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+# Interface for module authors
+
+Status: proposed
+
+## Context
+
+ClanModules define cross `machine` `services`.
+Services can be `instantiated` multiple times.
+To make configuration more natural each service instance has multiple `roles` (.i.e. `client`  `server`)
+
+Overall this makes it a 4-Dimensional problem where n-`machines` can have m-`services`, m-`services` can have k-`instances` and k-`instances` can have l-`roles`.
+
+### Problems
+
+Problems with the current way of writing clanModules.
+
+1. No way to retrieve the config of a single service instance
+    2. -> We can't really support multiple instances even though the nix module *could* support multiple instances
+    3. the "config" of a service MUST be evaluated against the "interface" outside of any machine.
+    -> Solution: evaluate each instance config and store it in seperate attributes
+3. Can't merge multiple config instances -> Example: https://pad.lassul.us/i9uSspVbRUmiBmsJ8OR1ug#
+    -> Propsed change: feed the instance config into a helper that defines the merge behavior
+5. Writing modules for multiple instances is cumbersome
+    -> Propsed change: Helper utility such as "perInstance"
+6. No clearly separated, role based interface that is usable via json based APIs
+    1. Currently we just import the module into a test machine and try to figure out what changes
+    -> Proposed change: enforce machine agnostic inventory interface definitions
+
+### Some immediate ideas
+
+#### simple idea
+
+Just use another nested attribute set
+
+- `clan.something.borgbackup.instance1 = config`
+- `clan.something.borgbackup.instance1 = config`
+
+Problems solved:
+
+- each instance has its own config
+
+Problems not solved:
+
+- Machine independent seperated interface
+- Still cumbersome to merge the instances
+- Unclear how to get to each config because the interface also depends on the role
+
+#### Improved Idea
+
+- PerInstance: utility that maps over all instances and defines what needs to be set for each instance and receives the instanceConfig as argument.
+- GlobalConfig: Some stuff that needs to be set statically on a machine if the service is enabled.
+
+## Proposed Change
+
+Change the ClanModule interface
+
+`client.nix`
+```nix
+{
+  # Allows backwards compatibility
+  _class = "clan";
+
+  # Allows importing other service modules
+  module.zerotier = {
+    # function that receives each instance along with its config and resolved roles
+    perInstance =
+      {
+        # serviceConfig is the config of this instance
+        # as described in interface attribute below.
+        # Merged without differing priority:
+        # service.<instanceName>.config
+        # + role.<roleName>.config
+        # + machine.<machineName>.config
+        serviceConfig,
+        # : string
+        instanceName,
+        # machine :: { name :: string , roles :: listOf string; }
+        machine,
+        # : { {roleName} :: { machines :: listOf string; } }
+        roles,
+      ... }:
+      {
+        nixosModule = {
+          config.debug."${instanceName}-client" = serviceConfig;
+        };
+      };
+    # Function that is appplied once for this role
+    perService = {
+      nixosModule =
+        { lib, ... }:
+        {
+          # Some shared code should be put into a shared file
+          # Which is then imported into all/some roles
+          imports = [
+            ../shared.nix
+          ];
+          options.debug = lib.mkOption {
+            type = lib.types.attrsOf lib.types.raw;
+          };
+        };
+    };
+
+    # Describes the settings how a `client` can be configured.
+    # These are the `options` of serviceConfig of a `client`. (Since: This file is called client.nix)
+    interface =
+      { lib, ... }:
+      {
+        options.foo = lib.mkOption {
+          type = lib.types.str;
+          default = "bar";
+        };
+      };
+  };
+}
+```
+
+## Not Changed! (existing inventory.services for api configuration of clanModules)
+
+```nix
+{
+    inventory.services = {
+        test."instance1" = {
+            roles.client.tags = [ "all" ];
+            machines.foo.config = { ... };
+            roles.client.config = { ... };
+
+        };
+        test."instance2" = {
+        roles.server.tags = [ "all" ];
+        config = { ... };
+        };
+    };
+}
+```
+
+We don't propse to change anything here. Because we had no problems with that interface yet.
+We even like it because it is very fast to evaluate and can be configured via UI.
+
+### Example of borgbackup before and after
+
+```nix
+# client.nix (BEFORE)
+{config, ...}:
+let
+  instances = config.clan.inventory.services.borgbackup or { };
+  # roles = { ${role_name} :: { machines :: [string] } }
+  allServers = lib.foldlAttrs (
+    acc: _instanceName: instanceConfig:
+    acc
+    ++ (
+      if builtins.elem machineName instanceConfig.roles.client.machines then
+        instanceConfig.roles.server.machines
+      else
+        [ ]
+    )
+  ) [ ] instances;
+
+  machineName = config.clan.core.settings.machine.name;
+
+  cfg = config.clan.borgbackup;
+in
+{
+#  ... Some nixos config
+}
+```
+
+```nix
+# client.nix (AFTER)
+{
+  _class = "clan";
+  perInstance = {instanceName, serviceConfig, machine, roles }: {
+    # allServers => roles.server;
+    # machineName => machine.name;
+    # cfg => serviceConfig;
+    nixosModule = {config, ...}: {
+    #   ...some nixos config
+    }
+    # Extendable. I.e. Instances can also define 'vars' or other things later.
+  };
+}
+```