XML
Extensible Markup Language: a verbose, tag-based format for structured data. Still used in enterprise systems, SOAP, and configuration files.
What is XML?
Extensible Markup Language: a verbose, tag-based format for structured data. Still used in enterprise systems, SOAP, and configuration files.
XML is a foundational concept that sits in the Core Fundamentals area of system design. Engineers reach for it whenever they need to reason about real-world trade-offs in that space — not just for textbook correctness, but because real production systems at companies like Netflix, Amazon, and Google make these decisions every day.
If you want to go deeper than this definition — with diagrams, code, and a quiz to lock it in — work through the "XML" lesson linked below. It walks through the why, the mechanism, the trade-offs, and how the giants actually use it in production.
Learn XML in depth
Full interactive lesson with diagrams, code examples, real-world references, and a quiz.
Open the XML lessonRelated lessons
Lessons that touch on XML as part of a larger topic.
SOAP
The enterprise protocol that dominated before REST. XML-based, strictly typed, and still running critical infrastructure
intermediate · api design protocols
SAML
Security Assertion Markup Language, the XML-based protocol that powers enterprise single sign-on and identity federation
intermediate · security architecture
XML Schema
Formal validation rules for XML document structure and data types
foundation · core fundamentals
See also
Related glossary terms you might want to look up next.
JSON
JavaScript Object Notation: a lightweight text format for data interchange using key-value pairs and arrays. The lingua franca of web APIs.
REST API
An architectural style for building APIs using standard HTTP methods (GET, POST, PUT, DELETE). Resources are identified by URLs.
HTTP
The protocol powering the web. A request-response model where clients ask for resources and servers respond. Stateless by design.