diff --git a/flake.nix b/flake.nix
index d3112d8..4f02d49 100644
--- a/flake.nix
+++ b/flake.nix
@@ -102,8 +102,6 @@
       checks.x86_64-linux =
         let
           pkgs = nixpkgs.legacyPackages.x86_64-linux;
-          serviceIdentities = import ./lib/service-identities.nix { inherit lib; };
-          serviceIdentityTestsPass = lib.all (result: result) (lib.attrValues serviceIdentities.tests);
         in
         (lib.mapAttrs (
           name: host:
@@ -113,11 +111,6 @@
             throw "Host ${name} has evaluation warnings:\n${lib.concatStringsSep "\n" host.config.warnings}"
         ) self.nixosConfigurations)
         // {
-          service-identities = pkgs.runCommand "service-identities-check" { } ''
-            ${lib.optionalString (!serviceIdentityTestsPass) "exit 1"}
-            touch $out
-          '';
-
           copyparty-reverse-proxy-trust-config =
             let
               pikachuConfig = self.nixosConfigurations.pikachu.config;
@@ -128,6 +121,7 @@
                   system = "x86_64-linux";
                   specialArgs.my = self.lib;
                   modules = [
+                    self.lib.registry.module
                     ./modules/services/copyparty.nix
                     {
                       modules.services.copyparty = {
@@ -166,6 +160,7 @@
                   system = "x86_64-linux";
                   specialArgs.my = self.lib;
                   modules = [
+                    self.lib.registry.module
                     ./modules/services/copyparty.nix
                     {
                       modules.services.copyparty = {
diff --git a/guests/copyparty.nix b/guests/copyparty.nix
index 4d5b7fb..8657b8d 100644
--- a/guests/copyparty.nix
+++ b/guests/copyparty.nix
@@ -6,7 +6,6 @@
 }:
 let
   cfg = config.guests.copyparty;
-  serviceUid = my.serviceUid "copyparty";
 
   volumeType = lib.types.submodule {
     options = {
@@ -113,7 +112,7 @@ in
           ${cfg.stateDir} = {
             hostPath = cfg.statePath;
             create = {
-              owner = serviceUid;
+              owner = config.containers.copyparty.config.registry."stable-uid".copyparty.copyparty;
               group = "storage";
               mode = "0770";
             };
diff --git a/lib/default.nix b/lib/default.nix
index fc4b110..94b1e88 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -13,7 +13,6 @@ let
     mapAttrsToList
     ;
 
-  serviceIdentityRegistry = import ./service-identities.nix { inherit lib; };
   registry = import ./registry.nix { inherit lib self; };
 
   collectNixFiles =
@@ -90,6 +89,4 @@ in
 
   inherit (guests) deriveMac guest;
 
-  inherit (serviceIdentityRegistry) serviceUid;
-  serviceIdentities = serviceIdentityRegistry.identities;
 }
diff --git a/lib/registry.nix b/lib/registry.nix
index c0724a4..9805051 100644
--- a/lib/registry.nix
+++ b/lib/registry.nix
@@ -1,10 +1,17 @@
 { lib, self }:
 let
   inherit (lib)
+    all
+    concatLists
+    concatMapStringsSep
+    filter
     filterAttrs
+    foldl'
+    hasAttr
     hasSuffix
     mapAttrs
     mapAttrs'
+    mapAttrsToList
     mkOption
     nameValuePair
     removeSuffix
@@ -28,6 +35,67 @@ let
     in
     nameValuePair (registryName name) (import path { inherit lib; })
   ) (filterAttrs (name: type: type == "regular" && hasSuffix ".nix" name) (builtins.readDir (self + "/registry")));
+
+  rawDeclarations = registryName: hosts:
+    mapAttrs (_hostName: host: host.config.registry.${registryName} or { }) hosts;
+
+  replicatedEntries = registryName: raw:
+    let
+      entries = concatLists (
+        concatLists (
+          mapAttrsToList (
+            _evalHost: hostDeclarations:
+            mapAttrsToList (
+              hostName: declarations:
+              mapAttrsToList (entryName: value: {
+                path = "registry.${registryName}.${hostName}.${entryName}";
+                inherit entryName value;
+              }) declarations
+            ) hostDeclarations
+          ) raw
+        )
+      );
+    in
+    foldl' (
+      acc: entry:
+      if hasAttr entry.entryName acc then
+        if acc.${entry.entryName}.value == entry.value then
+          acc
+        else
+          throw "conflicting replicated registry entry ${entry.path} does not match ${acc.${entry.entryName}.path}"
+      else
+        acc // {
+          ${entry.entryName} = entry;
+        }
+    ) { } entries;
+
+  replicatedData = registryName: hosts: raw:
+    let
+      entries = mapAttrs (_entryName: entry: entry.value) (replicatedEntries registryName raw);
+    in
+    mapAttrs (_hostName: _host: entries) hosts;
+
+  validationAssertions = raw:
+    concatLists (
+      mapAttrsToList (
+        registryName: registry:
+        let
+          validate = registry.validate or (_raw: [ ]);
+        in
+        validate raw.${registryName}
+      ) definitions
+    );
+
+  assertValid = raw: value:
+    let
+      assertions = validationAssertions raw;
+      failures = filter (assertion: !assertion.assertion) assertions;
+    in
+    assert lib.assertMsg (all (assertion: assertion.assertion) assertions) (
+      concatMapStringsSep "\n" (assertion: assertion.message) failures
+    );
+    value;
+
   join = registry: host: entry:
     filterAttrs (_name: value: value != null) (
       mapAttrs (registryName: _registry: (registry.${registryName}.${host} or { }).${entry} or null) definitions
@@ -35,10 +103,17 @@ let
 
