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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="0347-0349.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0353-0355.html">Next</A></CENTER></P><A NAME="PAGENUM-350"><P>Page 350</P></A>regardless of the original format (rather than convert it, as is normally thecase in SGML applications), you can use the extracted information in acommon neutral format, and then you can merge it back into the source documentin the original format.</P><P>The OpenTag Initiative is very much in its infancy and a lot of work stillneeds to be done, but it opens up some extremely interesting future possibilitiesfor XML (some of which are discussed later in this chapter in the section"Looking to the Future"). The amazing thing is that amidst all the hue and cryabout HTML being too small, lacking power, lacking richness, being lessdesirable when SGML can do so much more, and many other less polite things, hereis an application that actually benefits from havingless!</P><H4><A NAME="ch18_ 11">Topic Navigation Maps</A></H4><P>You've just looked at the OpenTag Initiative and how it is contriving todo more with less. In this section, the last of our grand tour of existing XMLapplications, you learn about the other end of the spectrum. Topic MapNavigation is, in all honesty, an extremely complex XML application, one thatpushes even SGML far beyond its normal limits. To explain it here in all itsrichness would require several books and a fair understanding of not just SGMLbut also HyTime. However, because Topic Map Navigation uses XML'smechanisms to describe links, and it links XML documents, Topic MapNavigation does qualify as a true XML application (in fact, it might qualify as thefirst true XML extended-link application), and I will try to limit the discussionto that context. Be warned that it won't be easy. But by only skimming thesurface of the technical detail, you should be able to at least get a taste ofwhat Topic Navigation Maps could mean for the future of library science.</P><TABLE BGCOLOR="#FFFF99"><TR><TD>Note:</TD></TR><TR><TD><BLOCKQUOTE>Officially designated as ISO/IEC CD 13250, InformationProcessing_SGML Applications_Topic Navigation Maps, topic maps currently exist onlyin concept form, even though there are some excellent proofs of concept.(The proceedings of SGML Europe 97 were produced in topic map formusing HTML on a CD-ROM, and the EnLIGHTeN software package, fromthe French company of HighText, is a topic map tool without equal.) Thismeans that a lot of detail still needs to be worked out before a draft standard canbe submitted in 1998, and much is liable to change.</BLOCKQUOTE></TD></TR></TABLE><A NAME="PAGENUM-351"><P>Page 351</P></A><P>So what are topic maps? It's probably easier to start by looking at theproblem that topic maps are meant to solve. Imagine that you have a vast quantityof information. Now double that quantity, double it again, and thinkbig. Don't be shy; I'm talking about collections possibly as big as the whole Libraryof Congress. Putting all this source information into electronic form—or,better, into a structured electronic form such as SGML or XML—turns eachof the documents into a sort of database. Suddenly you are able toelectronically catalog, index, abstract, summarize, and ultimately link all thisinformation together. You are able to navigate this information.</P><P>Along come the librarians, the catalogers, the indexers, and the glossarywriters, and it looks like we will actually be able to make masses of data trulyaccessible as has never been done before. Wrong! The number of relationshipsbetween topics in a collection of books is literally infinite. Worse, there isno common agreement on the number and nature of the topics, and no consensus onthe nature of the links between them. Indexes are often one-off affairs, tied toa particular document and context; when indexes are expanded into thecontext of multiple document master indexes, they quickly become vague andinconsistent. The larger they become, the worse they become, as more peopleare required to cooperate. Add a new book to the collection (or even a newfield of knowledge such as genetic engineering), and existing indexes becomeinstantly obsolete. But little if any thought is given to making suchindexes maintainable, so there's a good chance that you have to restart theindexing each time from scratch.</P><P>Topic maps provide a formal, machine-executable way to create andmaintain information by classifying the information in documents into topics and,taking it one step further, classifying the topics themselves with relation toeach other.</P><P>This is all well and good but all pretty academic so far. How does it workin practice? Brace yourself because this bit gets technical. Topic maps rely onone essential feature of XML (and HyTime)—the capability to link two objectsby means of an external link description (an independentlink, called an ilink in HyTime, and an extendedlink in XML). A set of linked objects is called atopic. (Remember that—unlike HTML, for example—links can includemultiple sources and multiple targets.) Why not just call it a link? Links betweenobjects, such as index entries, are location identifiers where the link simplydescribes the location of a piece of information. Topics are far morepowerful than this. They can also describe the semantics of the link, such as the waya</P><A NAME="PAGENUM-352"><P>Page 352</P></A><P> glossary entry points to a location that is considered to be the definitionof something. Topic maps provide a means to add an element (called thesemantic assignment) to all the related objects and their anchors (the topics) andassign them a role, a description, or something significant that means thatthey can be merged with other topics, linked to other topics, maintained, andmachine generated.</P><P>Taking this to the next level of abstraction, these topics—or a particulararchitecture of topics that was designed for a particular document or a specificfield of knowledge—can be represented as a set of topics and a set of topicrelation ships, together called a topic map.</P><P>So why the fuss? Put in the simplest of terms, topic maps exploit the factthat in XML (and HyTime) the links between objects can be kept outside thedocuments themselves. When you describe the nature of these links, or at leastidentify them as topics, a relation can then be defined between the topicsthemselves—which is the topic map. You can have any number of topic maps for any setof documents, and topics can be included in any number of topic maps.Because everything is external to the documents themselves, topics can be madeand modified at the whim of the reader to suit any purpose, no matter howtransient. Even better, there is no restriction on the nature or significance ofthe relationship, and each reader can combine topics and other topic maps ina way that suits his or her particular needs at that time.</P><P>Before leaving the subject of topic maps, one last facet should beexamined. Topic Map Navigation will exploit all of XML's linking mechanisms. (Infact, it will probably exploit many if not all of HyTime's far more powerfuladdressing and locating mechanisms, but this is something that still mustbe decided on.) It will be possible to link any kind of electronic information,whether it's SGML, non-SGML, XML, non-XML, structured, or non-structured.</P><H4><A NAME="ch18_ 12">Looking to the Future</A></H4><P>It's been a long journey, but this grand tour of XML applications hastaken you from the simplest (OpenTag) to the most complex imaginable (TopicMap Navigation). It has covered commercial ground (Channel DefinitionFormat), and it has covered scientific territory (Math Markup Language). No tourlike this can ever be complete, though, because new applications for XML arebeing thought up and announced all the time (and this is even before theintroduction of any real support in the way of tools).</P><P><CENTER><A HREF="0347-0349.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0353-0355.html">Next</A></CENTER></P></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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