XML

XML (eXtensible Markup Language) is an extensible markup language for structuring and exchanging data between systems. It stores data as a hierarchy of tags (elements) with attributes. It is human-readable and strictly formalized for machine processing.

Purpose

• A universal data exchange format (for integration, documents, configurations).
• Suitable for complex hierarchies and mixing text with metadata.
• Has a rich ecosystem: XSD (schemas), DTD, XPath, XSLT, Namespaces.

Basic Concepts

• Element — A pair of tags with content: <title>Book</title>.
• Attribute — An additional property of an element: <price currency=”RUB”>990</price>.
• Root Element — The single top-level container of the document.
• Namespace — A prefix for distinguishing identically named tags:
  <xhtml:div xmlns:xhtml=”http://www.w3.org/1999/xhtml”>…</xhtml:div>.
• Well-formed — The document is syntactically correct (closed tags, one root, correct quotation marks).
• Valid — The document conforms to a schema (XSD/DTD), i.e., the structure and data types.

XML Example

xml

<?xml version=”1.0″ encoding=”UTF-8″?>

<order id=”A-1024″ date=”2025-11-04″>

<customer>

<name>Irina Petrova</name>

<email>irina@example.com</email>

</customer>

<items>

<item sku=”BK-001″ qty=”2″>

<title>Advanced SEO</title>

<price currency=”RUB”>990</price>

</item>

<item sku=”BK-002″ qty=”1″>

<title>Web Analytics</title>

<price currency=”RUB”>1290</price>

</item>

</items>

<total currency=”RUB”>3270</total>

</order>

Where It’s Used

Sitemap.xml, RSS/Atom, configurations (e.g., web.config), documents (DOCX, XLSX contain XML packages), SOAP/enterprise system integration, GIS/geodata (GML), graphs (GraphML), validation of complex formats via XSD.

Pros and Cons

Pros:

• Self-descriptive and extensible.
• Strict validation via schemas.
• Powerful query and transformation tools (XPath, XSLT).

Cons:

• Verbosity (more “weight” than JSON).
• Parsing and serialization are typically heavier.

XML vs. JSON (Briefly)

• JSON is more compact and convenient for web APIs, especially with JavaScript.
• XML is preferable when formal schemas, namespaces, complex types, and transformations are needed.

Best Practices

• Explicitly specify the encoding: <?xml version=”1.0″ encoding=”UTF-8″?>.
• Use XSD for validation and as contracts between systems.
• Introduce namespaces when integrating multiple tag vocabularies.
• Try to store data as content, and metadata as attributes, without mixing their roles.
• Keep the structure stable; coordinate changes through schema versioning.