<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.period</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>services.cron.systemCronJobs</option> (the list of
commands to be executed periodically by <command>cron</command>).</para>

<example xml:id='locate-example'><title>NixOS Module for the “locate” Service</title>
<programlisting>
{ config, lib, pkgs, ... }:

with lib;

let locatedb = "/var/cache/locatedb"; 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.
        '';
      };

      period = mkOption {
        type = types.str;
        default = "15 02 * * *";
        description = ''
          This option defines (in the format used by cron) when the
          locate database is updated.  The default is to update at
          02:15 at night every day.
        '';
      };

    };

  };

  config = {

    systemd.services.update-locatedb =
      { description = "Update Locate Database";
        path  = [ pkgs.su ];
        script =
          ''
            mkdir -m 0755 -p $(dirname ${locatedb})
            exec updatedb --localuser=nobody --output=${locatedb} --prunepaths='/tmp /var/tmp /run'
          '';
      };

    services.cron.systemCronJobs = optional config.services.locate.enable
      "${config.services.locate.period} root ${config.systemd.package}/bin/systemctl start update-locatedb.service";

  };
}</programlisting>
</example>

<xi:include href="option-declarations.xml" />
<xi:include href="option-def.xml" />

</chapter>
