Modular services

nixos/doc/manual/default.nix

@@ -13,6 +13,7 @@ let
   inherit (pkgs) buildPackages runCommand docbook_xsl_ns;
 
   inherit (pkgs.lib)
+    evalModules
     hasPrefix
     removePrefix
     flip
@@ -97,8 +98,35 @@ let
         ${testOptionsDoc.optionsJSON}/${common.outputPath}/options.json
     sed -e '/@PYTHON_MACHINE_METHODS@/ {' -e 'r ${testDriverMachineDocstrings}/machine-methods.md' -e 'd' -e '}' \
       -i ./development/writing-nixos-tests.section.md
+    substituteInPlace ./development/modular-services.md \
+      --replace-fail \
+        '@PORTABLE_SERVICE_OPTIONS@' \
+        ${portableServiceOptions.optionsJSON}/${common.outputPath}/options.json
+    substituteInPlace ./development/modular-services.md \
+      --replace-fail \
+        '@SYSTEMD_SERVICE_OPTIONS@' \
+        ${systemdServiceOptions.optionsJSON}/${common.outputPath}/options.json
   '';
 
+  portableServiceOptions = buildPackages.nixosOptionsDoc {
+    inherit (evalModules { modules = [ ../../modules/system/service/portable/service.nix ]; }) options;
+    inherit revision warningsAreErrors;
+    transformOptions = opt: opt // {
+      # Clean up declaration sites to not refer to the NixOS source tree.
+      declarations = map stripAnyPrefixes opt.declarations;
+    };
+  };
+
+  systemdServiceOptions = buildPackages.nixosOptionsDoc {
+    inherit (evalModules { modules = [ ../../modules/system/service/systemd/service.nix ]; }) options;
+    # TODO: filter out options that are not systemd-specific, maybe also change option prefix to just `service-opt-`?
+    inherit revision warningsAreErrors;
+    transformOptions = opt: opt // {
+      # Clean up declaration sites to not refer to the NixOS source tree.
+      declarations = map stripAnyPrefixes opt.declarations;
+    };
+  };
+
 in rec {
   inherit (optionsDoc) optionsJSON optionsNix optionsDocBook;
 

