depot/third_party/nixpkgs/nixos/doc/manual/development/replace-modules.section.md

Replace Modules

Modules that are imported can also be disabled. The option declarations, config implementation and the imports of a disabled module will be ignored, allowing another to take its place. This can be used to import a set of modules from another channel while keeping the rest of the system on a stable release.

disabledModules is a top level attribute like imports, options and config. It contains a list of modules that will be disabled. This can either be the full path to the module or a string with the filename relative to the modules path (eg. <nixpkgs/nixos/modules> for nixos).

This example will replace the existing postgresql module with the version defined in the nixos-unstable channel while keeping the rest of the modules and packages from the original nixos channel. This only overrides the module definition, this won't use postgresql from nixos-unstable unless explicitly configured to do so.

{ config, lib, pkgs, ... }:

{
  disabledModules = [ "services/databases/postgresql.nix" ];

  imports =
    [ # Use postgresql service from nixos-unstable channel.
      # sudo nix-channel --add https://nixos.org/channels/nixos-unstable nixos-unstable
      <nixos-unstable/nixos/modules/services/databases/postgresql.nix>
    ];

  services.postgresql.enable = true;
}

This example shows how to define a custom module as a replacement for an existing module. Importing this module will disable the original module without having to know its implementation details.

{ config, lib, pkgs, ... }:

with lib;

let
  cfg = config.programs.man;
in

{
  disabledModules = [ "services/programs/man.nix" ];

  options = {
    programs.man.enable = mkOption {
      type = types.bool;
      default = true;
      description = "Whether to enable manual pages.";
    };
  };

  config = mkIf cfg.enabled {
    warnings = [ "disabled manpages for production deployments." ];
  };
}