From 349721ac49c21fb2df09ba29c486a47719bd23e5 Mon Sep 17 00:00:00 2001
From: Rafal Studnicki <rafal@whatnot.com>
Date: Wed, 16 Jul 2025 14:27:35 +0200
Subject: [PATCH 1/7] Add registry partition_by: :pid | :key option

---
 lib/elixir/lib/registry.ex               | 172 ++++++++++++++++-------
 lib/elixir/test/elixir/registry_test.exs |  30 +++-
 2 files changed, 153 insertions(+), 49 deletions(-)

diff --git a/lib/elixir/lib/registry.ex b/lib/elixir/lib/registry.ex
index d35f82bc12a..522cab40082 100644
--- a/lib/elixir/lib/registry.ex
+++ b/lib/elixir/lib/registry.ex
@@ -231,6 +231,7 @@ defmodule Registry do
           | {:partitions, pos_integer}
           | {:listeners, [atom]}
           | {:meta, [{meta_key, meta_value}]}
+          | {:partition_by, :key | :pid}
 
   @typedoc """
   The message that the registry sends to listeners when a process registers or unregisters.
@@ -255,7 +256,7 @@ defmodule Registry do
 
   defp whereis_name(registry, key) do
     case key_info!(registry) do
-      {:unique, partitions, key_ets} ->
+      {:unique, partitions, key_ets, _} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
 
         case safe_lookup_second(key_ets, key) do
@@ -266,7 +267,7 @@ defmodule Registry do
             :undefined
         end
 
-      {kind, _, _} ->
+      {kind, _, _, _} ->
         raise ArgumentError, ":via is not supported for #{kind} registries"
     end
   end
@@ -329,6 +330,20 @@ defmodule Registry do
         {Registry, keys: :unique, name: MyApp.Registry, partitions: System.schedulers_online()}
       ], strategy: :one_for_one)
 
+  For `:duplicate` registries with many different keys (e.g., many topics with
+  few subscribers each), you can optimize key-based lookups by partitioning by key:
+
+      Registry.start_link(
+        keys: :duplicate,
+        name: MyApp.TopicRegistry,
+        partitions: System.schedulers_online(),
+        partition_by: :key
+      )
+
+  This allows key-based lookups to check only a single partition instead of
+  searching all partitions. Use the default `:pid` partitioning when you have
+  fewer keys with many entries each (e.g., one topic with many subscribers).
+
   ## Options
 
   The registry requires the following keys:
@@ -344,6 +359,19 @@ defmodule Registry do
       listener if the listener wants to be notified if the registered process
       crashes. Messages sent to listeners are of type `t:listener_message/0`.
     * `:meta` - a keyword list of metadata to be attached to the registry.
+    * `:partition_by` - the partitioning strategy for `:duplicate` registries.
+      Can be `:key` or `:pid`. Defaults to `:pid`.
+
+      Use `:pid` (default) when you have keys with many entries (e.g., one topic
+      with many subscribers). This is the traditional behavior and groups all
+      entries from the same process together.
+
+      Use `:key` when entries are spread across many different keys (e.g., many
+      topics with few subscribers each). This makes key-based lookups more
+      efficient as they only need to check a single partition instead of all
+      partitions.
+
+      Only supported for `:duplicate` registries.
 
   """
   @doc since: "1.5.0"
@@ -388,6 +416,17 @@ defmodule Registry do
             "expected :listeners to be a list of named processes, got: #{inspect(listeners)}"
     end
 
+    partition_by = Keyword.get(options, :partition_by, :pid)
+
+    if partition_by not in [:key, :pid] do
+      raise ArgumentError,
+            "expected :partition_by to be :key or :pid, got: #{inspect(partition_by)}"
+    end
+
+    if keys == :unique and partition_by == :key do
+      raise ArgumentError, ":partition_by :key is only supported for :duplicate registries"
+    end
+
     compressed = Keyword.get(options, :compressed, false)
 
     if not is_boolean(compressed) do
@@ -397,11 +436,19 @@ defmodule Registry do
 
     # The @info format must be kept in sync with Registry.Partition optimization.
     entries = [
-      {@all_info, {keys, partitions, nil, nil, listeners}},
-      {@key_info, {keys, partitions, nil}} | meta
+      {@all_info, {keys, partitions, nil, nil, listeners, partition_by}},
+      {@key_info, {keys, partitions, nil, partition_by}} | meta
     ]
 
-    Registry.Supervisor.start_link(keys, name, partitions, listeners, entries, compressed)
+    Registry.Supervisor.start_link(
+      keys,
+      name,
+      partitions,
+      listeners,
+      entries,
+      compressed,
+      partition_by
+    )
   end
 
   @doc false
@@ -450,7 +497,7 @@ defmodule Registry do
           {new_value :: term, old_value :: term} | :error
   def update_value(registry, key, callback) when is_atom(registry) and is_function(callback, 1) do
     case key_info!(registry) do
-      {:unique, partitions, key_ets} ->
+      {:unique, partitions, key_ets, _} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
 
         try do
@@ -467,7 +514,7 @@ defmodule Registry do
             :error
         end
 
-      {kind, _, _} ->
+      {kind, _, _, _} ->
         raise ArgumentError, "Registry.update_value/3 is not supported for #{kind} registries"
     end
   end
@@ -502,18 +549,18 @@ defmodule Registry do
       when is_atom(registry) and is_function(mfa_or_fun, 1)
       when is_atom(registry) and tuple_size(mfa_or_fun) == 3 do
     case key_info!(registry) do
-      {:unique, partitions, key_ets} ->
+      {:unique, partitions, key_ets, _} ->
         (key_ets || key_ets!(registry, key, partitions))
         |> safe_lookup_second(key)
         |> List.wrap()
         |> apply_non_empty_to_mfa_or_fun(mfa_or_fun)
 
-      {:duplicate, 1, key_ets} ->
+      {:duplicate, 1, key_ets, _} ->
         key_ets
         |> safe_lookup_second(key)
         |> apply_non_empty_to_mfa_or_fun(mfa_or_fun)
 
-      {:duplicate, partitions, _} ->
+      {:duplicate, partitions, _, _} ->
         if Keyword.get(opts, :parallel, false) do
           registry
           |> dispatch_parallel(key, mfa_or_fun, partitions)
@@ -614,7 +661,7 @@ defmodule Registry do
   @spec lookup(registry, key) :: [{pid, value}]
   def lookup(registry, key) when is_atom(registry) do
     case key_info!(registry) do
-      {:unique, partitions, key_ets} ->
+      {:unique, partitions, key_ets, _} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
 
         case safe_lookup_second(key_ets, key) do
@@ -625,10 +672,14 @@ defmodule Registry do
             []
         end
 
-      {:duplicate, 1, key_ets} ->
+      {:duplicate, 1, key_ets, _} ->
         safe_lookup_second(key_ets, key)
 
