create top-level switch whether a machine holds the application data

nixos/machines/default.nix

@@ -38,9 +38,7 @@
 
     nixosModule.lab = {
       disko.osDiskDevice = "/dev/sda";
-      backups.enable = true;
+      dataHost.enable = true;
-      networking.allowDMZConnectivity = true;
-      data-sharing.enable = true;
 
       dataDisk = {
         enable = true;

nixos/modules/default.nix

@@ -1,3 +1,7 @@
+{ lib, config, ... }:
+
+let cfg = config.lab.dataHost;
+in
 {
   imports = [
     ./terraform-database
@@ -9,4 +13,20 @@
     ./networking.nix
     ./data-sharing.nix
   ];
+
+  options.lab.dataHost.enable = lib.mkOption {
+    default = false;
+    type = lib.types.bool;
+    description = ''
+      Whether this machine holds application data.
+      This enables NFS and PostgreSQL to serve this data, and sets up backups.
+      Also enables networking on the DMZ to enable serving data.
+    '';
+  };
+
+  config.lab = lib.mkIf cfg.enable {
+    backups.enable = true;
+    data-sharing.enable = true;
+    networking.allowDMZConnectivity = true;
+  };
 }