Add Registry partition_by option for duplicate registries

lib/elixir/lib/registry.ex

@@ -187,15 +187,15 @@ 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
 
   @typedoc "The registry identifier"
   @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
@@ -266,8 +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
 
@@ -329,11 +329,24 @@ 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, :key},
+        name: MyApp.TopicRegistry,
+        partitions: System.schedulers_online()
+      )
+
+  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:
 
-    * `: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:
@@ -345,16 +358,40 @@ defmodule Registry do
       crashes. Messages sent to listeners are of type `t:listener_message/0`.
     * `:meta` - a keyword list of metadata to be attached to the registry.
 
+      For `:duplicate` registries, you can specify the partitioning strategy
+      directly in the `:keys` option:
+
+      * `: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.
+
+      * `{: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"
   @spec start_link([start_option]) :: {:ok, pid} | {:error, term}
   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
@@ -397,11 +434,18 @@ 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, {kind, partitions, nil, nil, listeners}},
+      {@key_info, {kind, partitions, nil}} | meta
     ]
 
-    Registry.Supervisor.start_link(keys, name, partitions, listeners, entries, compressed)
+    Registry.Supervisor.start_link(
+      kind,
+      name,
+      partitions,
+      listeners,
+      entries,
+      compressed
+    )
   end
 
   @doc false
@@ -468,7 +512,8 @@ defmodule Registry do
         end
 
       {kind, _, _} ->
-        raise ArgumentError, "Registry.update_value/3 is not supported for #{kind} registries"
+        raise ArgumentError,
+              "Registry.update_value/3 is not supported for #{inspect(kind)} registries"
     end
   end
 
@@ -508,12 +553,12 @@ defmodule Registry do
         |> 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)
@@ -625,10 +670,14 @@ defmodule Registry do
             []
         end
 
-      {:duplicate, 1, key_ets} ->
+      {{:duplicate, _}, 1, key_ets} ->
         safe_lookup_second(key_ets, key)
 
-      {:duplicate, partitions, _key_ets} ->
+      {{:duplicate, :key}, partitions, _key_ets} ->
+        partition = hash(key, partitions)
+        safe_lookup_second(key_ets!(registry, partition), key)
+
+      {{:duplicate, :pid}, partitions, _key_ets} ->
         for partition <- 0..(partitions - 1),
             pair <- safe_lookup_second(key_ets!(registry, partition), key),
             do: pair
@@ -749,10 +798,10 @@ defmodule Registry do
         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
@@ -795,16 +844,35 @@ defmodule Registry do
   @spec keys(registry, pid) :: [key]
   def keys(registry, pid) when is_atom(registry) and is_pid(pid) do
     {kind, partitions, _, pid_ets, _} = info!(registry)
-    {_, pid_ets} = pid_ets || pid_ets!(registry, pid, partitions)
 
-    keys =
-      try do
-        spec = [{{pid, :"$1", :"$2", :_}, [], [{{:"$1", :"$2"}}]}]
-        :ets.select(pid_ets, spec)
-      catch
-        :error, :badarg -> []
+    pid_etses =
+      if pid_ets do
+        {_, pid_ets} = pid_ets
+        [pid_ets]
+      else
+        case kind do
+          {:duplicate, :key} ->
+            for partition <- 0..(partitions - 1) do
+              {_, pid_ets} = pid_ets!(registry, partition)
+              pid_ets
+            end
+
+          _ ->
+            {_, pid_ets} = pid_ets!(registry, pid, partitions)
+            [pid_ets]
+        end
       end
 
+    keys =
+      Enum.flat_map(pid_etses, fn pid_ets ->
+        try do
+          spec = [{{pid, :"$1", :"$2", :_}, [], [{{:"$1", :"$2"}}]}]
+          :ets.select(pid_ets, spec)
+        catch
+          :error, :badarg -> []
+        end
+      end)
+
     # Handle the possibility of fake keys
     keys = gather_keys(keys, [], false)
 
@@ -882,8 +950,17 @@ 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, :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, :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
@@ -1121,7 +1198,7 @@ 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
@@ -1339,10 +1416,10 @@ defmodule Registry do
         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} ->
         Enum.sum_by(0..(partitions - 1), fn partition_index ->
           :ets.select_count(key_ets!(registry, partition_index), spec)
         end)
@@ -1512,7 +1589,12 @@ defmodule Registry do
     {hash(key, partitions), hash(pid, partitions)}
   end
 
-  defp partitions(:duplicate, _key, pid, partitions) do
+  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}
   end
@@ -1576,9 +1658,10 @@ 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
@@ -1633,6 +1716,7 @@ defmodule Registry.Partition 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)
 
@@ -1659,7 +1743,7 @@ 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