# Unit 4: XML (CA553) ## Introduction **XML (eXtensible Markup Language)** is a W3C standard for storing and transporting structured data with **user-defined tags**; platform independent, self-descriptive, strict syntax. - **Rules**: one **root element**, properly nested, all tags closed, case-sensitive, attribute values quoted, special characters escaped (`<`, `&`). - **Well-formed** (syntax correct) vs **valid** (conforms to DTD/schema). - Compared with HTML: XML describes data, HTML displays it. ```xml Ali85 ``` ## Displaying an XML Document Browsers show the tree by default. For formatting: **CSS** (``) or **XSLT** (transforms XML to HTML using templates and **XPath**). ## Data Interface with an XML Document Accessing XML data from programs: **XML parsers**, **data islands** and data binding in HTML, **DOM** and **SAX** APIs, **XPath/XQuery** for selecting nodes, and use as data exchange format for web services (SOAP) and configuration; **AJAX** retrieves XML (`responseXML`). ## Document Type Definitions (DTD) A **DTD** defines the legal structure: elements, attributes and their order. ```xml ]> ``` Content model symbols: `+` one or more, `*` zero or more, `?` optional, `|` choice. Data types `#PCDATA`, `CDATA`, ID. ### Internal and External DTD | Internal DTD | External DTD | |---|---| | Declared inside the XML file in DOCTYPE | Stored in separate `.dtd` file | | `` | `` | | Applies to one document | Shared by many documents | **XML Schema (XSD)** is a more powerful alternative supporting data types and namespaces. ## Parsers Using XML An **XML parser** reads XML, checks well-formedness (and validity) and exposes the content. - **DOM parser**: loads the entire document as a tree in memory; random access; heavy memory. - **SAX parser**: event-driven, sequential, low memory, read-only. - Others: **StAX** pull parser. Validating vs non-validating parsers. ## Client-Side and Server-Side Usage - **Client-side**: browser parsing with JavaScript DOM, XSLT presentation, AJAX data fetch, RSS/Atom feeds. - **Server-side**: parsing by PHP, Java or .NET to store/retrieve data, configuration files, web services (SOAP, REST/XML), data exchange between organisations. ## Likely Exam Questions 1. What is XML? List its syntax rules. (2 marks) 2. Differentiate XML and HTML. 3. What is a DTD? Explain internal and external DTD with an example. 4. Differentiate DOM and SAX parsers. 5. How can an XML document be displayed in a browser? 6. Explain client-side and server-side usage of XML.