How to add custom services in Nixos - 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

Adding a service requires that you first write a service definition for your service. That is, a nix file that declares the options of your service and provides an implementation.
Let's say our service is called foo, then we write a service definition for it an save it as the file foo.nix:
{ config, lib, pkgs, ... }:
with lib; # use the functions from lib, such as mkIf
let
# the values of the options set for the service by the user of the service
foocfg = config.services.foo;
in {
##### interface. here we define the options that users of our service can specify
options = {
# the options for our service will be located under services.foo
services.foo = {
enable = mkOption {
type = types.bool;
default = false;
description = ''
Whether to enable foo.
'';
};
barOption = {
type = types.str;
default = "qux";
description = ''
The bar option for foo.
'';
};
};
};
##### implementation
config = mkIf foocfg.enable { # only apply the following settings if enabled
# here all options that can be specified in configuration.nix may be used
# configure systemd services
# add system users
# write config files, just as an example here:
environment.etc."foo-bar" = {
text = foocfg.bar; # we can use values of options for this service here
};
};
For example, for Hydra, this file can be found here: https://github.com/NixOS/hydra/blob/dd32033657fc7d6a755c2feae1714148ee43fc7e/hydra-module.nix.
After having written the service definition, we can use it our main configuration like this:
{
network.description = "Web server";
webserver = { config, pkgs, ... }: {
imports = [ ./foo.nix ]; # import our service
services.mysql = {
enable = true;
package = pkgs.mysql51;
};
services.foo = {
enable = true;
bar = "hello nixos modules!";
};
};
}
Disclaimer: there might be some typos in this, I have not tested it.

Answer 2

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)