XMLd

XMLd is a term used for a family of XML-based data description and processing technologies designed to describe, validate, and transform XML data within distributed systems. In typical implementations, XMLd combines a declarative schema language with a lightweight runtime engine, enabling developers to specify data structures, constraints, and transformation rules in a consistent XML syntax and then apply those rules to XML documents at ingest, storage, or exchange points.

Core concepts in XMLd include the notion of a data schema, which defines element types, attributes, cardinality, and data types, and a set of transformation rules that map input XML to output XML or to other representations. Schemas support standard XML concepts such as namespaces and typing, and they can express constraints beyond what is possible with basic XML Schema, including conditional processing, cross-field validation, and data enrichment. XMLd runtimes typically support streaming or in-memory processing, incremental validation, and integration with XPath or XQuery for data navigation.

XMLd is often used in data integration, API gateways, and data pipelines where robust validation and flexible transformation are required without writing custom code. It aims to complement existing XML technologies by providing a higher-level, declarative approach to common data handling tasks, while remaining interoperable with XML Schema, XSLT, and other XML tooling through common data models and serializations.

Adoption varies by organization, and implementations may differ in syntax and feature sets. Some projects emphasize lightweight embedded engines for client applications, while others provide server-side services for enterprise data exchanges. As with any XML-centric approach, performance and maintenance considerations focus on schema complexity, namespace management, and integration with existing XML toolchains.