XML: Extensible Markup Language

XML (eXtensible Markup Language)

• XML is a markup language derived from SGML.
• It structures, organizes, and stores data, rather than controlling its presentation like HTML.
• Parsers, applications, or transformation tools must interpret and process XML data, unlike HTML, which web browsers directly render.
• Documents have tags providing extra information about the document sections.
• HTML focuses on displaying information to people.
• XML maintains the meaning of information.
• New tags and nested tag structures enable XML to effectively exchange data.
• XML serves as a useful data format for inter-application communication.
• It integrates information from various applications.
• Three essential characteristics: XML structures, stores, and exchanges data independently of its presentation.
• XML is extensible, users develop custom tags or languages.
• XML is a public standard developed by the WWW Consortium (W3C).

XML Syntax

• XML carries data and structure, independent of presentation.
• XML is extensible allowing for custom descriptive tags.
• XML is a public standard maintained by the W3C.
• XML syntax includes: elements, nesting and attributes
• Element syntax includes a start and end tag with a content, or use an empty-element tag.
• XML elements can contain multiple XML elements as children without overlapping.
• Only one root element is allowed in an XML document.
• Mixes of text with sub-elements are legal.

Attributes

• Specifies a single property for an element.
• Attributes are specified in name=value pairs within the element's starting tag.
• Attribute names are not defined with quotation marks, values must appear in them.
• Elements can have several attributes, each occurring only once.
• Attributes are part of the markup, sub-elements are part of the basic document.
• Information can be represented as either an attribute or an element.
• Attributes for short metadata; sub-elements for actual content.

XML References

• XML references add additional text or markup.
• XML references begin with "&" and end with ";".
• XML has two reference types: entity and character.
• Entity references use predefined special chars or custom reusable values.
• Character references use Unicode values to display characters directly.

XML Namespaces

• A mechanism uniquely identifies element and attribute names using a URI.
• XML namespaces prevent name conflicts when combining XML data from different sources.
• The syntax is defined as xmlns:prefix="URI".

XML CDATA

• Treat content as a string and prevent the parser from interpreting special characters.
• Syntax: character with markup

XML Documents and Schemas

• XML documents are created without associated schemas but schemas are essential for XML data exchange.
• DTD or XML defines structure, legal elements, and attributes.
• Applications use DTD or schemas to verify the data and can be internal or external.
• DTD constraints structure, define content type, define what attributes can or must occur inside each element.
• Define what sub-elements must occur and define now many times.
• Syntax: _

DTD Elements and Attributes

• DTD elements define the structure of the XML document.
• Content Types: child elements, parsed character data, text content, contain any content, cannot contain any content
• Attributes additional properties of elements.
• Attribute Types: Character data, unique identifier, ID reference, name tokens
• Default values: Required, implied, fixed values

Modifiers for Child Elements

• Operators specify rules for defining child elements: Sequence, Choice, Optional, Zero or More, One or More.

Bank DTD with ID and IDREF Attribute Types

• Elements have only one type ID attribute; values must be distinct in XML documents.
• IDREF attribute contains a set of ID values.
• IDREF attributes contain ID values of the same document elements.

Summary of XML

• XML is a flexible, structured markup language for organizing, storing, transmitting, and exchanging hierarchical data.
• Well-formed XML follows syntax rules, includes a root element, and uses correct case sensitivity.
• Valid XML conforms to a DTD or XSD, ensuring a predefined structure and constraints.
• XPath navigates and selects nodes using hierarchical paths.
• XQuery retrieves, manipulates, and transforms XML data.
• XSLT transforms XML into different formats.
• DOM & SAX Parsers provides methods for manipulating XML in programming languages.
• Real-world applications include configuration files, data exchange, and web services.