1 “Universal Data-Speak”: The eXtensible Markup Language Zack Ives CSE 590DB, Winter 2000 University of Washington 3 January 2000

2 What Is XML?  eXtensible Markup Language for data  Standard for publishing and interchange  “Cleaner” SGML for the Internet  Applications:  Data exchange over intranets, between companies  E-business  Native file formats (Word, SVG)  Publishing of data  Storage format for irregular data  …

3 What’s Special about XML?  Supported by almost everyone  Easy to parse (even with no info about the doc)  Can encode data with little or much structure  Supports data references inside & outside document  Presentation layer for publishing (XSL)  Document Object Model (DOM) for manipulating  Many, many tools

4 Basic XML Structures  Elements:  Open & close tags or “empty tag”  Ordered, nestable  Attributes:  Single-valued, unordered  Special types:  ID, IDREF, IDREFS  PCDATA/CDATA Publishing Object Data IBM Since its… Intro XML, … … …

5 Other XML Structures  Processing instructions: instructions for applications  CDATA sections: treat content as char data Whatever!!! ]]>  Comments: just like HTML  Entities: external resources and macros  &my-entity; (non-parameter entity)  %param-entity; (parameter entity for DTD declarations)

6 Document Type Descriptor  Inherited from SGML DTD standard  BNF grammar establishing constraints on element structure and content  Specification of attributes and their types  Definitions of entities

7 Example DTD

8 Shortcomings of DTDs Useful for documents, but not so good for data:  No support for structural re-use  Object-oriented-like structures aren’t supported  No support for data types  Can’t do data validation  Can have a single key item (ID), but:  No support for multi-attribute keys  No support for foreign keys (references to other keys)  No constraints on IDREFs (reference only a Section)

9 XML Schema  In XML format  Includes primitive data types (integers, strings, dates, etc.)  Supports value-based constraints (integers > 100)  User-definable structured types  Inheritance (extension or restriction)  Foreign keys  Element-type reference constraints

10 Sample XML Schema …

11 Subtyping in

12 Important XML Standards  XSL/XSLT *: presentation and transformation standards  RDF : resource description framework (meta-info such as ratings, categorizations, etc.)  Xpath/Xpointer/Xlink *: standard for linking to documents and elements within  Namespaces : for resolving name clashes  DOM : Document Object Model for manipulating XML documents  SAX : Simple API for XML parsing

13 Some Key XML Resources  W3C XML standards  SGML, XML standards  XML portal  xml.apache.org: Apache XML tools (Cocoon, Xerces, Xalan, etc.) xml.apache.org  java.sun.com/xml: Sun Java tools java.sun.com/xml  alphaworks.ibm.com: IBM tools alphaworks.ibm.com  tools, xCentral search  XML directory

14 Conclusions  XML is emerging as the standard for data publishing and exchange  Based on nested elements, references  DTDs and XML Schema provide constraints on structure  Later in this quarter:  Querying, presenting XML  Storing XML  Integrating XML