diff --git a/nixos/doc/manual/development/settings-options.xml b/nixos/doc/manual/development/settings-options.xml new file mode 100644 index 00000000000..84895adb444 --- /dev/null +++ b/nixos/doc/manual/development/settings-options.xml @@ -0,0 +1,179 @@ +
+ Options for Program Settings + + + Many programs have configuration files where program-specific settings can be declared. File formats can be separated into two categories: + + + + Nix-representable ones: These can trivially be mapped to a subset of Nix syntax. E.g. JSON is an example, since its values like {"foo":{"bar":10}} can be mapped directly to Nix: { foo = { bar = 10; }; }. Other examples are INI, YAML and TOML. The following section explains the convention for these settings. + + + + + 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 if true; then echo hi; fi doesn't have a trivial representation in Nix. + + + Currently there are no fixed conventions for these, but it is common to have a configFile option for setting the configuration file path directly. The default value of configFile can be an auto-generated file, with convenient options for controlling the contents. For example an option of type attrsOf str can be used for representing environment variables which generates a section like export FOO="foo". Often it can also be useful to also include an extraConfig option of type lines to allow arbitrary text after the autogenerated part of the file. + + + + +
+ Nix-representable Formats (JSON, YAML, TOML, INI, ...) + + By convention, formats like this are handled with a generic settings 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 pkgs.formats: + + + + pkgs.formats.json { } + + + + A function taking an empty attribute set (for future extensibility) and returning a set with JSON-specific attributes type and generate as specified below. + + + + + + pkgs.formats.yaml { } + + + + A function taking an empty attribute set (for future extensibility) and returning a set with YAML-specific attributes type and generate as specified below. + + + + + + pkgs.formats.ini { listsAsDuplicateKeys ? false, ... } + + + + A function taking an attribute set with values + + + + listsAsDuplicateKeys + + + + A boolean for controlling whether list values can be used to represent duplicate INI keys + + + + + It returns a set with INI-specific attributes type and generate as specified below. + + + + + + pkgs.formats.toml { } + + + + A function taking an empty attribute set (for future extensibility) and returning a set with TOML-specific attributes type and generate as specified below. + + + + + + + + These functions all return an attribute set with these values: + + + + type + + + + A module system type representing a value of the format + + + + + + generate filename jsonValue + + + + A function that can render a value of the format to a file. Returns a file path. + + + This function puts the value contents in the Nix store. So this should be avoided for secrets. + + + + + + + + + Module with conventional <literal>settings</literal> option + + 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. + + +{ options, config, lib, pkgs, ... }: +let + cfg = config.services.foo; + # Define the settings format used for this program + settingsFormat = pkgs.formats.json {}; +in { + + options.services.foo = { + enable = lib.mkEnableOption "foo service"; + + settings = lib.mkOption { + # Setting this type allows for correct merging behavior + type = settingsFormat.type; + default = {}; + description = '' + Configuration for foo, see + <link xlink:href="https://example.com/docs/foo"/> + for supported values. + ''; + }; + }; + + config = lib.mkIf cfg.enable { + # We can assign some default settings here to make the service work by just + # enabling it. We use `mkDefault` for values that can be changed without + # problems + services.foo.settings = { + # Fails at runtime without any value set + log_level = lib.mkDefault "WARN"; + + # We assume systemd's `StateDirectory` is used, so we require this value, + # therefore no mkDefault + data_path = "/var/lib/foo"; + + # Since we use this to create a user we need to know the default value at + # eval time + user = lib.mkDefault "foo"; + }; + + environment.etc."foo.json".source = + # The formats generator function takes a filename and the Nix value + # representing the format value and produces a filepath with that value + # rendered in the format + settingsFormat.generate "foo-config.json" cfg.settings; + + # We know that the `user` attribute exists because we set a default value + # for it above, allowing us to use it without worries here + users.users.${cfg.settings.user} = {} + + # ... + }; +} + + +
+ +
diff --git a/nixos/doc/manual/development/writing-modules.xml b/nixos/doc/manual/development/writing-modules.xml index bbf793bb0be..602f134f9cb 100644 --- a/nixos/doc/manual/development/writing-modules.xml +++ b/nixos/doc/manual/development/writing-modules.xml @@ -183,4 +183,5 @@ in { +