   collectData =
     { hosts }:
-    mapAttrs (
-      registryName: _registry:
-      mapAttrs (hostName: host: host.config.registry.${registryName}.${hostName} or { }) hosts
-    ) definitions;
+    let
+      raw = mapAttrs (registryName: _registry: rawDeclarations registryName hosts) definitions;
+      data = mapAttrs (
+        registryName: registry:
+        if registry.replicate or false then
+          replicatedData registryName hosts raw.${registryName}
+        else
+          mapAttrs (hostName: host: host.config.registry.${registryName}.${hostName} or { }) hosts
+      ) definitions;
+    in
+    assertValid raw data;
 
   withOperations = registry:
     registry
diff --git a/lib/service-identities.nix b/lib/service-identities.nix
deleted file mode 100644
index d389267..0000000
--- a/lib/service-identities.nix
+++ /dev/null
@@ -1,59 +0,0 @@
-{ lib }:
-let
-  defaultIdentities = {
-    actualbudget = 20001;
-    copyparty = 20002;
-  };
-
-  duplicateUidMessage = duplicates: ''
-    Duplicate stable service UID allocation(s): ${lib.concatStringsSep ", " (
-      map (entry: "${toString entry.uid} is assigned to ${lib.concatStringsSep ", " entry.names}") duplicates
-    )}
-  '';
-
-  mk = identities:
-    let
-      names = lib.attrNames identities;
-      namesByUid = lib.foldl' (
-        acc: name:
-        let
-          uid = toString identities.${name};
-        in
-        acc // {
-          ${uid} = (acc.${uid} or [ ]) ++ [ name ];
-        }
-      ) { } names;
-      duplicates = lib.mapAttrsToList (uid: namesForUid: {
-        uid = lib.toInt uid;
-        names = namesForUid;
-      }) (lib.filterAttrs (_uid: namesForUid: builtins.length namesForUid > 1) namesByUid);
-      checkedIdentities =
-        assert lib.assertMsg (duplicates == [ ]) (duplicateUidMessage duplicates);
-        identities;
-    in
-    rec {
-      inherit checkedIdentities;
-
-      serviceUid = name:
-        if builtins.hasAttr name checkedIdentities then
-          checkedIdentities.${name}
-        else
-          throw "Unknown stable service identity `${name}`. Known identities: ${lib.concatStringsSep ", " (lib.attrNames checkedIdentities)}";
-    };
-
-  default = mk defaultIdentities;
-
-  tests = {
-    actualbudgetUid = default.serviceUid "actualbudget" == 20001;
-    copypartyUid = default.serviceUid "copyparty" == 20002;
-    missingLookupFails = !(builtins.tryEval (default.serviceUid "missing-service")).success;
-    duplicateUidFails = !(builtins.tryEval (mk {
-      first = 20003;
-      second = 20003;
-    }).checkedIdentities).success;
-  };
-in
-default // {
-  identities = default.checkedIdentities;
-  inherit tests;
-}
diff --git a/modules/services/actualbudget.nix b/modules/services/actualbudget.nix
index 45a1d18..bc62bea 100644
--- a/modules/services/actualbudget.nix
+++ b/modules/services/actualbudget.nix
@@ -1,7 +1,6 @@
 {
   config,
   lib,
-  my,
   ...
 }:
 let
@@ -29,12 +28,6 @@ in
       description = "User account that runs Actual Budget.";
     };
 
