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<!DOCTYPE HTML PUBLIC "html.dtd"><HTML><HEAD><TITLE>Presenting XML:Resolution of the XML Specification: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=17 //--><!-- PAGES=0309-0330 //--><!-- UNASSIGNED1 //--><!-- UNASSIGNED2 //--><P><CENTER><A HREF="0309-0312.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0316-0318.html">Next</A></CENTER></P><A NAME="PAGENUM-313"><P>Page 313</P></A><H4><A NAME="ch17_ 5">Fragment Interchange</A></H4><P>Long before XML was even an idea, the SGML Open Consortium (a fee-<BR>paying group of SGML software vendors and other interested parties) wasasked to look at ways in which a fragment of an SGML document could besuccessfully parsed without access to the complete document. The matters thathad to be settled were what information would be necessary to identify thedocument fragment or provide whatever other context information might beneeded (such as automatic numbering), a notation in which to describe thatinformation, and a means for linking that information with the fragment.</P><P>The result is SGML Open Technical Resolution 9601:1996, published asa draft in November 1995 and released in its final form in November 1996.The technical resolution describes an SGML specification that identifies thecontext of a piece of an SGML document (the name of its SGML declaration, itsDTD, and its location in the hierarchical tree of SGML elements of the documentit came from). The "packaging" of the description with the fragment itself isquite simply done using the same MIME Multipart/Mixed identificationscheme used for e-mail messages made up of several parts.</P><P>Although it is a reasonably simple and quite elegant solution, thetechnical resolution is very much an SGML solution and, sadly, one that has notyet been implemented (and probably never will be).</P><P>So much for 1995/96's answer; on to the next one!</P><H4><A NAME="ch17_ 6">Web Collections</A></H4><P>As its name suggests, whereas the SGML Open Technical Resolution wasan SGML approach, the Web Collections angle is very definitely an Internetapproach, or rather an HTML approach.</P><P>Having gone through several versions, the original (pre-XML) version ofthe W3C working draft (February 1996, and the October 1996 IBM Haifarevision) intended to extend HTML by adding specific tags to identifycollections of documents and to specify the points in the collective document atwhich the individual comments are to be included. On the face of it, the schemewas quite workable, except for the fact that although they were able to suggestusing the HTML <META> tag and adding a tagID to URLs to provide most of the identification information needed, they were unable to suggest aworkable scheme for including informative data(meta-data) for "sets of sets" such aslists, without defining a completely new set of tags.</P><A NAME="PAGENUM-314"><P>Page 314</P></A><P>The Web Collections idea would probably have stalled at that point ifXML had not blown new life into it. In March 1997, a new proposal wassubmitted to W3C (with the editor's chair passing from IBM to Microsoft). Thisversion is like an idea reborn; Web Collections is suddenly a full-scale XMLapplication that promises to be expressible within HTML documents or instandalone documents.</P><P>Taking full advantage of XML's extensibility, the Web Collections syntaxcalls for a set of new elements to identify the collective document and theHTML pages that are included in it. To provide the "packaging" information,the authors propose creating a new MIME type (similar to the suggestion madein the pre-XML days) by using URLs in property element attributes(something that SGML couldn't do, but XML can) and by using XML's<LINK> element and its REL attribute to describe the relationship between the documents.</P><P>The proposal is still in a very rough draft form, but it shows a great dealof promise and, by using XML, there seem to be few, if any, technicalproblems in its path.</P><H4><A NAME="ch17_ 7">PICS-NG</A></H4><P>PICS New Generation, or PICS-NG for short, represents yet anotherapproach to the problem of collating XML documents and fragments of documents.This initiative comes from a totally different, even unexpected corner.</P><P>The Platform for Internet Content Selection was originally formed in 1995as a response to the increasing amount of "bad press" that the Internet hadbeen getting on account of too much pornography allegedly being accessibleto children on the Web. At the time, it looked as though the Web faced asimple choice: censorship or self-regulation. PICS is an attempt at self-regulation.On a purely voluntary basis, the author of the home page (or whatever first pagea visitor will see) adds a <META> element within the<HEAD> element in the HTML code to describe the nature of the content of the Web site (or moreprecisely, to state whether the content is unsuitable for minors). Then, usingPICS-aware software, parents and teachers can monitor, or even block, access to siteswith undesirable content.</P><P>It isn't a very large step to conclude that the kind of identification(labeling) done as part of PICS can also be put to other uses: to add digital signaturesto HTML documents for cataloging purposes, and by letting a group ofdocuments "point" to the same description set through a common URL, toallow multiple documents to be logically associated. In early 1997, a proposalwas</P><A NAME="PAGENUM-315"><P>Page 315</P></A><P>circulated to extend PICS in just this way and make it a more general, <BR>application-neutral method of describing content. The May 1997proposal, submitted to the W3C working groups concerned with these matters,formalized the syntax, worked out a few of the concepts a little further, andextended the original to turn it into an XML application.</P><P>There is still a lot of work to be done on this proposal; currently, theXML syntax is somewhat experimental, and a number of issues still need to beworked out (such as the matter of inheritance). There seems to have been littlethought paid to the possibility of handling partial documents, even though the useof XML to provide the extra element tags needed to describe the content givesan obvious pointer in the direction in which this could be achieved. It willbe interesting to see whether or not the relevant working groups pick up onthis and in what direction they take it.</P><H4><A NAME="ch17_ 8">MCF</A></H4><P>The Meta Content Framework Using XML (MCF for short, and lessaffectionately known in some circles as "The Netscape Proposal" becauseNetscape provided one of the editors) is one of the most recent additions to the listof candidates for fractional and composite HTML/XML documents, theproposal to W3C being dated June 1997.</P><P>MCF starts off with the novel proposition that one man's metadata isanother man's core data; it is simply a matter of context and does not justify anartificial distinction. The foundation of MCF is twofold: a set of labels(somewhat more extensive than any of the other schemes, but fundamentally the samein both purpose and function), and a mathematical model called a Directed <BR>Labeled Graph (DLG). Although there has been some criticism of thismathematical model, it certainly gives the MCF proposal the strongesttheoretical foundation, even though this does seem to be at the cost of simplicity. Iwon't go any deeper into the model, but full information about it is available onthe Internet, including a tutorial. One thing that makes MCF worth watchingis that, unlike the other candidates, XML is wonderfully suited for computermanipulation, including database storage.</P><H4><A NAME="ch17_ 9">XML-Data</A></H4><P>If MCF is Netscape's proposal, XML-Data is Microsoft's proposal.Beginning its life as a Microsoft position paper in June 1997, XML-Data wasformally submitted to the W3C on June 26, 1997.</P><P><CENTER><A HREF="0309-0312.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0316-0318.html">Next</A></CENTER></P></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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