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XML: a very brief introduction

XML: a very brief introduction. (and other related acronyms). John Miller, KU, February 28, 2002. General outline:. What is XML? How is it related to other stuff? the context other standards and tools Why is it potentially important to libraries? Examples Q & A.

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XML: a very brief introduction

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  1. XML: a very brief introduction (and other related acronyms) John Miller, KU, February 28, 2002

  2. General outline: • What is XML? • How is it related to other stuff? • the context • other standards and tools • Why is it potentially important to libraries? • Examples • Q & A

  3. It’s an acronym soup ... some of the ingredients

  4. XML: What is it? eXtensible Markup Language

  5. Language? • conveys meaning -- provides a meaning for others to understand your intent • has rules • has a syntax • but ... NOT a programming language

  6. Markup? • conveys meaning by “marking up” other text and data with tags • for example <name>John Miller</name> <city>Lawrence</city> <shoe_size>13</shoe_size> generically: <element>value</element>

  7. eXtensible? • “capable of being extended” • extend = “to increase the scope, meaning, or application of; broaden” • not tied to a single model or data definition • handles both text and data

  8. What else is it? • it is a family of technologies (more 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 appearance • combined with style sheets, it can be use to create formatted documents in any style you want

  9. What else is it? -- cont. • it is modular -- one can define a new document by combining and reusing other existing formats • it is an open standard, not tied to any one company or software • it 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)

  10. What else is it? -- cont. • To repeat ... it is a single system that can be used as the basis both for • storing, searching, formatting, & displaying TEXT • storing, searching, formatting, & displaying DATA

  11. but, there is some disagreement ... • “XML is not a a markup language -- it is a toolkit for creating, shaping, and using markup languages (Erik T. Ray, Learning XML, O’Reilly, 2001) • “XML is a markup language, and only a markup language. It’s 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 Harold & W. Scott Means, XML in a Nutshell, O’Reilly, 2001.)

  12. How is it related to other stuff? The Context [hint: it all depends on your ontology] Or, ... what the h*@# are RDF and the SemanticWeb?

  13. Definitions: W3C • World Wide Web Consortium • founded 1994 by Tim Berners-Lee (with MIT & CERN); now has 506 institutional 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 standards • www.w3c.org

  14. Definitions: ontology / ontologies • Webster's: “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 are ] “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.”

  15. Ontologies -- cont. • ... or, more precisely: “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.)

  16. Namespaces • What are they? • means of linking a tag to a metadata standard and/or DTD • spaces within which an ontology is defined • Why 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 ontologies • Example: Both HTML and Dublin Core have an element called <title> • format = <namespace : element> • for example: <dc:title>

  17. Definitions: RDF • Resource Description Framework • W3C: “Resource Description Framework (RDF) is a foundation for processing metadata; it provides interoperability between applications that exchange machine-understandable information 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.“

  18. the RDF data model • 3 types of objects: • resource -- web site; web page, individual tagged element on a page, etc. -- always named by a URI • property -- “a specific aspect, characteristic, attribute, or relation used to describe a resource” (W3C) -- also identifiable by a URI • statement -- combination of a resource, a property, and a value for the property

  19. RDF: statement example • “John Smith is the creator of www.xyz.edu”

  20. RDF -- cont. • So ... how can this framework be implemented? ... how can automated communication occur across the web? XML ! Another 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 relations.”

  21. an RDF “message” (“xmlns” = name space declaration) <?xml version="1.0"?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf- syntax-ns#" xmlns:s="http://description.org/schema/"> <rdf:Description about="http://www.lib.xyz.edu/"> <s:Creator>John Smith</s:Creator> </rdf:Description> </rdf:RDF>

  22. Definitions: URI vs. URL • URI: “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 specifications) associated with popular URI schemes: http, ftp, mailto, etc.” (W3C)

  23. Definitions: Semantic Web • SEMANTICS: “the study of meanings” -- “the meaning or relationship of meaning of a sign or set of signs” (Webster’s) • “The Semantic Web” by Tim Berners-Lee, James Hendler, & Ora Lassila. (see bibl. 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 Web pages, creating an environment where software agents roaming from page to page can readily carry out sophisticated tasks for users.”

  24. 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 that 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 Framework (RDF).”

  25. How is it related to other stuff? standards and tools

  26. XML: “well-formed” vs. “valid” • well-formed: follows the rules for XML internal structure and consistency • valid: follows a standard definition of the structure and content of a document, either a • Data Type Definition (DTD), or an • XML Schema

  27. 3 basic kinds of XML docs • well-formed, but unvalidated • well-formed and valid, based upon a Data Type Definition (DTD) -- either internal or external • well-formed and “valid”, based upon specifications in an XML Schema

  28. Requirements for being “well-formed” • a declaration at the top of a document signaling what it is: <?XML version=“1.0”?> • if conforming to a DTD, a declaration of that DTD: <!DOCTYPE TEI SYSTEM “teixlite.dtd”> • a root element: <document> or <letter> or <shoe> or ... • every start tag must have an end tag or, if empty, have a special format: <a></a> or <a/> • tags must nest cleanly: <a><b></b></a> • attribute values must be in quotation marks • tags are case-sensitive and must match • some characters must be rendered in a special way

  29. Valid? Data Type Definition (DTD) What is a DTD? It is a set of rules that define: • what elements may appears in a document • what elements must appear in a document • what elements may be repeated • the hierarchical relationship among elements • what attributes are allowed for each element • other structural requirements Generally, a separate document, but definitions also can be inside an XML document

