Zanzibar.
Google's globally consistent authorization system, described in a 2019 paper that became the architectural template for modern ReBAC and FGA implementations.
Google's internal Zanzibar implementation is not open source, but the paper is detailed enough that the open-source ecosystem rebuilt the architecture multiple times. OpenFGA (CNCF Sandbox project, originated at Auth0), SpiceDB (Authzed), Permify, Ory Keto, Auth0 FGA, and WorkOS FGA are all Zanzibar-derived. Each makes slightly different design choices around schema language, consistency model, and operational tooling.
Common questions
Is Zanzibar open source?
What are zookies?
Which products implement Zanzibar?