-      {:duplicate, partitions, _key_ets} ->
+      {:duplicate, partitions, _key_ets, :key} ->
+        partition = hash(key, partitions)
+        safe_lookup_second(key_ets!(registry, partition), key)
+
+      {:duplicate, partitions, _key_ets, :pid} ->
         for partition <- 0..(partitions - 1),
             pair <- safe_lookup_second(key_ets!(registry, partition), key),
             do: pair
@@ -689,7 +740,7 @@ defmodule Registry do
   @doc since: "1.18.0"
   def lock(registry, lock_key, function)
       when is_atom(registry) and is_function(function, 0) do
-    {_kind, partitions, _, pid_ets, _} = info!(registry)
+    {_kind, partitions, _, pid_ets, _, _} = info!(registry)
     {pid_server, _pid_ets} = pid_ets || pid_ets!(registry, lock_key, partitions)
     Registry.Partition.lock(pid_server, lock_key, function)
   end
@@ -745,14 +796,14 @@ defmodule Registry do
     spec = [{{:_, {:_, pattern}}, guards, [{:element, 2, :"$_"}]}]
 
     case key_info!(registry) do
-      {:unique, partitions, key_ets} ->
+      {:unique, partitions, key_ets, _} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
         :ets.select(key_ets, spec)
 
-      {:duplicate, 1, key_ets} ->
+      {:duplicate, 1, key_ets, _} ->
         :ets.select(key_ets, spec)
 
-      {:duplicate, partitions, _key_ets} ->
+      {:duplicate, partitions, _key_ets, _} ->
         for partition <- 0..(partitions - 1),
             pair <- :ets.select(key_ets!(registry, partition), spec),
             do: pair
@@ -794,7 +845,7 @@ defmodule Registry do
   @doc since: "1.4.0"
   @spec keys(registry, pid) :: [key]
   def keys(registry, pid) when is_atom(registry) and is_pid(pid) do
-    {kind, partitions, _, pid_ets, _} = info!(registry)
+    {kind, partitions, _, pid_ets, _, _} = info!(registry)
     {_, pid_ets} = pid_ets || pid_ets!(registry, pid, partitions)
 
     keys =
@@ -871,7 +922,7 @@ defmodule Registry do
   @spec values(registry, key, pid) :: [value]
   def values(registry, key, pid) when is_atom(registry) do
     case key_info!(registry) do
-      {:unique, partitions, key_ets} ->
+      {:unique, partitions, key_ets, _} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
 
         case safe_lookup_second(key_ets, key) do
@@ -882,8 +933,16 @@ defmodule Registry do
             []
         end
 
-      {:duplicate, partitions, key_ets} ->
-        key_ets = key_ets || key_ets!(registry, pid, partitions)
+      {:duplicate, 1, key_ets, _} ->
+        for {^pid, value} <- safe_lookup_second(key_ets, key), do: value
+
+      {:duplicate, partitions, _key_ets, :key} ->
+        partition = hash(key, partitions)
+        key_ets = key_ets!(registry, partition)
+        for {^pid, value} <- safe_lookup_second(key_ets, key), do: value
+
+      {:duplicate, partitions, _key_ets, :pid} ->
+        key_ets = key_ets!(registry, pid, partitions)
         for {^pid, value} <- safe_lookup_second(key_ets, key), do: value
     end
   end
@@ -930,8 +989,8 @@ defmodule Registry do
   @spec unregister(registry, key) :: :ok
   def unregister(registry, key) when is_atom(registry) do
     self = self()
-    {kind, partitions, key_ets, pid_ets, listeners} = info!(registry)
-    {key_partition, pid_partition} = partitions(kind, key, self, partitions)
+    {kind, partitions, key_ets, pid_ets, listeners, partition_by} = info!(registry)
+    {key_partition, pid_partition} = partitions(kind, key, self, partitions, partition_by)
     key_ets = key_ets || key_ets!(registry, key_partition)
     {pid_server, pid_ets} = pid_ets || pid_ets!(registry, pid_partition)
 
@@ -993,8 +1052,8 @@ defmodule Registry do
   def unregister_match(registry, key, pattern, guards \\ []) when is_list(guards) do
     self = self()
 
-    {kind, partitions, key_ets, pid_ets, listeners} = info!(registry)
-    {key_partition, pid_partition} = partitions(kind, key, self, partitions)
+    {kind, partitions, key_ets, pid_ets, listeners, partition_by} = info!(registry)
+    {key_partition, pid_partition} = partitions(kind, key, self, partitions, partition_by)
     key_ets = key_ets || key_ets!(registry, key_partition)
     {pid_server, pid_ets} = pid_ets || pid_ets!(registry, pid_partition)
 
@@ -1089,8 +1148,8 @@ defmodule Registry do
   @spec register(registry, key, value) :: {:ok, pid} | {:error, {:already_registered, pid}}
   def register(registry, key, value) when is_atom(registry) do
     self = self()
-    {kind, partitions, key_ets, pid_ets, listeners} = info!(registry)
-    {key_partition, pid_partition} = partitions(kind, key, self, partitions)
+    {kind, partitions, key_ets, pid_ets, listeners, partition_by} = info!(registry)
+    {key_partition, pid_partition} = partitions(kind, key, self, partitions, partition_by)
     key_ets = key_ets || key_ets!(registry, key_partition)
     {pid_server, pid_ets} = pid_ets || pid_ets!(registry, pid_partition)
 
@@ -1266,12 +1325,14 @@ defmodule Registry do
   @spec count(registry) :: non_neg_integer()
   def count(registry) when is_atom(registry) do
     case key_info!(registry) do
-      {_kind, partitions, nil} ->
-        Enum.sum_by(0..(partitions - 1), fn partition_index ->
+      {_kind, partitions, nil, _} ->
+        0..(partitions - 1)
+        |> Enum.map(fn partition_index ->
           safe_size(key_ets!(registry, partition_index))
         end)
+        |> Enum.sum()
 
-      {_kind, 1, key_ets} ->
+      {_kind, 1, key_ets, _} ->
         safe_size(key_ets)
     end
   end
@@ -1335,17 +1396,19 @@ defmodule Registry do
     spec = [{{:_, {:_, pattern}}, guards, [true]}]
 
     case key_info!(registry) do
-      {:unique, partitions, key_ets} ->
+      {:unique, partitions, key_ets, _} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
         :ets.select_count(key_ets, spec)
 
-      {:duplicate, 1, key_ets} ->
+      {:duplicate, 1, key_ets, _} ->
         :ets.select_count(key_ets, spec)
 
-      {:duplicate, partitions, _key_ets} ->
-        Enum.sum_by(0..(partitions - 1), fn partition_index ->
+      {:duplicate, partitions, _key_ets, _} ->
+        0..(partitions - 1)
+        |> Enum.map(fn partition_index ->
           :ets.select_count(key_ets!(registry, partition_index), spec)
         end)
+        |> Enum.sum()
     end
   end
 
