Optimize Shard.list and Shard.get_by_key

Previous list and get_by_key had to go through GenServer to acquire
values ets table and replicas information. In case GenServer was
processing an update (e.g. heartbeat, track, untrack) then list and
get_by_key functions were blocked until it was completed. We saw this
behaviour in our cluster where simple list/get_by_key calls were
sometimes taking over few hundred milliseconds.

Storing down replicas information in an ets table allows us to avoid
going through genserver and allows us to process list/get_by_key
immediately.

I removed dirty_list function which was not public / exposed and which
was trying to resolve the same issue. dirty_list was called dirty
because it didn't check for down_replicas. This solution checks
down_replicas and doesn't change the api interface.

This should also resolve phoenixframework#124

lib/phoenix/tracker/shard.ex

@@ -68,23 +68,24 @@ defmodule Phoenix.Tracker.Shard do
   end
 
   @spec list(pid | atom, topic) :: [presence]
-  def list(server_pid, topic) do
+  def list(server_pid, topic) when is_pid(server_pid) do
     server_pid
     |> GenServer.call({:list, topic})
     |> State.get_by_topic(topic)
   end
-
-  @doc false
-  def dirty_list(shard_name, topic) do
-    State.tracked_values(shard_name, topic, [])
+  def list(shard_name, topic) when is_atom(shard_name) do
+    State.get_by_topic(shard_name, topic)
   end
 
   @spec get_by_key(pid | atom, topic, term) :: [presence]
-  def get_by_key(server_pid, topic, key) do
+  def get_by_key(server_pid, topic, key) when is_pid(server_pid) do
     server_pid
     |> GenServer.call({:list, topic})
     |> State.get_by_key(topic, key)
   end
+  def get_by_key(shard_name, topic, key) when is_atom(shard_name) do
+    State.get_by_key(shard_name, topic, key)
+  end
 
   @spec graceful_permdown(pid) :: :ok
   def graceful_permdown(server_pid) do

lib/phoenix/tracker/state.ex

@@ -21,15 +21,15 @@ defmodule Phoenix.Tracker.State do
   @type pid_lookup :: {pid, topic, key}
 
   @type t :: %State{
-    replica:  name,
-    context:  context,
-    clouds:   clouds,
-    values:   values,
-    pids:     ets_id,
-    mode:     :unset | :delta | :normal,
-    delta:    :unset | delta,
-    replicas: %{name => :up | :down},
-    range:    {context, context}
+    replica:       name,
+    context:       context,
+    clouds:        clouds,
+    values:        values,
+    pids:          ets_id,
+    mode:          :unset | :delta | :normal,
+    delta:         :unset | delta,
+    down_replicas: ets_id,
+    range:         {context, context}
   }
 
   defstruct replica: nil,
@@ -39,7 +39,7 @@ defmodule Phoenix.Tracker.State do
             pids: nil,
             mode: :unset,
             delta: :unset,
-            replicas: %{},
+            down_replicas: nil,
             range: {%{}, %{}}
 
   @compile {:inline, tag: 1, clock: 1, put_tag: 2, delete_tag: 2, remove_delta_tag: 2}
@@ -61,7 +61,7 @@ defmodule Phoenix.Tracker.State do
       mode: :normal,
       values: :ets.new(shard_name, [:named_table, :protected, :ordered_set]),
       pids: :ets.new(:pids, [:duplicate_bag]),
-      replicas: %{replica => :up}})
+      down_replicas: :ets.new(down_replicas_table(shard_name), [:named_table, :protected, :bag])})
   end
 
   @doc """
@@ -123,21 +123,30 @@ defmodule Phoenix.Tracker.State do
   @doc """
   Returns a list of elements for the topic who belong to an online replica.
   """
-  @spec get_by_topic(t, topic) :: [key_meta]
+  @spec get_by_topic(t | atom, topic) :: [key_meta]
   def get_by_topic(%State{values: values} = state, topic) do
     tracked_values(values, topic, down_replicas(state))
   end
+  def get_by_topic(shard_name, topic) do
+    tracked_values(shard_name, topic, down_replicas(shard_name))
+  end
 
   @doc """
   Returns a list of elements for the topic who belong to an online replica.
   """
-  @spec get_by_key(t, topic, key) :: [key_meta]
+  @spec get_by_key(t | atom, topic, key) :: [key_meta]
   def get_by_key(%State{values: values} = state, topic, key) do
     case tracked_key(values, topic, key, down_replicas(state)) do
       [] -> []
       [_|_] = metas -> metas
     end
   end
+  def get_by_key(shard_name, topic, key) do
+    case tracked_key(shard_name, topic, key, down_replicas(shard_name)) do
+      [] -> []
+      [_|_] = metas -> metas
+    end
+  end
 
