured data around on intranets and the Internet. For server-based applications, there are few obstacles. For client-based applications, though, incompatible Microsoft and Netscape implementations of DHTML and browser document object models will cause big headaches.

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Where to Find

The W3C working draft:


Internet: 
http://www.w3.org/pub/WWW/TR/WD-xml.html
. 


Microsoft initiatives: 


Internet: 
http://www.microsoft.com/standards/xml/




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Web Work Flow, the XML Way

illustration_link (35 Kbytes)

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W3C Draft and Microsoft Initiatives in 1998

illustration_link (13 Kbytes)

AT A GLANCE: XML lets Web developers represent object data as tagged text, easily exchange data among cli ents and servers, and allow rich tools for parsing, validating, and rendering data in browsers.

WHO SUPPORTS IT: Sun, Microsoft, and Netscape. Software distributors: Microsoft, Marimba. Webcasters: PointCast, DataChannel. XML tools: ArborText, Inso.