@@ -1404,12 +1467,12 @@ defmodule Registry do
     spec = group_match_headers(spec, __ENV__.function)
 
     case key_info!(registry) do
-      {_kind, partitions, nil} ->
+      {_kind, partitions, nil, _} ->
         Enum.flat_map(0..(partitions - 1), fn partition_index ->
           :ets.select(key_ets!(registry, partition_index), spec)
         end)
 
-      {_kind, 1, key_ets} ->
+      {_kind, 1, key_ets, _} ->
         :ets.select(key_ets, spec)
     end
   end
@@ -1435,12 +1498,14 @@ defmodule Registry do
     spec = group_match_headers(spec, __ENV__.function)
 
     case key_info!(registry) do
-      {_kind, partitions, nil} ->
-        Enum.sum_by(0..(partitions - 1), fn partition_index ->
+      {_kind, partitions, nil, _} ->
+        0..(partitions - 1)
+        |> Enum.map(fn partition_index ->
           :ets.select_count(key_ets!(registry, partition_index), spec)
         end)
+        |> Enum.sum()
 
-      {_kind, 1, key_ets} ->
+      {_kind, 1, key_ets, _} ->
         :ets.select_count(key_ets, spec)
     end
   end
@@ -1508,11 +1573,16 @@ defmodule Registry do
     end
   end
 
-  defp partitions(:unique, key, pid, partitions) do
+  defp partitions(:unique, key, pid, partitions, _partition_by) do
     {hash(key, partitions), hash(pid, partitions)}
   end
 
-  defp partitions(:duplicate, _key, pid, partitions) do
+  defp partitions(:duplicate, key, _pid, partitions, :key) do
+    partition = hash(key, partitions)
+    {partition, partition}
+  end
+
+  defp partitions(:duplicate, _key, pid, partitions, :pid) do
     partition = hash(pid, partitions)
     {partition, partition}
   end
@@ -1546,12 +1616,12 @@ defmodule Registry.Supervisor do
   @moduledoc false
   use Supervisor
 
-  def start_link(kind, registry, partitions, listeners, entries, compressed) do
-    arg = {kind, registry, partitions, listeners, entries, compressed}
+  def start_link(kind, registry, partitions, listeners, entries, compressed, partition_by) do
+    arg = {kind, registry, partitions, listeners, entries, compressed, partition_by}
     Supervisor.start_link(__MODULE__, arg, name: registry)
   end
 
-  def init({kind, registry, partitions, listeners, entries, compressed}) do
+  def init({kind, registry, partitions, listeners, entries, compressed, partition_by}) do
     ^registry = :ets.new(registry, [:set, :public, :named_table, read_concurrency: true])
     true = :ets.insert(registry, entries)
 
@@ -1559,7 +1629,10 @@ defmodule Registry.Supervisor do
       for i <- 0..(partitions - 1) do
         key_partition = Registry.Partition.key_name(registry, i)
         pid_partition = Registry.Partition.pid_name(registry, i)
-        arg = {kind, registry, i, partitions, key_partition, pid_partition, listeners, compressed}
+
+        arg =
+          {kind, registry, i, partitions, key_partition, pid_partition, listeners, compressed,
+           partition_by}
 
         %{
           id: pid_partition,
@@ -1631,7 +1704,10 @@ defmodule Registry.Partition do
 
   ## Callbacks
 
-  def init({kind, registry, i, partitions, key_partition, pid_partition, listeners, compressed}) do
+  def init(
+        {kind, registry, i, partitions, key_partition, pid_partition, listeners, compressed,
+         partition_by}
+      ) do
     Process.flag(:trap_exit, true)
     key_ets = init_key_ets(kind, key_partition, compressed)
     pid_ets = init_pid_ets(kind, pid_partition)
@@ -1640,8 +1716,8 @@ defmodule Registry.Partition do
     # is to write the table information alongside the registry info.
     if partitions == 1 do
       entries = [
-        {@key_info, {kind, partitions, key_ets}},
-        {@all_info, {kind, partitions, key_ets, {self(), pid_ets}, listeners}}
+        {@key_info, {kind, partitions, key_ets, partition_by}},
+        {@all_info, {kind, partitions, key_ets, {self(), pid_ets}, listeners, partition_by}}
       ]
 
       true = :ets.insert(registry, entries)
diff --git a/lib/elixir/test/elixir/registry_test.exs b/lib/elixir/test/elixir/registry_test.exs
index d19264c3f97..bf4f5640883 100644
--- a/lib/elixir/test/elixir/registry_test.exs
+++ b/lib/elixir/test/elixir/registry_test.exs
@@ -931,6 +931,34 @@ defmodule Registry.Test do
                  {{"world", :_, :_}, [], [true]}
                ])
     end
+
+    test "works with partition_by: :key", %{partitions: partitions} do
+      name = :"test_partition_by_keys_#{partitions}"
+      opts = [keys: :duplicate, name: name, partitions: partitions, partition_by: :key]
+      {:ok, _} = start_supervised({Registry, opts})
+
+      {:ok, _} = Registry.register(name, "hello", :value1)
+      {:ok, _} = Registry.register(name, "hello", :value2)
+      {:ok, _} = Registry.register(name, "world", :value3)
+
+      assert 3 == Registry.count(name)
+      assert Registry.values(name, "hello", self()) |> Enum.sort() == [:value1, :value2]
+      assert Registry.values(name, "world", self()) == [:value3]
+    end
+
+    test "works with partition_by: :pid", %{partitions: partitions} do
+      name = :"test_partition_by_pids_#{partitions}"
+      opts = [keys: :duplicate, name: name, partitions: partitions, partition_by: :pid]
+      {:ok, _} = start_supervised({Registry, opts})
+
+      {:ok, _} = Registry.register(name, "hello", :value1)
+      {:ok, _} = Registry.register(name, "hello", :value2)
+      {:ok, _} = Registry.register(name, "world", :value3)
+
+      assert 3 == Registry.count(name)
+      assert Registry.values(name, "hello", self()) |> Enum.sort() == [:value1, :value2]
+      assert Registry.values(name, "world", self()) == [:value3]
+    end
   end
 
   # Note: those tests relies on internals
@@ -958,7 +986,7 @@ defmodule Registry.Test do
           assert :ets.tab2list(pid) == []
         end
       else
-        [{-1, {_, _, key, {partition, pid}, _}}] = :ets.lookup(registry, -1)
+        [{-1, {_, _, key, {partition, pid}, _, _}}] = :ets.lookup(registry, -1)
         GenServer.call(partition, :sync)
         assert :ets.tab2list(key) == []
         assert :ets.tab2list(pid) == []

From e7d9496d7cbc0e6b99031e9e5bc52af014c29f44 Mon Sep 17 00:00:00 2001
From: Rafal Studnicki <rafal@whatnot.com>
Date: Wed, 16 Jul 2025 15:10:04 +0200
Subject: [PATCH 2/7] Extend the clean up test with partition_by values