   @doc """
   Performs table lookup for tracked elements in the topic.
@@ -400,18 +409,18 @@ defmodule Phoenix.Tracker.State do
   Marks a replica as up in the set and returns rejoined users.
   """
   @spec replica_up(t, name) :: {t, joins :: [values], leaves :: []}
-  def replica_up(%State{replicas: replicas, context: ctx} = state, replica) do
-    {%State{state |
-            context: Map.put_new(ctx, replica, 0),
-            replicas: Map.put(replicas, replica, :up)}, replica_users(state, replica), []}
+  def replica_up(%State{down_replicas: down_replicas, context: ctx} = state, replica) do
+    :ets.delete_object(down_replicas, replica)
+    {%State{state | context: Map.put_new(ctx, replica, 0)}, replica_users(state, replica), []}
   end
 
   @doc """
   Marks a replica as down in the set and returns left users.
   """
   @spec replica_down(t, name) :: {t, joins:: [], leaves :: [values]}
-  def replica_down(%State{replicas: replicas} = state, replica) do
-    {%State{state | replicas: Map.put(replicas, replica, :down)}, [], replica_users(state, replica)}
+  def replica_down(%State{down_replicas: down_replicas} = state, replica) do
+    :ets.insert(down_replicas, replica)
+    {state, [], replica_users(state, replica)}
   end
 
   @doc """
@@ -562,10 +571,9 @@ defmodule Phoenix.Tracker.State do
            delta: %State{delta | range: {start_clock, new_end}}}
   end
 
-  @spec down_replicas(t) :: [name]
-  defp down_replicas(%State{replicas: replicas})  do
-    for {replica, :down} <- replicas, do: replica
-  end
+  @spec down_replicas(t | atom) :: [name]
+  defp down_replicas(%State{down_replicas: down_replicas}), do: :ets.tab2list(down_replicas)
+  defp down_replicas(shard_name), do: :ets.tab2list(down_replicas_table(shard_name))
 
   @spec replica_users(t, name) :: [value]
   defp replica_users(%State{values: values}, replica) do
@@ -582,4 +590,8 @@ defmodule Phoenix.Tracker.State do
   defp foldl({objects, cont}, acc, func) do
     foldl(:ets.select(cont), Enum.reduce(objects, acc, func), func)
   end
+
+  defp down_replicas_table(shard_name) do
+    :"#{shard_name}.down_replicas"
+  end
 end

test/phoenix/tracker/shard_replication_test.exs

@@ -47,6 +47,9 @@ defmodule Phoenix.Tracker.ShardReplicationTest do
     # node1 fulfills tranfer request and sends transfer_ack to primary
     assert_transfer_ack ref, from: @node1
     assert_heartbeat to: @node1, from: @primary
+
+    # small delay to ensure transfer_ack has been processed before calling list
+    :timer.sleep(10)
     assert [{"node1", _}] = list(shard, topic)
   end
 
@@ -92,6 +95,8 @@ defmodule Phoenix.Tracker.ShardReplicationTest do
     assert_heartbeat from: @node1
     assert_heartbeat from: @node2
 
+    # small delay to ensure transfer_ack has been processed before calling list
+    :timer.sleep(10)
     assert [{"node1", _}, {"node1.2", _}, {"node2", _}] = list(shard, topic)
   end
 
@@ -242,21 +247,15 @@ defmodule Phoenix.Tracker.ShardReplicationTest do
     assert_join ^topic, "node1", %{name: "s1"}
     assert %{@node1 => %Replica{status: :up}} = replicas(shard)
     assert [{"local1", _}, {"node1", _}] = list(shard, topic)
-    assert [{"local1", _}, {"node1", _}] = dirty_list(shard, topic)
 
     # nodedown
     Process.unlink(node_pid)
     Process.exit(node1_server, :kill)
     assert_leave ^topic, "node1", %{name: "s1"}
     assert %{@node1 => %Replica{status: :down}} = replicas(shard)
     assert [{"local1", _}] = list(shard, topic)
-    assert [{"local1", _}, {"node1", _}] = dirty_list(shard, topic)
-
-    :timer.sleep(@permdown + 2*@heartbeat)
-    assert [{"local1", _}] = dirty_list(shard, topic)
   end
 
-
   test "untrack with no tracked topic is a noop",
     %{shard: shard, topic: topic} do
     assert Shard.untrack(shard, self(), topic, "foo") == :ok
@@ -429,8 +428,4 @@ defmodule Phoenix.Tracker.ShardReplicationTest do
   defp list(shard, topic) do
     Enum.sort(Shard.list(shard, topic))
   end
-
-  defp dirty_list(shard, topic) do
-    Enum.sort(Shard.dirty_list(shard, topic))
-  end
 end