The packages themselves can't mutate, the nix store is read-only (except during installation). But some packages needs a global, mutable state (think databases)

The packages themselves can't mutate, the nix store is read-only (except during installation). But some packages needs a global, mutable state (think databases), which is fine. What isn't fine is that some packages expect that mutable space to be in certain paths, and things get messy. Gitlab comes to mind as a service that's been extremely painful to get working under nixos because it expects its install path to be mutable.