---
 lib/elixir/test/elixir/registry_test.exs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/elixir/test/elixir/registry_test.exs b/lib/elixir/test/elixir/registry_test.exs
index bf4f5640883..18fd5d0ee99 100644
--- a/lib/elixir/test/elixir/registry_test.exs
+++ b/lib/elixir/test/elixir/registry_test.exs
@@ -23,6 +23,7 @@ defmodule Registry.Test do
     listeners = List.wrap(config[:base_listener]) |> Enum.map(&:"#{&1}_#{partitions}")
     name = :"#{config.test}_#{partitions}"
     opts = [keys: keys, name: name, partitions: partitions, listeners: listeners]
+    opts = if config[:partition_by], do: opts ++ [partition_by: config.partition_by], else: opts
     {:ok, _} = start_supervised({Registry, opts})
     %{registry: name, listeners: listeners}
   end
@@ -962,9 +963,14 @@ defmodule Registry.Test do
   end
 
   # Note: those tests relies on internals
-  for keys <- [:unique, :duplicate] do
+  for {keys, partition_by} <- [
+        {:unique, nil},
+        {:duplicate, :pid},
+        {:duplicate, :key}
+      ] do
     @tag keys: keys
-    test "clean up #{keys} registry on process crash",
+    @tag partition_by: partition_by
+    test "clean up #{keys} registry on process crash with partition_by: #{partition_by || "default"}",
          %{registry: registry, partitions: partitions} do
       {_, task1} = register_task(registry, "hello", :value)
       {_, task2} = register_task(registry, "world", :value)

From 3959062b71e4152dc42b4855a85587698a1ae5ed Mon Sep 17 00:00:00 2001
From: Rafal Studnicki <rafal@whatnot.com>
Date: Thu, 17 Jul 2025 12:44:08 +0200
Subject: [PATCH 3/7] Get rid of extra partition_by option, store partition
 strategy as part of {:duplicate, startegy}

---
 lib/elixir/lib/registry.ex               | 180 +++++++++++------------
 lib/elixir/test/elixir/registry_test.exs |  31 ++--
 2 files changed, 106 insertions(+), 105 deletions(-)

diff --git a/lib/elixir/lib/registry.ex b/lib/elixir/lib/registry.ex
index 522cab40082..9cda3d97567 100644
--- a/lib/elixir/lib/registry.ex
+++ b/lib/elixir/lib/registry.ex
@@ -187,7 +187,7 @@ defmodule Registry do
   Note that the registry uses one ETS table plus two ETS tables per partition.
   """
 
-  @keys [:unique, :duplicate]
+  @keys [:unique, :duplicate, {:duplicate, :key}, {:duplicate, :pid}]
   @all_info -1
   @key_info -2
 
@@ -195,7 +195,7 @@ defmodule Registry do
   @type registry :: atom
 
   @typedoc "The type of the registry"
-  @type keys :: :unique | :duplicate
+  @type keys :: :unique | :duplicate | {:duplicate, :key} | {:duplicate, :pid}
 
   @typedoc "The type of keys allowed on registration"
   @type key :: term
@@ -231,7 +231,6 @@ defmodule Registry do
           | {:partitions, pos_integer}
           | {:listeners, [atom]}
           | {:meta, [{meta_key, meta_value}]}
-          | {:partition_by, :key | :pid}
 
   @typedoc """
   The message that the registry sends to listeners when a process registers or unregisters.
@@ -256,7 +255,7 @@ defmodule Registry do
 
   defp whereis_name(registry, key) do
     case key_info!(registry) do
-      {:unique, partitions, key_ets, _} ->
+      {:unique, partitions, key_ets} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
 
         case safe_lookup_second(key_ets, key) do
@@ -267,8 +266,8 @@ defmodule Registry do
             :undefined
         end
 
-      {kind, _, _, _} ->
-        raise ArgumentError, ":via is not supported for #{kind} registries"
+      {{:duplicate, _}, _, _} ->
+        raise ArgumentError, ":via is not supported for duplicate registries"
     end
   end
 
@@ -334,10 +333,9 @@ defmodule Registry do
   few subscribers each), you can optimize key-based lookups by partitioning by key:
 
       Registry.start_link(
-        keys: :duplicate,
+        keys: {:duplicate, :key},
         name: MyApp.TopicRegistry,
-        partitions: System.schedulers_online(),
-        partition_by: :key
+        partitions: System.schedulers_online()
       )
 
   This allows key-based lookups to check only a single partition instead of
@@ -348,7 +346,7 @@ defmodule Registry do
 
   The registry requires the following keys:
 
-    * `:keys` - chooses if keys are `:unique` or `:duplicate`
+    * `:keys` - chooses if keys are `:unique`, `:duplicate`, `{:duplicate, :key}`, or `{:duplicate, :pid}`
     * `:name` - the name of the registry and its tables
 
   The following keys are optional:
@@ -359,19 +357,18 @@ defmodule Registry do
       listener if the listener wants to be notified if the registered process
       crashes. Messages sent to listeners are of type `t:listener_message/0`.
     * `:meta` - a keyword list of metadata to be attached to the registry.
-    * `:partition_by` - the partitioning strategy for `:duplicate` registries.
-      Can be `:key` or `:pid`. Defaults to `:pid`.
 
-      Use `:pid` (default) when you have keys with many entries (e.g., one topic
-      with many subscribers). This is the traditional behavior and groups all
-      entries from the same process together.
+      For `:duplicate` registries, you can specify the partitioning strategy
+      directly in the `:keys` option:
 
-      Use `:key` when entries are spread across many different keys (e.g., many
-      topics with few subscribers each). This makes key-based lookups more
-      efficient as they only need to check a single partition instead of all
-      partitions.
+      * `:duplicate` or `{:duplicate, :pid}` - Use `:pid` partitioning (default)
+        when you have keys with many entries (e.g., one topic with many subscribers).
+        This is the traditional behavior and groups all entries from the same process together.
 
-      Only supported for `:duplicate` registries.
+      * `{:duplicate, :key}` - Use `:key` partitioning when entries are spread across
+        many different keys (e.g., many topics with few subscribers each). This makes
+        key-based lookups more efficient as they only need to check a single partition
+        instead of all partitions.
 