nixos/doc/manual/development/development.md

@@ -12,4 +12,5 @@ writing-documentation.chapter.md
 nixos-tests.chapter.md
 developing-the-test-driver.chapter.md
 testing-installer.chapter.md
+modular-services.md
 ```

nixos/doc/manual/development/modular-services.md

@@ -0,0 +1,91 @@
+
+# Modular Services {#modular-services}
+
+Status: in development. This functionality is new in NixOS 25.05, and significant changes should be expected. We'd love to hear your feedback in <!-- FIXME PR link -->
+
+Traditionally, NixOS services were defined using sets of options *in* modules, not *as* modules. This made them non-modular, resulting in problems with composability, reuse, and portability.
+
+A *modular service* is a [module] that defines values for a core set of options, including which program to run.
+
+NixOS provides two options into which such modules can be plugged:
+
+- `system.services.<name>`
+- an option for user services (TBD)
+
+Crucially, these options have the type [`attrsOf`] [`submodule`].
+The name of the service is the attribute name corresponding to `attrsOf`.
+<!-- ^ This is how composition is *always* provided, instead of a difficult thing (but this is reference docs, not a changelog) -->
+The `submodule` is pre-loaded with two modules:
+- a generic module that is intended to be portable
+- a module with systemd-specific options, whose values or defaults derive from the generic module's option values.
+
+So note that the default value of `system.services.<name>` is not a complete service. It requires that the user provide a value, and this is typically done by importing a module. For example:
+
+<!-- Not using typical example syntax, because reading this is *not* optional, and should it should not be folded closed. -->
+```nix
+{
+  system.services.httpd = {
+    imports = [ nixpkgs.modules.services.foo ];
+    foo.settings = {
+      # ...
+    };
+  };
+}
+```
+
+## Portability {#modular-service-portability}
+
+It is possible to write service modules that are portable. This is done by either avoiding the `systemd` option tree, or by defining process-manager-specific definitions in an optional way:
+
+```nix
+{ config, options, lib, ... }: {
+  _class = "service";
+  config = {
+    process.executable = "${lib.getExe config.foo.program}";
+  } // lib.optionalAttrs (options?systemd) {
+    # ... systemd-specific definitions ...
+  };
+}
+```
+
+This way, the module can be loaded into a configuration manager that does not use systemd, and the `systemd` definitions will be ignored.
+Similarly, other configuration managers can declare their own options for services to customize.
+
+## Composition and Ownership {#modular-service-composition}
+
+Compared to traditional services, modular services are inherently more composable, by virtue of being modules and receiving a user-provided name when imported.
+However, composition can not end there, because services need to be able to interact with each other.
+This can be achieved in two ways:
+1. Users can link services together by providing the necessary NixOS configuration.
+2. Services can be compositions of other services.
+
+These aren't mutually exclusive. In fact, it is a good practice when developing services to first write them as individual services, and then compose them into a higher-level composition. Each of these services is a valid modular service, including their composition.
+
+## Migration {#modular-service-migration}
+
+Many services could be migrated to the modular service system, but even when the modular service system is mature, it is not necessary to migrate all services.
+For instance, many system-wide services are a mandatory part of a desktop system, and it doesn't make sense to have multiple instances of them.
+Moving their logic into separate Nix files may still be beneficial for the efficient evaluation of configurations that don't use those services, but that is a rather minor benefit, unless modular services potentially become the standard way to define services.
+
+<!-- TODO example of a single-instance service -->
+
+## Portable Service Options {#modular-service-options-portable}
+
+```{=include=} options
+id-prefix: service-opt-
+list-id: service-options
+source: @PORTABLE_SERVICE_OPTIONS@
+```
+
+## Systemd-specific Service Options {#modular-service-options-systemd}
+
+```{=include=} options
+id-prefix: systemd-service-opt-
+list-id: systemd-service-options
+source: @SYSTEMD_SERVICE_OPTIONS@
+```
+
+[module]: https://nixos.org/manual/nixpkgs/stable/index.html#module-system
+<!-- TODO: more anchors -->
+[`attrsOf`]: #sec-option-types-composed
+[`submodule`]: #sec-option-types-submodule

nixos/doc/manual/redirects.json

@@ -2,6 +2,24 @@
   "book-nixos-manual": [
     "index.html#book-nixos-manual"
   ],
+  "modular-service-composition": [
+    "index.html#modular-service-composition"
+  ],
+  "modular-service-migration": [
+    "index.html#modular-service-migration"
+  ],
+  "modular-service-options-portable": [
+    "index.html#modular-service-options-portable"
+  ],
+  "modular-service-options-systemd": [
+    "index.html#modular-service-options-systemd"
+  ],
+  "modular-service-portability": [
+    "index.html#modular-service-portability"
+  ],
+  "modular-services": [