-    uid = lib.mkOption {
-      type = lib.types.ints.positive;
-      default = my.serviceUid "actualbudget";
-      description = "Stable uid for the Actual Budget service user.";
-    };
-
     group = lib.mkOption {
       type = lib.types.str;
       default = "storage";
@@ -44,9 +37,11 @@ in
   };
 
   config = lib.mkIf cfg.enable {
+    registry."stable-uid".${config.networking.hostName}.actualbudget = 20001;
+
     users.users.${cfg.user} = {
       isSystemUser = true;
-      uid = cfg.uid;
+      uid = config.registry."stable-uid".${config.networking.hostName}.actualbudget;
       group = cfg.group;
     };
 
diff --git a/modules/services/copyparty.nix b/modules/services/copyparty.nix
index d57174d..248eca1 100644
--- a/modules/services/copyparty.nix
+++ b/modules/services/copyparty.nix
@@ -1,7 +1,6 @@
 {
   config,
   lib,
-  my,
   pkgs,
   ...
 }:
@@ -102,12 +101,6 @@ in
       description = "User account that runs Copyparty.";
     };
 
-    uid = lib.mkOption {
-      type = lib.types.ints.positive;
-      default = my.serviceUid "copyparty";
-      description = "Stable uid for the Copyparty service user.";
-    };
-
     group = lib.mkOption {
       type = lib.types.str;
       default = "storage";
@@ -149,6 +142,8 @@ in
   };
 
   config = lib.mkIf cfg.enable {
+    registry."stable-uid".${config.networking.hostName}.copyparty = 20002;
+
     assertions = [
       {
         assertion = cfg.volumes != { };
@@ -162,7 +157,7 @@ in
 
     users.users.${cfg.user} = {
       isSystemUser = true;
-      uid = cfg.uid;
+      uid = config.registry."stable-uid".${config.networking.hostName}.copyparty;
       group = cfg.group;
       home = cfg.stateDir;
     };
diff --git a/registry/stable-uid.nix b/registry/stable-uid.nix
new file mode 100644
index 0000000..56b33da
--- /dev/null
+++ b/registry/stable-uid.nix
@@ -0,0 +1,99 @@
+{ lib }:
+let
+  inherit (lib)
+    concatLists
+    concatStringsSep
+    foldl'
+    groupBy
+    mapAttrsToList
+    unique
+    ;
+
+  declarations = raw:
+    concatLists (
+      mapAttrsToList (
+        _evalHost: hostDeclarations:
+        concatLists (
+          mapAttrsToList (
+            hostName: entries:
+            mapAttrsToList (serviceName: uid: {
+              path = "${hostName}.${serviceName}";
+              inherit serviceName uid;
+            }) entries
+          ) hostDeclarations
+        )
+      ) raw
+    );
+
+  duplicateServices = entries:
+    let
+      byService = groupBy (entry: entry.serviceName) entries;
+    in
+    foldl' (
+      acc: serviceName:
+      let
+        serviceEntries = byService.${serviceName};
+        uids = unique (map (entry: entry.uid) serviceEntries);
+      in
+      if builtins.length uids > 1 then
+        acc ++ [ {
+          inherit serviceName uids;
+          paths = map (entry: entry.path) serviceEntries;
+        } ]
+      else
+        acc
+    ) [ ] (builtins.attrNames byService);
+
+  duplicateUids = entries:
+    let
+      byUid = groupBy (entry: toString entry.uid) entries;
+    in
+    foldl' (
+      acc: uid:
+      let
+        uidEntries = byUid.${uid};
+        serviceNames = unique (map (entry: entry.serviceName) uidEntries);
+      in
+      if builtins.length serviceNames > 1 then
+        acc ++ [ {
+          uid = lib.toInt uid;
+          inherit serviceNames;
+          paths = map (entry: entry.path) uidEntries;
+        } ]
+      else
+        acc
+    ) [ ] (builtins.attrNames byUid);
+
+  renderServiceConflict = conflict:
+    "${conflict.serviceName} is assigned UIDs ${concatStringsSep ", " (map toString conflict.uids)} by ${concatStringsSep ", " conflict.paths}";
+
+  renderUidConflict = conflict:
+    "${toString conflict.uid} is assigned to ${concatStringsSep ", " conflict.serviceNames} by ${concatStringsSep ", " conflict.paths}";
+in
+{
+  # Next stable UID: 20003
+  replicate = true;
+
+  type = lib.types.ints.positive;
+
+  validate = raw:
+    let
+      entries = declarations raw;
+      serviceConflicts = duplicateServices entries;
+      uidConflicts = duplicateUids entries;
+    in
+    [
+      {
+        assertion = serviceConflicts == [ ];
+        message = ''
+          Conflicting stable UID allocation(s): ${concatStringsSep "\n" (map renderServiceConflict serviceConflicts)}
+        '';
+      }
+      {
+        assertion = uidConflicts == [ ];
+        message = ''
+          Duplicate stable UID allocation(s): ${concatStringsSep "\n" (map renderUidConflict uidConflicts)}
+        '';
+      }
+    ];
+}