NixOS: declare all state locations

[roberth]

Project description

Provide options that declare where all system state is kept. This serves a number of purposes:

• provide structured documentation
• let you query your system configuration to tell you where it thinks all your system state is kept
• let other modules use this information to provide useful functionality such as
  • backups
  • killing state in other locations (e.g. Impermanence, which would be great to support natively in NixOS)
  • if not derived directly, provide reporting on whether your backups or impermanence setup are ok

Example module config:

  config = mkMerge [
    (mkIf cfg.enable ...)
    {
      state."my-module" = {
        locations = [
          "/var/lib/my-module";
        ];
        enable = cfg.enable;
      };
      # (already exists)
      meta.maintainers = ...;
    }
  ];

Some discussion can be found in

• https://github.com/NixOS/nixpkgs/pull/273384#issuecomment-1851914680

---

Add a :+1: reaction to issues you find important.

[jansol]

I assume by Impermanence you mean nix-community/impermanence?

[rnhmjoj]

Great idea! NixOS could do better with handling its state and this seems a step in the right direction.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/what-about-state-management/37082/2

[gytis-ivaskevicius]

How about we do this the other way around? Configure applications to use certain state directory like /volumes/postgres whenever possible

this would allow users to easily find data directories and configure mountpoints/snapshotting and since its a custom path - it would allow us to easily find other state locations which are not properly configured

Not sure if there is much point in doing so, just a thought