+    "index.html#modular-services"
+  ],
   "module-services-crab-hole": [
     "index.html#module-services-crab-hole"
   ],

nixos/modules/misc/assertions.nix

@@ -32,5 +32,7 @@
     };
 
   };
-  # impl of assertions is in <nixpkgs/nixos/modules/system/activation/top-level.nix>
+  # impl of assertions is in
+  # - <nixpkgs/nixos/modules/system/activation/top-level.nix>
+  # - <nixpkgs/nixos/modules/system/service/portable/lib.nix>
 }

nixos/modules/module-list.nix

@@ -1715,6 +1715,8 @@
   ./system/boot/tmp.nix
   ./system/boot/uvesafb.nix
   ./system/etc/etc-activation.nix
+  ./system/service/systemd/system.nix
+  ./system/service/systemd/user.nix
   ./tasks/auto-upgrade.nix
   ./tasks/bcache.nix
   ./tasks/cpu-freq.nix

nixos/modules/system/service/README.md

@@ -0,0 +1,28 @@
+
+# Modular Services
+
+This directory defines a modular service infrastructure for NixOS.
+See the [Modular Services chapter] in the manual [[source]](../../doc/manual/development/modular-services.md).
+
+[Modular Services chapter]: https://nixos.org/manual/nixos/unstable/#modular-services
+
+# Design decision log
+
+- `system.services.<name>`. Alternatives considered
+  - `systemServices`: similar to does not allow importing a composition of services into `system`. Not sure if that's a good idea in the first place, but I've kept the possibility open.
+  - `services.abstract`: used in https://github.com/NixOS/nixpkgs/pull/267111, but too weird. Service modules should fit naturally into the configuration system.
+    Also "abstract" is wrong, because it has submodules - in other words, evalModules results, concrete services - not abstract at all.
+  - `services.modular`: only slightly better than `services.abstract`, but still weird
+
+- No `daemon.*` options. https://github.com/NixOS/nixpkgs/pull/267111/files#r1723206521
+
+- For now, do not add an `enable` option, because it's ambiguous. Does it disable at the Nix level (not generate anything) or at the systemd level (generate a service that is disabled)?
+
+- Move all process options into a `process` option tree. Putting this at the root is messy, because we also have sub-services at that level. Those are rather distinct. Grouping them "by kind" should raise fewer questions.
+
+- `modules/system/service/systemd/system.nix` has `system` twice. Not great, but
+  - they have different meanings
+    1. These are system-provided modules, provided by the configuration manager
+    2. `systemd/system` configures SystemD _system units_.
+  - This reserves `modules/service` for actual service modules, at least until those are lifted out of NixOS, potentially
+

nixos/modules/system/service/portable/lib.nix

@@ -0,0 +1,33 @@
+{ lib, ... }:
+let
+  inherit (lib) concatLists mapAttrsToList showOption;
+in
+rec {
+  flattenMapServicesConfigToList =
+    f: loc: config:
+    f loc config
+    ++ concatLists (
+      mapAttrsToList (
+        k: v:
+        flattenMapServicesConfigToList f (
+          loc
+          ++ [
+            "services"
+            k
+          ]
+        ) v
+      ) config.services
+    );
+
+  getWarnings = flattenMapServicesConfigToList (
+    loc: config: map (msg: "in ${showOption loc}: ${msg}") config.warnings
+  );
+
+  getAssertions = flattenMapServicesConfigToList (
+    loc: config:
+    map (ass: {
+      message = "in ${showOption loc}: ${ass.message}";
+      assertion = ass.assertion;
+    }) config.assertions
+  );
+}

nixos/modules/system/service/portable/service.nix

@@ -0,0 +1,63 @@
+{
+  lib,
+  config,
+  options,
+  ...
+}:
+let
+  inherit (lib) mkOption types;
+  pathOrStr = types.coercedTo types.path (x: "${x}") types.str;
+  program =
+    types.coercedTo (
+      types.package
+      // {
+        # require mainProgram for this conversion
+        check = v: v.type or null == "derivation" && v ? meta.mainProgram;
+      }
+    ) lib.getExe pathOrStr
+    // {
+      description = "main program, path or command";
+      descriptionClass = "conjunction";
+    };
+in
+{
+  # https://nixos.org/manual/nixos/unstable/#modular-services
+  _class = "service";
+  imports = [
+    ../../../misc/assertions.nix
+  ];
+  options = {
+    services = mkOption {
+      type = types.attrsOf (
+        types.submoduleWith {
+          modules = [
+            ./service.nix
+          ];
+        }
+      );
+      description = ''
+        A collection of [modular services](https://nixos.org/manual/nixos/unstable/#modular-services) that are configured in one go.
+
+        You could consider the sub-service relationship to be an ownership relation.
+        It **does not** automatically create any other relationship between services (e.g. systemd slices), unless perhaps such a behavior is explicitly defined and enabled in another option.
+      '';
+      default = { };
+      visible = "shallow";
+    };
+    process = {
+      executable = mkOption {
+        type = program;
+        description = ''
+          The path to the executable that will be run when the service is started.
+        '';
+      };
+      args = lib.mkOption {
+        type = types.listOf pathOrStr;
+        description = ''
+          Arguments to pass to the `executable`.
+        '';
+        default = [ ];
+      };
+    };
+  };
+}