  30. Valid? XML Schema • “an XML language for describing and containing the content of XML documents” (W3C) • a schema document is itself an XML document • an alternative to a DTD -- both can exist (as alternatives) for a particular format, but only one is needed

  31. XSL & XSLT • eXtensible 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 Objects • eXtensible 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 HTML web pages from XML data.” (W3C)

  32. 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 instruction • want to store XML data in one format and present it in a different form • want to present same XML data in many different ways

  33. CSS • Cascading Style Sheets • a 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 style • defines margins, positioning, fonts, color, size, box and list properties, etc. • not NEEDED to render XML-based HTML, but useful

  34. How do XSL and CSS compare? • XSL uses XML notation; CSS uses its own • CSS formatting following the document “object tree”; XSL formatting can radically move objects around • both can be used to directly format XML documents • XSL transforms while it formats

  35. CSS & XSL (cont.) CSSXSL Can be used with HTML? yes no Can be used with XML? yes yes Transformation language? no yes Syntax CSS XML -- from W3C’s “What are style sheets” @ http://www.w3.org/Style)

  36. Xlink & Xpointer • XML replacements for the HTML tags: • Xlink replaces <a href ...> • Xpointer replace <a name .. • in XML, ANY element can have a linking capability • in XML, can link to any point in a document with a tag • in XML, a link imports rather than transfers

  37. DTD or Schema XML doc XSL doc XSLT transformer software HTML doc CSS

  38. SGML • Standard Generalized Markup Language • an ISO standard for defining the structural descriptions of electronic documents • “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 removes all the optional features which make SGML too complex to program for in a Web environment.” (from Peter Flynn’s “XML FAQ” @ http://www.ucc.ie/xml/#sgml) • valid, well-formed XML is valid SGML, but not necessarily vice versa

  39. XHTML • eXtensible Hypertext Markup Language • the successor to HTML, almost the same 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 for authors is the norm, not the exception.”(Chuck Musciano & Bill Kennedy, HTML & XHTML: the Definitive Guide, O’Reilly, 2000)

  40. Why is XML potentially important to libraries?

  41. Library-related uses • Cross-database searching -- integration of multiple data definitions • Digital library metadata (EAD, VRA, etc.) • Textual markup for presentation • Public Interface design (e.g., OPAC) • -- send XML (formatted by XSL), not HTML

  42. Library-related uses -- cont. • exchanging data & metadata • between libraries • an alternative to MARC ? • an alternative to Z39.50 ? • NCIP (National Circulation Interchange Protocol) • between libraries and vendors • between library system & other campus systems (e.g., Voyager and PeopleSoft) • Open Archives Initiative (OAI) metadata harvesting

  43. Example: Endeavor ENCompass • Federated Searching • Multi-protocol searching • Z39.50 • HTTP • XML Gateways • Search and Navigation • Web based, using XSL technology for ultimate customization of displays

  44. Collection of XML docs query query XML search software Perl / CGI control program results results validation form DTD or Schema query results as XML doc XSL doc HTML search form HTML response html XSLT transformer software Example of an XML-based search system Browser CSS happy user

  45. Data vs. Metadata • Like HTML, XML can contain both data and metadata • metadata can be explicit (i.e., between <metadata></metadata> tags) • metadata can be individual elements • metadata also can appear as attributes • but ... at least with text, what’s the difference once everything is tagged according to content & structure? -- one person’s metadata is another’s data

  46. Some Common metadata schemes • DC: Dublin Core • VRA Core: Visual Resources Association • EAD: Encoded Archival Description • TEI: Text Encoding Initiative (also TEI Lite) • MARC: MAchine-Readable Cataloging • CSDGM: Content Standard for Digital Geospatial Metadata • FGDC: Federal Geographic Data Committee metadata

  47. Example: DTD <!-- This is a sample DTD for a record/CD collection --> <!ELEMENTMYMUSIC (album+)> <!ELEMENTalbum (title, ((artist+,genre+) | (genre+, artist+)) year_produced, year_purchased?, label?, song_list?) > <ATTLISTalbum id ID #REQUIRED ref IDREF #IMPLIED condition (n.p. | bad | worn | good | excellent) “n.p.” > <ELEMENTtitle (#PCDATA)> <ELEMENTartist (#PCDATA)> <ELEMENTgenre (#PCDATA)> <ATTLISTgenre+ type (folk | rock | country | blues | jazz | classical ) #REQUIRED> ...

  48. Example: XML <MYMUSIC> <album id="LedZeppelin-1969-1" condition="worn"> <title>Led Zeppelin II</title> <artist>Led Zeppelin</artist> <genre type="rock">Rock</genre> <year_produced>1969</year_produced> <year_purchased>1988</year_purchased> <label>Atlantic Records</label> <song_list> <song length="5:34">Whole Lotta Love</song> <song length="6:19">The Lemon Song</song> ... <song length="4:24">Ramble On</song> <song length="4:21">Moby Dick</song> <song length="4:19">Bring It On Home</song> </song_list> </album>

  49. Example: XSL (part 1) <?xml version="1.0"?> <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> <xsl:output method="html" indent="yes"/> <xsl:template match="/"> <html> <head><title>My Music</title></head> <body> <xsl:apply-templates select="MYMUSIC/album"> <xsl:sort select="year_produced"/> </xsl:apply-templates> </body> </html> </xsl:template>

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