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1、XML: a very brief introduction(and other related acronyms)John Miller, KU, February 28, 2002General outline:What is XML?How is it related to other stuff?the contextother standards and toolsWhy is it potentially important to libraries?ExamplesQ & AIts an acronym soup . XML W3C RDF XSL SGML TEI XSLT H

2、TML EAD DTD XHTML MARC XML Schema DOM VRA CSS PICS DCsome of the ingredientsXML: What is it? eXtensible Markup LanguageLanguage?conveys meaning - provides a meaning for others to understand your intenthas ruleshas a syntaxbut . NOT a programming languageMarkup?conveys meaning by “marking up” other t

3、ext and data with tagsfor exampleJohn MillerLawrence13generically:valueeXtensible?“capable of being extended” extend = “to increase the scope, meaning, or application of; broaden”not tied to a single model or data definitionhandles both text and data What else is it?it is a family of technologies (m

4、ore later)it is a “simplified” version or subset of SGML (more later)it is a means of separating the description of document structure from document appearancecombined with style sheets, it can be use to create formatted documents in any style you wantWhat else is it? - cont.it is modular - one can

5、define a new document by combining and reusing other existing formatsit is an open standard, not tied to any one company or softwareit can be read both by humans and by programs (unlike a MARC record, for example)perhaps most importantly, “it is the basis for RDF and the Semantic Web” (more later)Wh

6、at else is it? - cont.To repeat . it is a single system that can be used as the basis both forstoring, searching, formatting, & displaying TEXTstoring, searching, formatting, & displaying DATA but, there is some disagreement .“XML is not a a markup language - it is a toolkit for creating, shaping, a

7、nd using markup languages (Erik T. Ray, Learning XML, OReilly, 2001)“XML is a markup language, and only a markup language. Its important to remember this fact. The XML hype has become so extreme that some people expect XML to do everything up, and including, washing the family dog.” (Elliotte Rusty

8、Harold & W. Scott Means, XML in a Nutshell, OReilly, 2001.)How is it related to other stuff? Or, . what the h*# are RDF and the Semantic Web?hint: it all depends on your ontologyThe ContextDefinitions: W3CWorld Wide Web Consortiumfounded 1994 by Tim Berners-Lee (with MIT & CERN); now has 506 institu

9、tional members; TBL still leads“. develops interoperable technologies (specifications, guidelines, software, and tools) to lead the Web to its full potential as a forum for information, commerce, communication, and collective understanding.”XML, XSL, CSS, HTML, and many others are W3C standardsDefin

10、itions: ontology / ontologiesWebsters: “a branch of metaphysics concerned with the nature and relations of being”W3C: “Formal descriptions of terms in a certain area (shopping or manufacturing, for example) are called ontologies and are a necessary part of the semantic web.”TBL et al.: Ontologies ar

11、e “collections of statements written in a language such as RDF that define the relations between concepts and specify logical rules for reasoning about them. Computers will understand the meaning of semantic data on a Web page by following links to specified ontologies.”Ontologies - cont. or, more p

12、recisely:“Artificial-intelligence and Web researchers have co-opted the term for their own jargon, and for them an ontology is a document or file that formally defines the relations among terms. The most typical kind of ontology for the Web has a taxonomy and a set of inference rules.” (TBL et al.)

13、NamespacesWhat are they?means of linking a tag to a metadata standard and/or DTDspaces within which an ontology is definedWhy are they needed? XML is modular: can combine portions of different XML documents that conform to different DTDs into a single document, i.e., which use different ontologiesEx

14、ample: Both HTML and Dublin Core have an element called format = for example: Definitions: RDFResource Description FrameworkW3C: “Resource Description Framework (RDF) is a foundation for processing metadata; it provides interoperability between applications that exchange machine-understandable infor

15、mation on the Web. RDF emphasizes facilities to enable automated processing of Web resources.”and . “The broad goal of RDF is to define a mechanism for describing resources that makes no assumptions about a particular application domain, nor defines (a priori) the semantics of any application domain

16、.“the RDF data model3 types of objects:resource - web site; web page, individual tagged element on a page, etc. - always named by a URIproperty - “a specific aspect, characteristic, attribute, or relation used to describe a resource” (W3C) - also identifiable by a URIstatement - combination of a res

