Nix registry

A quick note about a small trick I implemented in my NixOS configurations.

I started using flakes a few months ago, and quickly began using them daily. Notably, I moved the NixOS configurations for all my machines in a single flake. But there has been a caveat in my workflow: using flakes for almost every project I compile made me end up with lots of different versions of Nixpkgs lying around. It became very annoying when compiling LaTeX documents: two LaTeX projects created a few hours apart could be using different versions of Nixpkgs, and thus different versions of TexLive. Downloading a few gigabytes every time I set up a LaTeX project was not very optimal. I began copying lock files between projects, but it only made me more frustrated.

Nix uses a registry to resolve flake URIs. For instance, it resolves the special flake URI “nixpkgs” to “github:nixpkgs/nixos-unstable”. We can configure the registry system-wide to map “nixpkgs” to a specific revision of the repository. Using the “rev” attribute of a flake input, we can even point it to the same revision as the one used by our flake-defined configuration.

Just be sure not to use the “nixpkgs” flake URL in your configuration flake, otherwise nix flake update won’t update nixpkgs!

The module to configure the Nix registry looks like this:

{

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
  };

  outputs = { self, nixpkgs, }: {

    nixosModules.default = {
      nix.registry.nixpkgs = {
        from = {
          type = "indirect";
          id = "nixpkgs";
        };
        to = {
          type = "github";
          owner = "nixos";
          repo = "nixpkgs";
          inherit (nixpkgs) rev;
        };
      };
    };

  };

}

The Nix registry can also be configured at the user level, and this module can be used as a home-manager module.