PhoenixmlDb is a modern, cross-platform embedded XML/JSON document database for .NET. Store, query, and transform documents with the full power of XQuery 4.0 and XSLT 4.0 — no Saxon dependency, no license fees

Metadata

Every document in PhoenixmlDb can carry unlimited metadata — arbitrary key-value pairs stored alongside the document content. Metadata is stored in a dedicated #LMDB database, indexed for fast lookups, and participates in ACID transactions.

https://phoenixml.dev/phoenixmldb/metadata