17、ource, a property, and a value for the propertyRDF: statement example“John Smith is the creator of ”subject (resource)predicate (property)creatorobject (value/literal)John SmithRDF - cont.So . how can this framework be implemented? . how can automated communication occur across the web? XML !Another

18、 definition of RDF:Scientific American: “A scheme for defining information on the Web. RDF provides the technology for expressing the meaning of terms and concepts in a form that computers can readily process. RDF can use XML for its syntax and URIs to specify entities, concepts, properties and rela

19、tions.” John Smith an RDF “message” (“xmlns” = name space declaration)Definitions: URI vs. URLURI: “Uniform Resource Identifier. The generic set of all names/addresses that are short strings that refer to resources.” (W3C)URL: “Uniform Resource Locator. An informal term (no longer used in technical

20、specifications) associated with popular URI schemes: http, ftp, mailto, etc.” (W3C) Definitions: Semantic WebSEMANTICS: “the study of meanings” - “the meaning or relationship of meaning of a sign or set of signs” (Websters)“The Semantic Web” by Tim Berners-Lee, James Hendler, & Ora Lassila. (see bib

21、l. at end) Some quotes from the article:“The Semantic Web is not a separate Web but an extension of the current one, in which information is given well-defined meaning, better enabling computers and people to work in cooperation. “The Semantic Web will bring structure to the meaningful content of We

22、b pages, creating an environment where software agents roaming from page to page can readily carry out sophisticated tasks for users.”Semantic web - cont.More quotes:“For the semantic web to function, computers must have access to structured collections of information and sets of inference rules tha

23、t they can use to conduct automated reasoning.”“Human language thrives when using the same term to mean somewhat different things, but automation does not.”“Two important technologies for developing the Semantic Web are already in place: eXtensible Markup Language (XML) and the Resource Description

24、Framework (RDF).” How is it related to other stuff? standards and toolsXML: “well-formed” vs. “valid” well-formed: follows the rules for XML internal structure and consistencyvalid: follows a standard definition of the structure and content of a document, either a Data Type Definition (DTD), or anXM

25、L Schema3 basic kinds of XML docswell-formed, but unvalidatedwell-formed and valid, based upon a Data Type Definition (DTD) - either internal or externalwell-formed and “valid”, based upon specifications in an XML SchemaRequirements for being “well-formed”a declaration at the top of a document signa

26、ling what it is: if conforming to a DTD, a declaration of that DTD: a root element: or or or .every start tag must have an end tag or, if empty, have a special format: or tags must nest cleanly: attribute values must be in quotation markstags are case-sensitive and must matchsome characters must be

27、rendered in a special wayValid? Data Type Definition (DTD)What is a DTD? It is a set of rules that define:what elements may appears in a documentwhat elements must appear in a documentwhat elements may be repeatedthe hierarchical relationship among elementswhat attributes are allowed for each elemen

28、tother structural requirements Generally, a separate document, but definitions also can be inside an XML documentValid? XML Schema“an XML language for describing and containing the content of XML documents” (W3C)a schema document is itself an XML documentan alternative to a DTD - both can exist (as

29、alternatives) for a particular format, but only one is neededXSL & XSLTeXtensible Stylesheet Language“XSL is a language for expressing style sheets. An XSL style sheet is, like with CSS, a file that describes how to display an XML document of a given type” (W3C)includes XSL FO: XSL Formatting Object

30、seXtensible Stylesheet Language for Transformations“Originally intended to perform complex styling operations, like the generation of tables of contents and indexes, it is now used as a general purpose XML processing language. XSLT is thus widely used for purposes other than XSL, like generating HTM

31、L web pages from XML data.” (W3C) Stylesheets - why do we need them? XML is not a fixed tag set - a generic processor/browser has no idea what the tags “mean”XML markup generally does not include any formatting instructionwant to store XML data in one format and present it in a different formwant to

32、 present same XML data in many different waysCSSCascading Style Sheetsa simple styling language defining and attaching styles to HTML (or XML) elements. Each element type and each of its occurrences within a document can be given a unique styledefines margins, positioning, fonts, color, size, box an

33、d list properties, etc.not NEEDED to render XML-based HTML, but useful How do XSL and CSS compare?XSL uses XML notation; CSS uses its ownCSS formatting following the document “object tree”; XSL formatting can radically move objects aroundboth can be used to directly format XML documentsXSL transform

