document
The JSON path of the string member that names each document, or null for a store without document keys (SPEC.md §3.3): an RFC 9535 singular query without negative indexes, such as $.id or $['meta']['key'], as type paths are (SPEC.md §6.1), and not $ itself.
In a store created with one, every document must have a string of at most MAX_DOCUMENT_KEY_BYTES UTF-8 bytes there, or it's rejected (RejectReason.KEY_MISSING, RejectReason.LIMIT_EXCEEDED), and no two visible documents have the same key: an insert of a key that a visible document has throws WriteConflictException, and JsonStore.upsert replaces that document instead. JsonStore.get finds a document by its key, DocumentKeyQuery reads documents in key order, and SearchHit.documentKey names a hit's document.