What Is XML: A Comprehensive Guide

Last Updated: 25 Mar, 2025

What is XML?

XML (Extensible Markup Language) is a markup language and file format used for storing, transmitting, and reconstructing data. It provides a structured approach to encoding information, making it both human-readable and machine-readable.

Key Features of XML

1. Defined by Open Standards

• Established by the World Wide Web Consortium (W3C) in 1998 under the XML 1.0 Specification.
• Follows strict rules to ensure data is structured and easy to interpret.

2. Design Goals

• XML is designed with simplicity, generality, and usability across different platforms.
• Uses a text-based format, making it easy to read and edit.

3. Unicode Support

• XML supports Unicode, allowing it to store and process text in multiple languages.

4. Versatility

• Initially created for documents, XML is now widely used for data representation across various domains, including web services, APIs, and configuration files.

5. Schema and APIs

• XML structures can be defined using Document Type Definition (DTD), XML Schema (XSD), or Relax NG.
• Various APIs, such as DOM (Document Object Model) and SAX (Simple API for XML), enable XML processing.

6. Common Uses of XML

• Data exchange between different systems (e.g., web services, APIs).
• Configuration files in software applications.
• Storing structured information (e.g., RSS feeds, SVG graphics, document formats like DOCX).
• Web development (e.g., XHTML, SOAP, AJAX-based applications).

XML Overview

1. XML’s Main Purpose: Serialization

Serialization in XML means:

• Storing data in a structured format.
• Transmitting data between different systems.
• Reconstructing data in a usable form.

When two systems need to exchange information, XML provides a standardized file format for interoperability. It acts as a “lingua franca” for structured data exchange.

2. XML as a Markup Language

XML is a markup language, meaning it:

• Labels data with tags for clear structure.
• Organizes information hierarchically.
• Categorizes data with metadata.

Example:

<book>
    <title>XML Basics</title>
    <author>John Doe</author>
</book>

Here, <book>, <title>, and <author> are tags that define data elements.

3. XML Schema (XSD) and Validation

An XML Schema (XSD) defines structural rules for XML documents.

XML documents can be:

• Well-formed → Follows basic syntax rules (correct nesting, proper closing tags).
• Valid → Conforms to schema rules (mandatory elements, correct data types).

Example of an XSD Rule:

<xs:element name="title" type="xs:string"/>

This ensures that <title> must contain a string value.

4. XML Media Types (RFC 7303)

The IETF RFC 7303 defines XML media types for data transmission:

• application/xml → Standard XML data.
• application/xml-external-parsed-entity → Parsed XML entities.
• application/xml-dtd → XML DTD documents.

For XML-based formats, the +xml suffix is used (e.g., image/svg+xml for SVG images).

5. XML in Networked Applications (RFC 3470 / IETF BCP 70)

The IETF BCP 70 (RFC 3470) outlines best practices for XML use in network-based applications, ensuring seamless data interoperability across systems.

1. Applications of XML

XML is a flexible markup language designed for data exchange over the Internet. Its syntax has influenced numerous document formats and communication protocols, making it a foundational technology in many industries.

Common XML-Based Formats:

• Web Feeds: RSS, Atom
• Office Documents: Office Open XML, OpenDocument
• Graphics and 3D Models: SVG, COLLADA
• Web Technologies: XHTML
• Data Exchange Protocols: SOAP, XMPP
• AJAX: Uses XML for asynchronous data exchange

Industry Standards Using XML:

• Healthcare: HL7 (Health Level 7)
• Travel Industry: OpenTravel Alliance
• Finance: FpML (Financial products Markup Language)
• Real Estate & Mortgage: MISMO
• Government Data Exchange: National Information Exchange Model

XML in Science:

In meteorology, XML is used for operational weather data representation following IWXXM standards. Similarly, Darwin Information Typing Architecture (DITA) is an XML standard widely used in publishing.

2. Key Terminology in XML

Understanding the core terminology in XML is crucial for working with XML documents effectively.

Character

An XML document is a sequence of Unicode characters. XML 1.1 allows almost all Unicode characters, except the Null character (U+0000), which is strictly forbidden.

Processor and Application

• The XML processor (or parser) reads and analyzes the XML markup.
• The application receives structured data from the processor and acts on it.

Markup and Content

XML divides characters into markup and content:

• Markup: Includes elements, attributes, and entities (e.g., <tag> and &).
• Content: The actual data within elements.
• CDATA Sections: Allow text that shouldn’t be parsed as markup, enclosed within <![CDATA[ ... ]]>.

Tags and Elements

• Tags: Markup constructs enclosed in < >.
  • <section> (Start-tag)
  • </section> (End-tag)
  • <line-break /> (Empty-element tag)
• Elements: Logical components defined by tags.
  • Example: <greeting>Hello, world!</greeting>

Attributes

Attributes provide metadata for elements using name-value pairs. Example:

<img src="image.jpg" alt="Description" />

• src="image.jpg" and alt="Description" are attributes of the <img> element.

3. Characters and Escaping in XML

Valid Unicode Characters in XML

XML documents only allow specific Unicode characters to ensure proper parsing.

Allowed Characters in XML 1.0:

• Control characters:
  • U+0009 (Tab)
  • U+000A (Line Feed)
  • U+000D (Carriage Return)
• Text characters:
  • U+0020–U+D7FF
  • U+E000–U+FFFD
  • U+10000–U+10FFFF

XML 1.1 Enhancements:

• Allows all U+0001–U+001F control characters but requires them to be escaped.
• U+0000 (Null character) remains forbidden.

4. Encoding in XML

Common XML Encodings:

XML supports Unicode-based encodings, including:

• UTF-8 (Recommended default)
• UTF-16 (Must be supported by all XML parsers)
• ASCII, ISO-8859-X (Legacy encodings)

Encoding Declaration in XML:

The XML header can specify encoding:

<?xml version="1.0" encoding="UTF-8"?>

• If no encoding is specified, UTF-8 is assumed.
• UTF-16 requires a BOM (Byte Order Mark) for proper detection.

5. Escaping Special Characters in XML

Since XML syntax reserves certain characters, they must be escaped to avoid conflicts.

Predefined XML Entities:

Numeric Character References:

For special characters that cannot be typed, use numeric references:

• Decimal:

  &#20013;  <!-- Chinese character "中" -->
  

• Hexadecimal:

  &#x4E2D;  <!-- Chinese character "中" -->
  

• Example:

  <message>I &lt;3 J&#xF6;rg</message>
  

  Displays as: I <3 Jörg

Handling Binary Data:

XML does not support raw binary data. Instead, use Base64 encoding.

6. Comments in XML

XML Comment Syntax:

<!-- This is a valid XML comment -->

Rules for XML Comments:

• Cannot appear before the XML declaration.
• Cannot contain -- (double hyphen) inside the comment.
• Entity references are ignored in comments.

Example:

<!-- This is a comment with < and & symbols -->

Conclusion

XML is an essential technology for structured data exchange, powering web feeds, office documents, scientific data, and industry standards. Understanding key XML terminology, encoding rules, and escaping mechanisms ensures well-formed XML documents that function correctly across different systems.

By following best practices in character encoding and escaping, developers can avoid parsing errors and ensure compatibility in XML-based applications.