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nixos/docs: Add documentation for settings options
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Silvan Mosberger
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179
nixos/doc/manual/development/settings-options.xml
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179
nixos/doc/manual/development/settings-options.xml
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<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-settings-options">
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<title>Options for Program Settings</title>
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<para>
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Many programs have configuration files where program-specific settings can be declared. File formats can be separated into two categories:
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<itemizedlist>
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<listitem>
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<para>
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Nix-representable ones: These can trivially be mapped to a subset of Nix syntax. E.g. JSON is an example, since its values like <literal>{"foo":{"bar":10}}</literal> can be mapped directly to Nix: <literal>{ foo = { bar = 10; }; }</literal>. Other examples are INI, YAML and TOML. The following section explains the convention for these settings.
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</para>
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</listitem>
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<listitem>
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<para>
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Non-nix-representable ones: These can't be trivially mapped to a subset of Nix syntax. Most generic programming languages are in this group, e.g. bash, since the statement <literal>if true; then echo hi; fi</literal> doesn't have a trivial representation in Nix.
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</para>
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<para>
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Currently there are no fixed conventions for these, but it is common to have a <literal>configFile</literal> option for setting the configuration file path directly. The default value of <literal>configFile</literal> can be an auto-generated file, with convenient options for controlling the contents. For example an option of type <literal>attrsOf str</literal> can be used for representing environment variables which generates a section like <literal>export FOO="foo"</literal>. Often it can also be useful to also include an <literal>extraConfig</literal> option of type <literal>lines</literal> to allow arbitrary text after the autogenerated part of the file.
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</para>
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</listitem>
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</itemizedlist>
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</para>
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<section xml:id="sec-settings-nix-representable">
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<title>Nix-representable Formats (JSON, YAML, TOML, INI, ...)</title>
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<para>
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By convention, formats like this are handled with a generic <literal>settings</literal> option, representing the full program configuration as a Nix value. The type of this option should represent the format. The most common formats have a predefined type and string generator already declared under <literal>pkgs.formats</literal>:
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<variablelist>
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<varlistentry>
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<term>
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<varname>pkgs.formats.json</varname> { }
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</term>
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<listitem>
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<para>
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A function taking an empty attribute set (for future extensibility) and returning a set with JSON-specific attributes <varname>type</varname> and <varname>generate</varname> as specified <link linkend='pkgs-formats-result'>below</link>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>pkgs.formats.yaml</varname> { }
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</term>
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<listitem>
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<para>
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A function taking an empty attribute set (for future extensibility) and returning a set with YAML-specific attributes <varname>type</varname> and <varname>generate</varname> as specified <link linkend='pkgs-formats-result'>below</link>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>pkgs.formats.ini</varname> { <replaceable>listsAsDuplicateKeys</replaceable> ? false, ... }
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</term>
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<listitem>
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<para>
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A function taking an attribute set with values
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<variablelist>
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<varlistentry>
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<term>
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<varname>listsAsDuplicateKeys</varname>
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</term>
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<listitem>
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<para>
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A boolean for controlling whether list values can be used to represent duplicate INI keys
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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It returns a set with INI-specific attributes <varname>type</varname> and <varname>generate</varname> as specified <link linkend='pkgs-formats-result'>below</link>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>pkgs.formats.toml</varname> { }
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</term>
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<listitem>
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<para>
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A function taking an empty attribute set (for future extensibility) and returning a set with TOML-specific attributes <varname>type</varname> and <varname>generate</varname> as specified <link linkend='pkgs-formats-result'>below</link>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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<para xml:id="pkgs-formats-result">
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These functions all return an attribute set with these values:
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<variablelist>
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<varlistentry>
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<term>
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<varname>type</varname>
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</term>
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<listitem>
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<para>
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A module system type representing a value of the format
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<varname>generate</varname> <replaceable>filename</replaceable> <replaceable>jsonValue</replaceable>
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</term>
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<listitem>
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<para>
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A function that can render a value of the format to a file. Returns a file path.
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<note>
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<para>
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This function puts the value contents in the Nix store. So this should be avoided for secrets.
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</para>
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</note>
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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<example xml:id="ex-settings-nix-representable">
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<title>Module with conventional <literal>settings</literal> option</title>
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<para>
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The following shows a module for an example program that uses a JSON configuration file. It demonstrates how above values can be used, along with some other related best practices. See the comments for explanations.
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</para>
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<programlisting>
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{ options, config, lib, pkgs, ... }:
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let
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cfg = config.services.foo;
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# Define the settings format used for this program
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settingsFormat = pkgs.formats.json {};
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in {
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options.services.foo = {
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enable = lib.mkEnableOption "foo service";
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settings = lib.mkOption {
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# Setting this type allows for correct merging behavior
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type = settingsFormat.type;
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default = {};
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description = ''
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Configuration for foo, see
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<link xlink:href="https://example.com/docs/foo"/>
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for supported values.
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'';
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};
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};
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config = lib.mkIf cfg.enable {
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# We can assign some default settings here to make the service work by just
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# enabling it. We use `mkDefault` for values that can be changed without
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# problems
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services.foo.settings = {
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# Fails at runtime without any value set
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log_level = lib.mkDefault "WARN";
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# We assume systemd's `StateDirectory` is used, so we require this value,
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# therefore no mkDefault
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data_path = "/var/lib/foo";
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# Since we use this to create a user we need to know the default value at
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# eval time
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user = lib.mkDefault "foo";
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};
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environment.etc."foo.json".source =
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# The formats generator function takes a filename and the Nix value
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# representing the format value and produces a filepath with that value
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# rendered in the format
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settingsFormat.generate "foo-config.json" cfg.settings;
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# We know that the `user` attribute exists because we set a default value
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# for it above, allowing us to use it without worries here
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users.users.${cfg.settings.user} = {}
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# ...
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};
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}
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</programlisting>
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</example>
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</section>
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</section>
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@ -183,4 +183,5 @@ in {
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<xi:include href="meta-attributes.xml" />
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<xi:include href="importing-modules.xml" />
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<xi:include href="replace-modules.xml" />
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<xi:include href="settings-options.xml" />
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</chapter>
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