eXistdb

eXist-db is an open-source XML database and XQuery application server. It provides a native XML storage engine and a powerful XQuery processor designed to store, query, and manage XML documents. The system uses a document-oriented model with collections and resources and supports standard XML technologies such as XPath, XQuery, and XSLT.

Access and interfaces: It exposes HTTP-based interfaces including a REST API, WebDAV for file-like access, and a web-based Admin Console for configuration and monitoring.

Querying and indexing: The platform supports XQuery as the primary query language, including the XQuery Update Facility for modifying data. It provides built-in indexing capabilities (path, value, and full-text indexes) to accelerate queries and updates. It also supports XML transformations via XSLT.

Architecture and usage: eXist-db runs on the Java Virtual Machine, making it cross-platform. It can run as a stand-alone server or be embedded in Java applications. It is designed for XML-centric workloads such as digital humanities, metadata stores, content management, and data integration.

License and availability: The project is open-source and released under the GNU Affero General Public License (AGPL) version 3, with community and commercial support options.