McGraw-Hill/Irwin © 2004 by The McGraw-Hill Companies, Inc. All rights reserved. An Overview of XML Ellen Pearlman Eileen Mullin Programming the Web Using XML

1-2 Learning Objectives 1.The history of XML 2.Understanding XML’s importance working with multiple platforms 3.Content and data sharing with XML 4.Using XML 5.Drawbacks of XML

1-3 The Evolution of Markup Languages Markup languages contain not only the codes that enable computers to interpret data, but also descriptive information to help humans understand the function of each element in a document. Markup languages have indicators called tags. Tags are first read and then processed by an appropriately enabled processor.

1-4 Learning the History: The Many Incarnations of SGML Into XML Researcher Charles Goldfarb coined the term “markup language” in the late 1960s while developing a project at IBM to automate the processing of legal documents. IBM’s adoption of GML (Generalized Markup Language) eventually led to the development of Standard Generalized Markup Language (SGML).

1-5 Meta-languages SGML is actually a meta-language, that is, a language that describes other text markup languages. XML’s function as its own meta-language allows it to create XML-based languages that create either documents or files used by a variety of organizations, developers and consolidated industries.

1-6 The Document Type Definition (DTD) While not a markup system itself, SGML is a programming language used to build working languages. The grammar of such a language is called its Document Type Definition (DTD) and contains its parts of speech. –XML refers to these parts of speech as the attributes, entities, elements, and notations. –A markup language’s DTD provides information about that document's structure and formatting.

1-7 Elements and Hierarchies Elements in a DTD are often allowed to contain other elements, a feature known as a hierarchy. This type of element hierarchy can also be envisioned as a branching tree diagram.

1-8 Parsers and Validation To make sure the grammar of a markup language is correct, a parser is used. A parser is a grammar checker for markup languages. A parser compares an HTML or XML document against the grammar in a process called validation, to ensure there are no mistakes.

1-9 How Validation Works If a document follows the rules listed in its DTD (or Schema), then it is said to be valid. If the document has markup errors that contradict the rules of the DTD (or Schema), then it would be labeled invalid.

1-10 Advances in Markup Languages Over Time

1-11 The Origins of HTML By the early 1990s, the Internet provided a ready platform for a hypertext mechanism, but a tagging language was needed to provide a linking mechanism as well as basic text formatting. In 1991 researcher Tim Berners-Lee at the European Laboratory for Particle Physics (CERN) released a hypertext markup language called HTML using SGML syntax.

1-12 Introducing XML XML was originally developed by the XML Working Group in XML became a full W3C recommendation in February XML documents contain either parsed or unparsed character data. Some of the data is composed of characters and some is composed of markup, which describes the layout and structure of information.

1-13 Differences Between XML and HTML HTML used a certain set of predefined tags, which were augmented in subsequent versions, until it morphed into an application of XML with XHTML. Whereas HTML displays information, XML can supply its own tags and lets developers structure and define what type of information goes into their documents. Its primary focus is to describe data or information.

1-14 The Goal of Standards The ultimate goal of these standards is to make machine-understandable data on the Web fully sharable by automated programs and people. The Web needs to have the ability to scale, or grow larger or smaller based on the needs of the user. As new programs are written, older programs can be integrated into newer versions.

1-15 Creating One Document For Multiple Platforms and Devices XML is an open standard that very clearly spells out data structures and their associated content. XML does not overload applications with legacy, or leftover data, nor does it depend on a proprietary or specialized system. –A proprietary system means it is accessible only to a special group or uses a special kind of technology.

1-16 DTDs and Schemas Even as XML 1.0 was being written in February 1998, developers at the World Wide Web Consortium (W3C) were creating a Schema Recommendation to set the framework of a new modeling definition, called the XML Schema Definition Language (XSD). In XML, a schema is used to define a class of XML documents. An instance document describes an XML document that conforms to a particular schema.

1-17 Repurposing Information with XML XML Data Repository XML Stylesheet Internet Explorer Browser Wireless Phone PDA

1-18 XML Categories by Industry

1-19 Sample XML Tags baseball Yankees Derek Jeter 45

1-20 XML Tag Basics < opens an XML tag > closes an XML tag <> encloses an opening XML statement encloses an ending XML statement

1-21 XML and Metadata XML works with metadata or data about itself, which means it can know when to link to other like-minded pages if necessary. The best way to think of metadata is think about our analogy of the card catalogue. It contains information that points you towards the information you are looking for, but is not the actual information itself.

1-22 Metadata and Middleware With metadata, XML data can then be filtered by the appropriate application once the XML structure has defined the appropriate category. Developers can choose how much information to actually display. This process allows seamless integration with complex middleware architecture. –Middleware defines any programming that mediates between two separate and different programs.

1-23 Content Sharing With XML: Taxonomies Managing and categorizing content necessitates a taxonomy. Taxonomies are usually created by a combination of methods including working with librarians, users and analysts in the field. Taxonomies also use automatic spidering of terms (searching with Web with an artificial agent) and finally the use of sophisticated linguistic analysis tools.

1-24 Content Sharing With XML: Leveraging Data Repositories

1-25 Limitations of DTDs The problem with DTDs is that their syntax is not particularly flexible. Documents in XML follow one syntax and DTDs follow another. Both DTDs and Schemas perform validation against data, but DTDs are not capable of complying with C++ and Java classes. The most difficult aspect of a DTD is that it employs its own syntax, which is not the same as XML’s syntax.

1-26 Categories of XML Development From W3C

1-27 The End