XML Parser, DTD and Schema

XML Parser

An XML parser reads an XML document and converts its textual representation into a form that a program can process. It checks XML syntax and, for validating parsers, can also check the document against a DTD or schema.

Types

• DOM parser: loads the document into an in-memory tree; convenient for random access but uses more memory.
• SAX parser: processes XML sequentially through events; efficient for large documents but does not keep the whole tree in memory.

DTD

A Document Type Definition (DTD) defines the legal structure and elements of an XML document.

<!DOCTYPE student [
  <!ELEMENT student (name, roll)>
  <!ELEMENT name (#PCDATA)>
  <!ELEMENT roll (#PCDATA)>
]>

XML Schema (XSD)

An XML Schema Definition (XSD) also defines XML structure, but provides richer data types, namespaces and more precise constraints.

<xs:element name="roll" type="xs:integer"/>

DTD vs Schema

DTD	XML Schema
Older validation mechanism	XML-based validation language
Limited data types	Rich built-in data types
Less expressive	More expressive constraints
Syntax is not XML	Uses XML syntax

Conclusion: The parser processes XML; DTD and XSD provide rules used to validate its structure.