34、s while it formatsCSS & XSL (cont.)CSS XSLCan be used with HTML? yes noCan be used with XML? yes yesTransformation language? no yesSyntax CSS XML- from W3Cs “What are style sheets” /Style)Xlink & XpointerXML replacements for the HTML tags:Xlink replaces Xpointer replace a name .in XML, ANY element c

35、an have a linking capabilityin XML, can link to any point in a document with a tagin XML, a link imports rather than transfersXMLdocXSLdocXSLT transformersoftwareHTMLdocDTDorSchemaCSSSGMLStandard Generalized Markup Language an ISO standard for defining the structural descriptions of electronic docum

36、ents“SGML is very large, powerful, and complex. It has been in heavy industrial and commercial use for over a decade, and there is a significant body of expertise and software to go with it. XML is a lightweight cut-down version of SGML which keeps enough of its functionality to make it useful but r

37、emoves all the optional features which make SGML too complex to program for in a Web environment.” (from Peter Flynns “XML FAQ” http:/www.ucc.ie/xml/#sgml)valid, well-formed XML is valid SGML, but not necessarily vice versaXHTMLeXtensible Hypertext Markup Languagethe successor to HTML, almost the sa

38、me as HTML 4.01 (their DTDs are identical except for some differences between SGML and XML)HTML defined according to XML rules - the HTML spec is a specific XML DTD/schema“The emergence of XHTML is just another chapter in the often tumultuous history of HTML and the World Wide Web, where confusion f

39、or authors is the norm, not the exception.” (Chuck Musciano & Bill Kennedy, HTML & XHTML: the Definitive Guide, OReilly, 2000)Why is XML potentially important to libraries?Library-related usesCross-database searching - integration of multiple data definitionsDigital library metadata (EAD, VRA, etc.)

40、Textual markup for presentationPublic Interface design (e.g., OPAC)- send XML (formatted by XSL), not HTMLLibrary-related uses - cont.exchanging data & metadatabetween librariesan alternative to MARC ?an alternative to Z39.50 ?NCIP (National Circulation Interchange Protocol)between libraries and ven

41、dorsbetween library system & other campus systems (e.g., Voyager and PeopleSoft)Open Archives Initiative (OAI) metadata harvestingExample: Endeavor ENCompassFederated SearchingMulti-protocol searchingZ39.50HTTPXML GatewaysSearch and NavigationWeb based, using XSL technology for ultimate customizatio

42、n of displaysXSLdocXSLT transformersoftwareHTMLsearchformDTD or SchemaCSS Collection of XML docsPerl / CGIcontrolprogramXMLsearchsoftwareHTMLresponse query queryresultsresultsresultsasXML docBrowserformqueryExample ofan XML-basedsearch systemvalidationhappy userhtmlData vs. MetadataLike HTML, XML ca

43、n contain both data and metadatametadata can be explicit (i.e., between tags)metadata can be individual elementsmetadata also can appear as attributesbut . at least with text, whats the difference once everything is tagged according to content & structure? - one persons metadata is anothers dataSome

44、 Common metadata schemesDC: Dublin CoreVRA Core: Visual Resources AssociationEAD: Encoded Archival DescriptionTEI: Text Encoding Initiative (also TEI Lite)MARC: MAchine-Readable CatalogingCSDGM: Content Standard for Digital Geospatial Metadata FGDC: Federal Geographic Data Committee metadataExample:

45、 DTD.Example: XMLLed Zeppelin IILed ZeppelinRock19691988Atlantic RecordsWhole Lotta LoveThe Lemon Song.Ramble OnMoby DickBring It On HomeExample: XSL (part 1) My Music Example: XSL (part 2). Led Zeppelin IILed Zeppelin1969 Whole Lotta Love What Is and What Should Never Be The Lemon Song Thank You He

46、artbreaker Living Loving Maid (Shes Just a Woman) Ramble On Moby Dick Bring It On HomeExample: the resulting HTML displayDublin Core examples:DTD declaration: RDF/namespace declaration: MARC & XMLMARC: MAchine-Readable Cataloginga structure / communications format, not a set of cataloging ruleslike XML, elements indicate structure/meaning rather than presentation - c