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<!DOCTYPE HTML PUBLIC "html.dtd"><HTML><HEAD><TITLE>Presenting XML:Potential Applications of XML:EarthWeb Inc.-</TITLE><META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW"><SCRIPT><!--function displayWindow(url, width, height) { var Win = window.open(url,"displayWindow",'width=' + width +',height=' + height + ',resizable=1,scrollbars=yes');}//--></SCRIPT></HEAD><BODY BGCOLOR="#FFFFFF" VLINK="#DD0000" TEXT="#000000" LINK="#DD0000" ALINK="#FF0000"><TD WIDTH="540" VALIGN="TOP"><!-- <CENTER><TABLE><TR><TD><FORM METHOD="GET" ACTION="http://search.itknowledge.com/excite/cgi-bin/AT-foldocsearch.cgi"><INPUT NAME="search" SIZE="20" VALUE=""><BR><CENTER><INPUT NAME="searchButton" TYPE="submit" VALUE="Glossary Search"></CENTER><INPUT NAME="source" TYPE="hidden" VALUE="local" CHECKED> <INPUT NAME="bltext" TYPE="hidden" VALUE="Back to Search"><INPUT NAME="sp" TYPE="hidden" VALUE="sp"></FORM></TD><TD><IMG SRC="http://www.itknowledge.com/images/dotclear.gif" WIDTH="15" HEIGHT="1"></TD><TD><FORM METHOD="POST" ACTION="http://search.itknowledge.com/excite/cgi-bin/AT-subscriptionsearch.cgi"><INPUT NAME="search" SIZE="20" VALUE=""><BR><CENTER><INPUT NAME="searchButton" TYPE="submit" VALUE=" Book Search "></CENTER><INPUT NAME="source" TYPE="hidden" VALUE="local" CHECKED> <INPUT NAME="backlink" TYPE="hidden" VALUE="http://search.itknowledge.com:80/excite/AT-subscriptionquery.html"><INPUT NAME="bltext" TYPE="hidden" VALUE="Back to Search"><INPUT NAME="sp" TYPE="hidden" VALUE="sp"></FORM></TD></TR></TABLE></CENTER> --><!-- ISBN=1575213346 //--><!-- TITLE=Presenting XML//--><!-- AUTHOR=Richard Light//--><!-- PUBLISHER=Macmillan Computer Publishing//--><!-- IMPRINT=Sams//--><!-- CHAPTER=18 //--><!-- PAGES=0331-0356 //--><!-- UNASSIGNED1 //--><!-- UNASSIGNED2 //--><P><CENTER><A HREF="../ch17/0328-0330.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0335-0337.html">Next</A></CENTER></P><A NAME="PAGENUM-331"><P>Page 331</P></A><H3><A NAME="ch18_ 1">CHAPTER 18</A></H3><H2>Potential Applications of XML</H2><B>by Simon North</B><P>Although this chapter is called "potential" applicationsof XML, some of the applications that you learn about inthis chapter are not potential applications at all; they are trueworking implementations (such as Channel DefinitionFormat). Others are concrete applications that are firmly foundedon XML but are either not yet complete or have not yetbeen released to the public (for example, Sun's AnswerBookand Math Markup Language).</P><P>Of the true "potential" applications of XML that you'llmeet in this chapter, some (such as Health Level 7 andHandheld Markup Language) are in such an early stage ofdevelopment that it is not even certain that they will become XMLapplications rather than applications of full SGML (oreven HyTime). In one case (Topic Map Navigation), thereverse is true; even</P><A NAME="PAGENUM-332"><P>Page 332</P></A><P> at this early stage we can safely say that it is an XMLapplication, although we can really only guess what the final form of that application <BR>will be.</P><P>Whatever their state of development, the applications that you'll meethere give an interesting, even exciting, glimpse of some of the things that canand will be done with XML. No mean feat for a standard that isn't even complete!</P><H3><A NAME="ch18_ 2">CDF</A></H3><P>The Channel Definition Format is currently hovering somewherebetween being a proprietary Microsoft standard and an open WWW standard.The market has not yet made its choice, although some of the leadingcompanies have committed themselves to supporting it. Whatever the market finallydecides, CDF is an example of an XML application that has already arrived. Itis here now, on the Internet, built into the latest release of Microsoft'sInternet Explorer (version 4.0). If you haven't already seen it, you probably will seeit soon.</P><H4><A NAME="ch18_ 3">Push Media</A></H4><P>Ever since 1993, when "The Internet" really started to mean somethingto people outside academic and military circles, companies have beendesperately looking for a business model that would serve as the foundation for trueInternet commerce.</P><P>During the last few years there have been many attempts, ranging fromclosed systems such as CompuServe and America Online (AOL) to so-calledcontent providers such as the ill-fated Microsoft Network (MSN).</P><P>When you surf the Web, the normal procedure is to start your Webbrowser (which is, statistically, most likely Netscape's Navigator/Communicatoror Microsoft's Internet Explorer). You then enter the address (URL) of aWeb page or pick an address that you have saved in your Bookmark or FavoritePlaces list. Needless to say, this is a pretty active process where you are incomplete control. The experience can be made a more recognizable one (in termsof broadcasting) by adding advertising to the pages you visit in the form ofbanners, but the economics of charging for their positioning and use hasraised problems that have not yet been resolved.