Good examples of authentication/authorization subsystem/microservice implementation?

I'm looking for an open source project to learn how to properly implement authentication and access control. What could you recommend? Any language would suffice, though Java or Go is preferred.

I'm likely looking for something where the access policies are stored separately (as in two different data sources) from the main entity repository.

Also it should be possible to set a policy for a group of entities (as in RBAC) and for a single entity based on its ID.