   """
   @doc since: "1.5.0"
@@ -379,10 +376,22 @@ defmodule Registry do
   def start_link(options) do
     keys = Keyword.get(options, :keys)
 
-    if keys not in @keys do
-      raise ArgumentError,
-            "expected :keys to be given and be one of :unique or :duplicate, got: #{inspect(keys)}"
-    end
+    # Validate and normalize keys format
+    kind =
+      case keys do
+        {:duplicate, partition_strategy} when partition_strategy in [:key, :pid] ->
+          {:duplicate, partition_strategy}
+
+        :unique ->
+          :unique
+
+        :duplicate ->
+          {:duplicate, :pid}
+
+        _ ->
+          raise ArgumentError,
+                "expected :keys to be given and be one of :unique, :duplicate, {:duplicate, :key}, or {:duplicate, :pid}, got: #{inspect(keys)}"
+      end
 
     name =
       case Keyword.fetch(options, :name) do
@@ -416,17 +425,6 @@ defmodule Registry do
             "expected :listeners to be a list of named processes, got: #{inspect(listeners)}"
     end
 
-    partition_by = Keyword.get(options, :partition_by, :pid)
-
-    if partition_by not in [:key, :pid] do
-      raise ArgumentError,
-            "expected :partition_by to be :key or :pid, got: #{inspect(partition_by)}"
-    end
-
-    if keys == :unique and partition_by == :key do
-      raise ArgumentError, ":partition_by :key is only supported for :duplicate registries"
-    end
-
     compressed = Keyword.get(options, :compressed, false)
 
     if not is_boolean(compressed) do
@@ -436,18 +434,17 @@ defmodule Registry do
 
     # The @info format must be kept in sync with Registry.Partition optimization.
     entries = [
-      {@all_info, {keys, partitions, nil, nil, listeners, partition_by}},
-      {@key_info, {keys, partitions, nil, partition_by}} | meta
+      {@all_info, {kind, partitions, nil, nil, listeners}},
+      {@key_info, {kind, partitions, nil}} | meta
     ]
 
     Registry.Supervisor.start_link(
-      keys,
+      kind,
       name,
       partitions,
       listeners,
       entries,
-      compressed,
-      partition_by
+      compressed
     )
   end
 
@@ -497,7 +494,7 @@ defmodule Registry do
           {new_value :: term, old_value :: term} | :error
   def update_value(registry, key, callback) when is_atom(registry) and is_function(callback, 1) do
     case key_info!(registry) do
-      {:unique, partitions, key_ets, _} ->
+      {:unique, partitions, key_ets} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
 
         try do
@@ -549,18 +546,18 @@ defmodule Registry do
       when is_atom(registry) and is_function(mfa_or_fun, 1)
       when is_atom(registry) and tuple_size(mfa_or_fun) == 3 do
     case key_info!(registry) do
-      {:unique, partitions, key_ets, _} ->
+      {:unique, partitions, key_ets} ->
         (key_ets || key_ets!(registry, key, partitions))
         |> safe_lookup_second(key)
         |> List.wrap()
         |> apply_non_empty_to_mfa_or_fun(mfa_or_fun)
 
-      {:duplicate, 1, key_ets, _} ->
+      {{:duplicate, _}, 1, key_ets} ->
         key_ets
         |> safe_lookup_second(key)
         |> apply_non_empty_to_mfa_or_fun(mfa_or_fun)
 
-      {:duplicate, partitions, _, _} ->
+      {{:duplicate, _}, partitions, _} ->
         if Keyword.get(opts, :parallel, false) do
           registry
           |> dispatch_parallel(key, mfa_or_fun, partitions)
@@ -661,7 +658,7 @@ defmodule Registry do
   @spec lookup(registry, key) :: [{pid, value}]
   def lookup(registry, key) when is_atom(registry) do
     case key_info!(registry) do
-      {:unique, partitions, key_ets, _} ->
+      {:unique, partitions, key_ets} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
 
         case safe_lookup_second(key_ets, key) do
@@ -672,14 +669,14 @@ defmodule Registry do
             []
         end
 
-      {:duplicate, 1, key_ets, _} ->
+      {{:duplicate, _}, 1, key_ets} ->
         safe_lookup_second(key_ets, key)
 
-      {:duplicate, partitions, _key_ets, :key} ->
+      {{:duplicate, :key}, partitions, _key_ets} ->
         partition = hash(key, partitions)
         safe_lookup_second(key_ets!(registry, partition), key)
 
-      {:duplicate, partitions, _key_ets, :pid} ->
+      {{:duplicate, :pid}, partitions, _key_ets} ->
         for partition <- 0..(partitions - 1),
             pair <- safe_lookup_second(key_ets!(registry, partition), key),
             do: pair
@@ -740,7 +737,7 @@ defmodule Registry do
   @doc since: "1.18.0"
   def lock(registry, lock_key, function)
       when is_atom(registry) and is_function(function, 0) do
-    {_kind, partitions, _, pid_ets, _, _} = info!(registry)
+    {_kind, partitions, _, pid_ets, _} = info!(registry)
     {pid_server, _pid_ets} = pid_ets || pid_ets!(registry, lock_key, partitions)
     Registry.Partition.lock(pid_server, lock_key, function)
   end
@@ -796,14 +793,14 @@ defmodule Registry do
     spec = [{{:_, {:_, pattern}}, guards, [{:element, 2, :"$_"}]}]
 
     case key_info!(registry) do
-      {:unique, partitions, key_ets, _} ->
+      {:unique, partitions, key_ets} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
         :ets.select(key_ets, spec)
 
-      {:duplicate, 1, key_ets, _} ->
+      {{:duplicate, _}, 1, key_ets} ->
         :ets.select(key_ets, spec)
 
-      {:duplicate, partitions, _key_ets, _} ->
+      {{:duplicate, _}, partitions, _key_ets} ->
         for partition <- 0..(partitions - 1),
             pair <- :ets.select(key_ets!(registry, partition), spec),
             do: pair
@@ -845,7 +842,7 @@ defmodule Registry do
   @doc since: "1.4.0"
   @spec keys(registry, pid) :: [key]
   def keys(registry, pid) when is_atom(registry) and is_pid(pid) do
-    {kind, partitions, _, pid_ets, _, _} = info!(registry)
+    {kind, partitions, _, pid_ets, _} = info!(registry)
     {_, pid_ets} = pid_ets || pid_ets!(registry, pid, partitions)
 
     keys =
@@ -922,7 +919,7 @@ defmodule Registry do
   @spec values(registry, key, pid) :: [value]
   def values(registry, key, pid) when is_atom(registry) do
     case key_info!(registry) do
-      {:unique, partitions, key_ets, _} ->
+      {:unique, partitions, key_ets} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
 
         case safe_lookup_second(key_ets, key) do
@@ -933,16 +930,17 @@ defmodule Registry do
             []
         end
 
-      {:duplicate, 1, key_ets, _} ->
+      {{:duplicate, _}, 1, key_ets} ->
         for {^pid, value} <- safe_lookup_second(key_ets, key), do: value
 
-      {:duplicate, partitions, _key_ets, :key} ->
+      {{:duplicate, :key}, partitions, _key_ets} ->
         partition = hash(key, partitions)
         key_ets = key_ets!(registry, partition)
         for {^pid, value} <- safe_lookup_second(key_ets, key), do: value
 
