Why XML ? Problems with HTML HTML design - HTML is intended for presentation of information as Web pages. - HTML contains a fixed set of markup tags. This design is not appropriate for data: - Tags don’t convey meaning of the data inside the tags. - Tags are not extensible.

Motivation for XML The need for a more SGML-like markup language for representing metadata. The XML design  Separates syntax (structural representation) from  semantics - and only considers syntax. - There is no fixed set of markup tags - we may define our own tags, according to the information. The objective is to have XML as the universal format for structuring information. A formal specification of XML can be found at:

Simple XML Example John Doe Introduction to XML 12 June XYZ Foo Bar Introduction to XSL 12 June ABC XML by itself is just hierarchically structured text - We need some sort of “grammar” (for a “Book” in the example) to check for correctness - A “stylesheet” is needed to define how the data will be shown Make up your own tags Sub-elements (properties) of “Book”

DTD [Document Type Definition] It is used to describe certain classes of XML documents (similar to grammars for other languages), and thus enforces constraints on the structure of the XML document. A Valid XML document is one, which conforms to a given DTD. Example DTD: Sub-Elements have to occur in the order specified Attribute “ID” of element “Book” is a required field Title is a “Parsed Character Data” i.e. string. “+” implies one or more Authors The Bookstore element can have 0 or more Books

Problems with XML DTDs It's not XML syntax You write your XML document using one syntax and the DTD using another syntax -> inconsistent, more work for the parsers. Limited set of primitive datatypes Desire a set of datatypes compatible with those found in databases –One of the main weaknesses of DTD is its lack of support for data types beyond character strings (PCDATA). Limited support for applying constraints. Can support only constraints like “+” (1 or more occurences), “?” (0 or 1 occurences), “*” (0 or more occurences), etc. No facility for providing constraints like those found in databases (enumerations, ranges, string length, etc.)

XML Schema XML Schemas are an improvement over DTDs: –Enhanced datatypes. Wider range of primitive data types, supporting those found in databases (string, boolean, decimal, integer, date, etc.) Can create your own datatypes (complexType) –Written in the same syntax as XML documents. –Object-oriented Can derive new type definitions on the basis of old ones (refinement) –Can have further constraints on the range of data values. Examples: maxlength, precision, enumeration, maxInclusive (upper bound), minInclusive (lower bound), encoding (applies only to binary)

An important diversion: Namespaces What is a Namespace ? The Namespace of an element, is the scope within which, it (and thus it’s name) is valid. (Ex. A basic block { … } in C) Why do we need Namespaces ?  If elements were defined within a global scope, it becomes a problem when combining elements from multiple documents. Name collision is hard to avoid. Modularity: If such a markup vocabulary exists which is well understood and for which there is useful software available, it is better to reuse this rather than make it again. Namespaces in XML: An XML namespace is a collection of names, identified by a URI reference. Names from XML namespaces may appear as qualified names, which contain a single colon, separating the name into a prefix and a local part. The prefix, which is mapped to a URI reference, selects a namespace

XML Schema [example revisited] <xsd:schema xmlns:xsd=“ targetNamespace=" xmlns=“ Prefix “xsd” refers to the XMLSchema namespace “xmlns” refers to the default namespace Defining the Element “Bookstore” as a complex Type Containing a sequence of 1 or more “Book” elements When referring to another Element, use “ref” The Author can be 1 or more Element definitions Notice the use of more meaningful data types

XSL [XML Stylesheet Language] Title Author Date ISBN An Example: TitleAuthorDateISBN Introduction to XMLJohn Doe12 June Introduction to XSLFoo Bar12 June Result: (Notice, that some fields have been filtered out from the XML file) Match the Root Element What you print out, on a Match of the root element Go through Each “Book” Element (inside a “Bookstore” Element) And, print out their Title, Author, Date, and ISBN “xsl” prefix refers to the XSL namespace

Tools/Software XML Spy By far, the most comprehensive editor. Handles XML files, DTD’s, XSL files, as well as XSD (XML Schema). Unfortunately only a 30 day trial version. XML Notepad Microsoft XML Notepad is a simple application for building and editing small sets of XML-based data. Freeware. XML Pro XML Pro is a top-notch XML editor but it doesn’t include as many features as XML Spy. Shareware. You can also validate your XML files by just opening them with IE5.0 or above. It checks if the XML file is well-formed or not, and also validates against a DTD (if specified on the DOCTYPE declaration Some nice & short Tutorials on XML/XSL/DTD/XML Schemas can be found at: