<chapter xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        xml:id="sec-writing-modules">
 <title>Writing NixOS Modules</title>
 <para>
  NixOS has a modular system for declarative configuration. This system
  combines multiple <emphasis>modules</emphasis> to produce the full system
  configuration. One of the modules that constitute the configuration is
  <filename>/etc/nixos/configuration.nix</filename>. Most of the others live in
  the
  <link
xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/modules"><filename>nixos/modules</filename></link>
  subdirectory of the Nixpkgs tree.
 </para>
 <para>
  Each NixOS module is a file that handles one logical aspect of the
  configuration, such as a specific kind of hardware, a service, or network
  settings. A module configuration does not have to handle everything from
  scratch; it can use the functionality provided by other modules for its
  implementation. Thus a module can <emphasis>declare</emphasis> options that
  can be used by other modules, and conversely can <emphasis>define</emphasis>
  options provided by other modules in its own implementation. For example, the
  module
  <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/security/pam.nix"><filename>pam.nix</filename></link>
  declares the option <option>security.pam.services</option> that allows other
  modules (e.g.
  <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/networking/ssh/sshd.nix"><filename>sshd.nix</filename></link>)
  to define PAM services; and it defines the option
  <option>environment.etc</option> (declared by
  <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/system/etc/etc.nix"><filename>etc.nix</filename></link>)
  to cause files to be created in <filename>/etc/pam.d</filename>.
 </para>
 <para xml:id="para-module-syn">
  In <xref
linkend="sec-configuration-syntax"/>, we saw the following structure
  of NixOS modules:
<programlisting>
{ config, pkgs, ... }:

{ <replaceable>option definitions</replaceable>
}
</programlisting>
  This is actually an <emphasis>abbreviated</emphasis> form of module that only
  defines options, but does not declare any. The structure of full NixOS
  modules is shown in <xref linkend='ex-module-syntax' />.
 </para>
 <example xml:id='ex-module-syntax'>
  <title>Structure of NixOS Modules</title>
<programlisting>
{ config, pkgs, ... }: <co xml:id='module-syntax-1' />

{
  imports =
    [ <replaceable>paths of other modules</replaceable> <co xml:id='module-syntax-2' />
    ];

  options = {
    <replaceable>option declarations</replaceable> <co xml:id='module-syntax-3' />
  };

  config = {
    <replaceable>option definitions</replaceable> <co xml:id='module-syntax-4' />
  };
}</programlisting>
 </example>
 <para>
  The meaning of each part is as follows.
  <calloutlist>
   <callout arearefs='module-syntax-1'>
    <para>
     This line makes the current Nix expression a function. The variable
     <varname>pkgs</varname> contains Nixpkgs, while <varname>config</varname>
     contains the full system configuration. This line can be omitted if there
     is no reference to <varname>pkgs</varname> and <varname>config</varname>
     inside the module.
    </para>
   </callout>
   <callout arearefs='module-syntax-2'>
    <para>
     This list enumerates the paths to other NixOS modules that should be
     included in the evaluation of the system configuration. A default set of
     modules is defined in the file
     <filename>modules/module-list.nix</filename>. These don't need to be added
     in the import list.
    </para>
   </callout>
   <callout arearefs='module-syntax-3'>
    <para>
     The attribute <varname>options</varname> is a nested set of
     <emphasis>option declarations</emphasis> (described below).
    </para>
   </callout>
   <callout arearefs='module-syntax-4'>
    <para>
     The attribute <varname>config</varname> is a nested set of
     <emphasis>option definitions</emphasis> (also described below).
    </para>
   </callout>
  </calloutlist>
 </para>
 <para>
  <xref linkend='locate-example' /> shows a module that handles the regular
  update of the “locate” database, an index of all files in the file
  system. This module declares two options that can be defined by other modules
  (typically the user’s <filename>configuration.nix</filename>):
  <option>services.locate.enable</option> (whether the database should be
  updated) and <option>services.locate.interval</option> (when the update
  should be done). It implements its functionality by defining two options
  declared by other modules: <option>systemd.services</option> (the set of all
  systemd services) and <option>systemd.timers</option> (the list of commands
  to be executed periodically by <command>systemd</command>).
 </para>
 <example xml:id='locate-example'>
  <title>NixOS Module for the “locate” Service</title>
<programlisting>
{ config, lib, pkgs, ... }:

with lib;

let
  cfg = config.services.locate;
in {
  options.services.locate = {
    enable = mkOption {
      type = types.bool;
      default = false;
      description = ''
        If enabled, NixOS will periodically update the database of
        files used by the <command>locate</command> command.
      '';
    };

    interval = mkOption {
      type = types.str;
      default = "02:15";
      example = "hourly";
      description = ''
        Update the locate database at this interval. Updates by
        default at 2:15 AM every day.

        The format is described in
        <citerefentry><refentrytitle>systemd.time</refentrytitle>
        <manvolnum>7</manvolnum></citerefentry>.
      '';
    };

    # Other options omitted for documentation
  };

  config = {
    systemd.services.update-locatedb =
      { description = "Update Locate Database";
        path  = [ pkgs.su ];
        script =
          ''
            mkdir -m 0755 -p $(dirname ${toString cfg.output})
            exec updatedb \
              --localuser=${cfg.localuser} \
              ${optionalString (!cfg.includeStore) "--prunepaths='/nix/store'"} \
              --output=${toString cfg.output} ${concatStringsSep " " cfg.extraFlags}
          '';
      };

    systemd.timers.update-locatedb = mkIf cfg.enable
      { description = "Update timer for locate database";
        partOf      = [ "update-locatedb.service" ];
        wantedBy    = [ "timers.target" ];
        timerConfig.OnCalendar = cfg.interval;
      };
  };
}
</programlisting>
 </example>
 <xi:include href="option-declarations.xml" />
 <xi:include href="option-types.xml" />
 <xi:include href="option-def.xml" />
 <xi:include href="assertions.xml" />
 <xi:include href="meta-attributes.xml" />
 <xi:include href="importing-modules.xml" />
 <xi:include href="replace-modules.xml" />
</chapter>