-      {:duplicate, partitions, _key_ets, :pid} ->
-        key_ets = key_ets!(registry, pid, partitions)
+      {{:duplicate, :pid}, partitions, _key_ets} ->
+        partition = hash(pid, partitions)
+        key_ets = key_ets!(registry, partition)
         for {^pid, value} <- safe_lookup_second(key_ets, key), do: value
     end
   end
@@ -989,8 +987,8 @@ defmodule Registry do
   @spec unregister(registry, key) :: :ok
   def unregister(registry, key) when is_atom(registry) do
     self = self()
-    {kind, partitions, key_ets, pid_ets, listeners, partition_by} = info!(registry)
-    {key_partition, pid_partition} = partitions(kind, key, self, partitions, partition_by)
+    {kind, partitions, key_ets, pid_ets, listeners} = info!(registry)
+    {key_partition, pid_partition} = partitions(kind, key, self, partitions)
     key_ets = key_ets || key_ets!(registry, key_partition)
     {pid_server, pid_ets} = pid_ets || pid_ets!(registry, pid_partition)
 
@@ -1052,8 +1050,8 @@ defmodule Registry do
   def unregister_match(registry, key, pattern, guards \\ []) when is_list(guards) do
     self = self()
 
-    {kind, partitions, key_ets, pid_ets, listeners, partition_by} = info!(registry)
-    {key_partition, pid_partition} = partitions(kind, key, self, partitions, partition_by)
+    {kind, partitions, key_ets, pid_ets, listeners} = info!(registry)
+    {key_partition, pid_partition} = partitions(kind, key, self, partitions)
     key_ets = key_ets || key_ets!(registry, key_partition)
     {pid_server, pid_ets} = pid_ets || pid_ets!(registry, pid_partition)
 
@@ -1148,8 +1146,8 @@ defmodule Registry do
   @spec register(registry, key, value) :: {:ok, pid} | {:error, {:already_registered, pid}}
   def register(registry, key, value) when is_atom(registry) do
     self = self()
-    {kind, partitions, key_ets, pid_ets, listeners, partition_by} = info!(registry)
-    {key_partition, pid_partition} = partitions(kind, key, self, partitions, partition_by)
+    {kind, partitions, key_ets, pid_ets, listeners} = info!(registry)
+    {key_partition, pid_partition} = partitions(kind, key, self, partitions)
     key_ets = key_ets || key_ets!(registry, key_partition)
     {pid_server, pid_ets} = pid_ets || pid_ets!(registry, pid_partition)
 
@@ -1180,7 +1178,7 @@ defmodule Registry do
     end
   end
 
-  defp register_key(:duplicate, key_ets, _key, entry) do
+  defp register_key({:duplicate, _}, key_ets, _key, entry) do
     true = :ets.insert(key_ets, entry)
     :ok
   end
@@ -1325,14 +1323,14 @@ defmodule Registry do
   @spec count(registry) :: non_neg_integer()
   def count(registry) when is_atom(registry) do
     case key_info!(registry) do
-      {_kind, partitions, nil, _} ->
+      {_kind, partitions, nil} ->
         0..(partitions - 1)
         |> Enum.map(fn partition_index ->
           safe_size(key_ets!(registry, partition_index))
         end)
         |> Enum.sum()
 
-      {_kind, 1, key_ets, _} ->
+      {_kind, 1, key_ets} ->
         safe_size(key_ets)
     end
   end
@@ -1396,14 +1394,14 @@ defmodule Registry do
     spec = [{{:_, {:_, pattern}}, guards, [true]}]
 
     case key_info!(registry) do
-      {:unique, partitions, key_ets, _} ->
+      {:unique, partitions, key_ets} ->
         key_ets = key_ets || key_ets!(registry, key, partitions)
         :ets.select_count(key_ets, spec)
 
-      {:duplicate, 1, key_ets, _} ->
+      {{:duplicate, _}, 1, key_ets} ->
         :ets.select_count(key_ets, spec)
 
-      {:duplicate, partitions, _key_ets, _} ->
+      {{:duplicate, _}, partitions, _key_ets} ->
         0..(partitions - 1)
         |> Enum.map(fn partition_index ->
           :ets.select_count(key_ets!(registry, partition_index), spec)
@@ -1467,12 +1465,12 @@ defmodule Registry do
     spec = group_match_headers(spec, __ENV__.function)
 
     case key_info!(registry) do
-      {_kind, partitions, nil, _} ->
+      {_kind, partitions, nil} ->
         Enum.flat_map(0..(partitions - 1), fn partition_index ->
           :ets.select(key_ets!(registry, partition_index), spec)
         end)
 
-      {_kind, 1, key_ets, _} ->
+      {_kind, 1, key_ets} ->
         :ets.select(key_ets, spec)
     end
   end
@@ -1498,14 +1496,14 @@ defmodule Registry do
     spec = group_match_headers(spec, __ENV__.function)
 
     case key_info!(registry) do
-      {_kind, partitions, nil, _} ->
+      {_kind, partitions, nil} ->
         0..(partitions - 1)
         |> Enum.map(fn partition_index ->
           :ets.select_count(key_ets!(registry, partition_index), spec)
         end)
         |> Enum.sum()
 
-      {_kind, 1, key_ets, _} ->
+      {_kind, 1, key_ets} ->
         :ets.select_count(key_ets, spec)
     end
   end
@@ -1573,16 +1571,14 @@ defmodule Registry do
     end
   end
 
-  defp partitions(:unique, key, pid, partitions, _partition_by) do
+  defp partitions(:unique, key, pid, partitions) do
     {hash(key, partitions), hash(pid, partitions)}
   end
-
-  defp partitions(:duplicate, key, _pid, partitions, :key) do
+  defp partitions({:duplicate, :key}, key, _pid, partitions) do
     partition = hash(key, partitions)
     {partition, partition}
   end
-
-  defp partitions(:duplicate, _key, pid, partitions, :pid) do
+  defp partitions({:duplicate, :pid}, _key, pid, partitions) do
     partition = hash(pid, partitions)
     {partition, partition}
   end
@@ -1616,12 +1612,12 @@ defmodule Registry.Supervisor do
   @moduledoc false
   use Supervisor
 
-  def start_link(kind, registry, partitions, listeners, entries, compressed, partition_by) do
-    arg = {kind, registry, partitions, listeners, entries, compressed, partition_by}
+  def start_link(kind, registry, partitions, listeners, entries, compressed) do
+    arg = {kind, registry, partitions, listeners, entries, compressed}
     Supervisor.start_link(__MODULE__, arg, name: registry)
   end
 
-  def init({kind, registry, partitions, listeners, entries, compressed, partition_by}) do
+  def init({kind, registry, partitions, listeners, entries, compressed}) do
     ^registry = :ets.new(registry, [:set, :public, :named_table, read_concurrency: true])
     true = :ets.insert(registry, entries)
 
@@ -1631,8 +1627,7 @@ defmodule Registry.Supervisor do
         pid_partition = Registry.Partition.pid_name(registry, i)
 
         arg =
-          {kind, registry, i, partitions, key_partition, pid_partition, listeners, compressed,
-           partition_by}
+          {kind, registry, i, partitions, key_partition, pid_partition, listeners, compressed}
 
