Depends on how the state is stored. If it's in configuration, Nix generated it and it lives immutable in the Nix store, so Nix will just point out it to the old

Depends on how the state is stored. If it's in configuration, Nix generated it and it lives immutable in the Nix store, so Nix will just point out it to the old version on rollback. If it's something like the content of a SQL database, which lives outside the Nix store and which Nix did not generate, you need some other tool (like a filesystem snapshot, maybe) to perform the rollback. I think CoW filesystems sometimes have performance issues with DBs, though, so I'm not sure that's always the approach you'd take. The Nix ecosystem does have a fairly mature tool for managing stateful components that live outside the Nix store, though: https://github.com/svanderburg/dysnomia https://github.com/svanderburg/dysnomia It's been around for a long time. Idk who all is using it