</P><A NAME="PAGENUM-333"><P>Page 333</P></A><P>A new variant on banner advertising is beginning to appear now in theform of so-called interstitials. Interstitials are small advertisements that aredisplayed when you move from one Web page to another. The basic idea behindthis type of Internet commerce remains as familiar as the billboards that lineour streets and highways.</P><P>A business model that the media industry does recognize is traditionalbroadcasting. Translate that idea into World Wide Web terms and it becomeswhat Microsoft has aptly called webcasting.</P><P>Instead of browsing from Web site to Web site and from Web page toWeb page within that site, you take out asubscription to a particular content provider. Instead ofpulling Web pages to yourself, you allow someone else topush those pages to you when they decide. After you have made yoursubscription choices, you can sit back and passively enjoy the information as it iswebcasted or pushed to your screen, almost as if you were watching television.</P><P>This is not to say that these services are bad; indeed, their success is astrange testimony to their merit. In a little more than a year, the original (andbest known) providers such as Pointcast and Ncompass have been quicklyjoined by more than 30 other companies (including AOL, CompuServe, andMSN). Push media can now provide you with up-to-the-minute sports, news,entertainment, and financial information at a speed and efficiency level thatvery few television companies will be able to match. It will be up to us todecide whether we want to open our desktops to yet another source of potentialinformation overload, or whether we will welcome it as an opportunity tocut down on information overload by publicly electing to receive certain typesof information in preference to others.</P><P>It probably comes as no surprise that, with so many different companiescompeting for a place in this new market, there is no standardization amongthem and the methods they've adopted. No two webcasters are able to deal witheach other's information.</P><P>For a while, it looked as though things were going to continue like thisuntil, on March 9, 1997, Microsoft submitted a formal proposal to the W3C(the World Wide Web Consortium, one of the nearest things the WWW has toa standards body). Then on March 12, 1997, Microsoft made a publicpress announcement about its new Channel Definition Format (CDF) specification.</P><A NAME="PAGENUM-334"><P>Page 334</P></A><H4><A NAME="ch18_ 4">The Channel Definition Format</A></H4><P>The Channel Definition Format (CDF) specification is an XMLapplication that defines collections of frequently updated information, calledchannels.</P><P>The CDF XML Document Type Definition (DTD) is not yet complete;the latest release is version 0.31, which was updated on May 28, 1997.However, the basics of the DTD are reasonably stable and are already supported byversion 4.0 of Microsoft's Internet Explorer.</P><P>The heart of the CDF XML DTD is the Channel element, which cancontain a number of other elements that describe the information it contains anddefines how and when the information it contains is to be updated:</P><!-- CODE //--><PRE><!ELEMENT Channel (Abstract, Channel, Item, Log, Login, Logo, Logtarget, Schedule, Time)* ></PRE><!-- END CODE //--><P>It would be far beyond the scope of this book to go into the full details ofthe CDF XML DTD here, but it is worth looking at some of the highlights.The elements that the CDF DTD defines allow a channel to do a wide varietyof things with your computer's screen display. Here are some examples:</P><UL><LI> The<Usage> element can specify whether the information is toappear in the browser window as a screen saver or as a desktop componenton Microsoft Internet Explorer's Active Desktop (introduced inversion 4.0).<LI> The<Abstract> element identifies a tooltip text that will bedisplayed on the right of an icon when the cursor is placed over thechannel icon.</UL><P>Detailed information concerning the Channel Definition Format can bedownloaded from Microsoft's standards Web pages and from its InternetExplorer Web pages. (See Appendix B for the URL addresses.)</P><H4><A NAME="ch18_ 5">Sun's AnswerBook</A></H4><P>The quantity of technical documentation supplied with large computersystems has always been daunting. In some cases, it has even become totallyoverwhelming. Long-time (ex-mainframe) computer users would be familiar with theterms "Big Grey Wall" and "Big Red Wall," recalling the sight of a set ofDigital Equipment's system documentation arranged in a copious bookshelf insome air-conditioned computer room. The cost of authoring, producing, andshipping such massive amounts of paper documentation has become amajor</P><P><CENTER><A HREF="../ch17/0328-0330.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0335-0337.html">Next</A></CENTER></P></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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