         %{
           id: pid_partition,
@@ -1649,9 +1644,10 @@ defmodule Registry.Supervisor do
   defp strategy_for_kind(:unique), do: :one_for_all
 
   # Duplicate registries have both key and pid partitions hashed
-  # by pid. This means that, if a PID partition crashes, all of
+  # by key ({:duplicate, :key}) or pid ({:duplicate, :pid}).
+  # This means that, if a PID or key partition crashes, all of
   # its associated entries are in its sibling table, so we crash one.
-  defp strategy_for_kind(:duplicate), do: :one_for_one
+  defp strategy_for_kind({:duplicate, _}), do: :one_for_one
 end
 
 defmodule Registry.Partition do
@@ -1704,11 +1700,9 @@ defmodule Registry.Partition do
 
   ## Callbacks
 
-  def init(
-        {kind, registry, i, partitions, key_partition, pid_partition, listeners, compressed,
-         partition_by}
-      ) do
+  def init({kind, registry, i, partitions, key_partition, pid_partition, listeners, compressed}) do
     Process.flag(:trap_exit, true)
+
     key_ets = init_key_ets(kind, key_partition, compressed)
     pid_ets = init_pid_ets(kind, pid_partition)
 
@@ -1716,8 +1710,8 @@ defmodule Registry.Partition do
     # is to write the table information alongside the registry info.
     if partitions == 1 do
       entries = [
-        {@key_info, {kind, partitions, key_ets, partition_by}},
-        {@all_info, {kind, partitions, key_ets, {self(), pid_ets}, listeners, partition_by}}
+        {@key_info, {kind, partitions, key_ets}},
+        {@all_info, {kind, partitions, key_ets, {self(), pid_ets}, listeners}}
       ]
 
       true = :ets.insert(registry, entries)
@@ -1735,7 +1729,7 @@ defmodule Registry.Partition do
     :ets.new(key_partition, compression_opt(opts, compressed))
   end
 
-  defp init_key_ets(:duplicate, key_partition, compressed) do
+  defp init_key_ets({:duplicate, _}, key_partition, compressed) do
     opts = [:duplicate_bag, :public, read_concurrency: true, write_concurrency: true]
     :ets.new(key_partition, compression_opt(opts, compressed))
   end
diff --git a/lib/elixir/test/elixir/registry_test.exs b/lib/elixir/test/elixir/registry_test.exs
index 18fd5d0ee99..31341ed479e 100644
--- a/lib/elixir/test/elixir/registry_test.exs
+++ b/lib/elixir/test/elixir/registry_test.exs
@@ -23,7 +23,6 @@ defmodule Registry.Test do
     listeners = List.wrap(config[:base_listener]) |> Enum.map(&:"#{&1}_#{partitions}")
     name = :"#{config.test}_#{partitions}"
     opts = [keys: keys, name: name, partitions: partitions, listeners: listeners]
-    opts = if config[:partition_by], do: opts ++ [partition_by: config.partition_by], else: opts
     {:ok, _} = start_supervised({Registry, opts})
     %{registry: name, listeners: listeners}
   end
@@ -933,9 +932,9 @@ defmodule Registry.Test do
                ])
     end
 
-    test "works with partition_by: :key", %{partitions: partitions} do
-      name = :"test_partition_by_keys_#{partitions}"
-      opts = [keys: :duplicate, name: name, partitions: partitions, partition_by: :key]
+    test "works with tuple syntax {:duplicate, :key}", %{partitions: partitions} do
+      name = :"test_tuple_keys_#{partitions}"
+      opts = [keys: {:duplicate, :key}, name: name, partitions: partitions]
       {:ok, _} = start_supervised({Registry, opts})
 
       {:ok, _} = Registry.register(name, "hello", :value1)
@@ -947,9 +946,9 @@ defmodule Registry.Test do
       assert Registry.values(name, "world", self()) == [:value3]
     end
 
-    test "works with partition_by: :pid", %{partitions: partitions} do
-      name = :"test_partition_by_pids_#{partitions}"
-      opts = [keys: :duplicate, name: name, partitions: partitions, partition_by: :pid]
+    test "works with tuple syntax {:duplicate, :pid}", %{partitions: partitions} do
+      name = :"test_tuple_pids_#{partitions}"
+      opts = [keys: {:duplicate, :pid}, name: name, partitions: partitions]
       {:ok, _} = start_supervised({Registry, opts})
 
       {:ok, _} = Registry.register(name, "hello", :value1)
@@ -960,17 +959,25 @@ defmodule Registry.Test do
       assert Registry.values(name, "hello", self()) |> Enum.sort() == [:value1, :value2]
       assert Registry.values(name, "world", self()) == [:value3]
     end
+
+    test "rejects invalid tuple syntax", %{partitions: partitions} do
+      name = :"test_invalid_tuple_#{partitions}"
+
+      assert_raise ArgumentError, ~r/expected :keys to be given and be one of/, fn ->
+        Registry.start_link(keys: {:duplicate, :invalid}, name: name, partitions: partitions)
+      end
+    end
   end
 
   # Note: those tests relies on internals
-  for {keys, partition_by} <- [
-        {:unique, nil},
+  for keys <- [
+        :unique,
+        :duplicate,
         {:duplicate, :pid},
         {:duplicate, :key}
       ] do
     @tag keys: keys
-    @tag partition_by: partition_by
-    test "clean up #{keys} registry on process crash with partition_by: #{partition_by || "default"}",
+    test "clean up #{inspect(keys)} registry on process crash",
          %{registry: registry, partitions: partitions} do
       {_, task1} = register_task(registry, "hello", :value)
       {_, task2} = register_task(registry, "world", :value)
@@ -992,7 +999,7 @@ defmodule Registry.Test do
           assert :ets.tab2list(pid) == []
         end
       else
-        [{-1, {_, _, key, {partition, pid}, _, _}}] = :ets.lookup(registry, -1)
+        [{-1, {_, _, key, {partition, pid}, _}}] = :ets.lookup(registry, -1)
         GenServer.call(partition, :sync)
         assert :ets.tab2list(key) == []
         assert :ets.tab2list(pid) == []

From fa31cdcaacad06c633fd143eb52030e58bfe2f03 Mon Sep 17 00:00:00 2001
From: Rafal Studnicki <rafal@whatnot.com>
Date: Thu, 17 Jul 2025 12:55:42 +0200
Subject: [PATCH 4/7] Test various duplicate registry settings in for loop

