How to add custom services in Nixos

Question

using nixops one can easily configure services like:

{
  network.description = "Web server";
  webserver = { config, pkgs, ... }:

    {
     services.mysql = {
      enable = true;
      package = pkgs.mysql51;
    };

but i want to extend services. for example by using override as done for pkgs below:

  let
    myfoo = callPackage ...
  in
  pkgs = pkgs.override {
    overrides = self: super: {
      myfoo-core = myfoo;
    };
  }

question

how to do that for services?

Answer 1

according to aszlig, we can do this:

configuration.nix

{ config, lib, ... }:

{
  disabledModules = [ "services/monitoring/nagios.nix" ];

  options.services.nagios.enable = lib.mkOption {
    # Make sure that this option type conflicts with the one in
    # the original NixOS module for illustration purposes.
    type = lib.types.str;
    default = "of course";
    description = "Really enable nagios?";
  };

  config = lib.mkIf (config.services.nagios.enable == "of course") {
    systemd.services.nagios = {
      description = "my own shiny nagios service...";
    };
  };
}

evaluate it

 $ nix-instantiate --eval '<nixpkgs/nixos>' --arg configuration ./test-disable.nix -A config.systemd.services.nagios.description
"my own shiny nagios service..."



versus without disabledModules:

 $ nix-instantiate --eval '<nixpkgs/nixos>' --arg configuration ./test-disable.nix -A config.systemd.services.nagios.description
error: The option `services.nagios.enable' in `/home/aszlig/test-disable.nix' is already declared in `/nix/var/nix/profiles/per-user/root/channels/vuizvui/nixpkgs/nixos/modules/services/monitoring/nagios.nix'.
(use '--show-trace' to show detailed location information)