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),
• or an attribute set containing a specific key attribute.

The latter allows some modules to be disabled, despite them being distributed via attributes instead of file paths. The key should be globally unique, so it is recommended to include a file path in it, or rely on a framework to do it for you.

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, ... }:

let
  inherit (lib) mkIf mkOption types;
  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." ];
  };
}