---
 lib/elixir/lib/registry.ex               |  2 ++
 lib/elixir/test/elixir/registry_test.exs | 39 +++++++++++-------------
 2 files changed, 19 insertions(+), 22 deletions(-)

diff --git a/lib/elixir/lib/registry.ex b/lib/elixir/lib/registry.ex
index 9cda3d97567..e32cb54e350 100644
--- a/lib/elixir/lib/registry.ex
+++ b/lib/elixir/lib/registry.ex
@@ -1574,10 +1574,12 @@ defmodule Registry do
   defp partitions(:unique, key, pid, partitions) do
     {hash(key, partitions), hash(pid, partitions)}
   end
+
   defp partitions({:duplicate, :key}, key, _pid, partitions) do
     partition = hash(key, partitions)
     {partition, partition}
   end
+
   defp partitions({:duplicate, :pid}, _key, pid, partitions) do
     partition = hash(pid, partitions)
     {partition, partition}
diff --git a/lib/elixir/test/elixir/registry_test.exs b/lib/elixir/test/elixir/registry_test.exs
index 31341ed479e..a035d5adbb8 100644
--- a/lib/elixir/test/elixir/registry_test.exs
+++ b/lib/elixir/test/elixir/registry_test.exs
@@ -932,23 +932,26 @@ defmodule Registry.Test do
                ])
     end
 
-    test "works with tuple syntax {:duplicate, :key}", %{partitions: partitions} do
-      name = :"test_tuple_keys_#{partitions}"
-      opts = [keys: {:duplicate, :key}, name: name, partitions: partitions]
-      {:ok, _} = start_supervised({Registry, opts})
-
-      {:ok, _} = Registry.register(name, "hello", :value1)
-      {:ok, _} = Registry.register(name, "hello", :value2)
-      {:ok, _} = Registry.register(name, "world", :value3)
+    test "rejects invalid tuple syntax", %{partitions: partitions} do
+      name = :"test_invalid_tuple_#{partitions}"
 
-      assert 3 == Registry.count(name)
-      assert Registry.values(name, "hello", self()) |> Enum.sort() == [:value1, :value2]
-      assert Registry.values(name, "world", self()) == [:value3]
+      assert_raise ArgumentError, ~r/expected :keys to be given and be one of/, fn ->
+        Registry.start_link(keys: {:duplicate, :invalid}, name: name, partitions: partitions)
+      end
     end
+  end
 
-    test "works with tuple syntax {:duplicate, :pid}", %{partitions: partitions} do
-      name = :"test_tuple_pids_#{partitions}"
-      opts = [keys: {:duplicate, :pid}, name: name, partitions: partitions]
+  for {keys, partitions} <- [
+        {{:duplicate, :key}, 1},
+        {{:duplicate, :key}, 8},
+        {{:duplicate, :pid}, 1},
+        {{:duplicate, :pid}, 8}
+      ] do
+    @tag keys: keys, partitions: partitions
+    test "works with tuple syntax #{inspect(keys)} with #{partitions} partitions",
+         %{keys: keys, partitions: partitions} do
+      name = :"test_tuple_#{elem(keys, 1)}_#{partitions}"
+      opts = [keys: keys, name: name, partitions: partitions]
       {:ok, _} = start_supervised({Registry, opts})
 
       {:ok, _} = Registry.register(name, "hello", :value1)
@@ -959,14 +962,6 @@ defmodule Registry.Test do
       assert Registry.values(name, "hello", self()) |> Enum.sort() == [:value1, :value2]
       assert Registry.values(name, "world", self()) == [:value3]
     end
-
-    test "rejects invalid tuple syntax", %{partitions: partitions} do
-      name = :"test_invalid_tuple_#{partitions}"
-
-      assert_raise ArgumentError, ~r/expected :keys to be given and be one of/, fn ->
-        Registry.start_link(keys: {:duplicate, :invalid}, name: name, partitions: partitions)
-      end
-    end
   end
 
   # Note: those tests relies on internals

From 09d5ea3ae6a1c341c9d8bb0b14f89b8e785ba71f Mon Sep 17 00:00:00 2001
From: Rafal Studnicki <rafal@whatnot.com>
Date: Thu, 17 Jul 2025 13:00:08 +0200
Subject: [PATCH 5/7] Restore Enum.sum_by used in Registry.count

---
 lib/elixir/lib/registry.ex | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/lib/elixir/lib/registry.ex b/lib/elixir/lib/registry.ex
index e32cb54e350..abb874ee018 100644
--- a/lib/elixir/lib/registry.ex
+++ b/lib/elixir/lib/registry.ex
@@ -1324,11 +1324,9 @@ defmodule Registry do
   def count(registry) when is_atom(registry) do
     case key_info!(registry) do
       {_kind, partitions, nil} ->
-        0..(partitions - 1)
-        |> Enum.map(fn partition_index ->
+        Enum.sum_by(0..(partitions - 1), fn partition_index ->
           safe_size(key_ets!(registry, partition_index))
         end)
-        |> Enum.sum()
 
       {_kind, 1, key_ets} ->
         safe_size(key_ets)

From 2da924c71dc6cc01a8e9180ca343f2d7225caf04 Mon Sep 17 00:00:00 2001
From: Rafal Studnicki <rafal@whatnot.com>
Date: Thu, 17 Jul 2025 13:01:16 +0200
Subject: [PATCH 6/7] Restore Enum.sum_by used in Registry.count_match

---
 lib/elixir/lib/registry.ex | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/lib/elixir/lib/registry.ex b/lib/elixir/lib/registry.ex
index abb874ee018..41e8dd1452b 100644
--- a/lib/elixir/lib/registry.ex
+++ b/lib/elixir/lib/registry.ex
@@ -1400,11 +1400,9 @@ defmodule Registry do
         :ets.select_count(key_ets, spec)
 
       {{:duplicate, _}, partitions, _key_ets} ->
-        0..(partitions - 1)
-        |> Enum.map(fn partition_index ->
+        Enum.sum_by(0..(partitions - 1), fn partition_index ->
           :ets.select_count(key_ets!(registry, partition_index), spec)
         end)
-        |> Enum.sum()
     end
   end
 

From b83745cd8ec105fed5658c4ad31613e6692f194a Mon Sep 17 00:00:00 2001
From: Rafal Studnicki <rafal@whatnot.com>
Date: Thu, 17 Jul 2025 13:02:28 +0200
Subject: [PATCH 7/7] Restore Enum.sum_by used in Registry.count_select

---
 lib/elixir/lib/registry.ex | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/lib/elixir/lib/registry.ex b/lib/elixir/lib/registry.ex
index 41e8dd1452b..451645b79bf 100644
--- a/lib/elixir/lib/registry.ex
+++ b/lib/elixir/lib/registry.ex
@@ -1493,11 +1493,9 @@ defmodule Registry do
 
     case key_info!(registry) do
       {_kind, partitions, nil} ->
-        0..(partitions - 1)
-        |> Enum.map(fn partition_index ->
+        Enum.sum_by(0..(partitions - 1), fn partition_index ->
           :ets.select_count(key_ets!(registry, partition_index), spec)
         end)
-        |> Enum.sum()
 
       {_kind, 1, key_ets} ->
         :ets.select_count(key_ets, spec)