Document versions: an HLC timestamp plus the id of the database that authored the write.

Versions replace rev-tree revisions ("Gen-Hash") as the identity of a document write. The HLC gives causally meaningful last-write-wins ordering; the author id breaks ties deterministically and identifies the writer. Authorship is per-database (each database carries a source id, see barrel_db_server), not per-node: two databases on one node share the HLC clock, so node-level authorship would totally order their writes and hide concurrency. The API _rev token is <hex(hlc)>@<author>: the hex is fixed width, so lexicographic token order equals causal order.

The winner among sibling versions is the maximum under compare/2, a commutative rule: any replica that sees the same version set picks the same winner.