From df3f2dae8d051e7f17660ddfdce479f340b25770 Mon Sep 17 00:00:00 2001
From: Steffen Beyer <steffen@beyer.io>
Date: Fri, 13 Mar 2026 20:53:43 +0100
Subject: [PATCH] Add ETS subscription index with candidate narrowing

---
 PROGRESS.md                                 |   4 +-
 lib/parrhesia/subscriptions/index.ex        | 433 ++++++++++++++++++++
 lib/parrhesia/subscriptions/supervisor.ex   |   6 +-
 lib/parrhesia/web/connection.ex             |  87 +++-
 test/parrhesia/subscriptions/index_test.exs |  81 ++++
 5 files changed, 605 insertions(+), 6 deletions(-)
 create mode 100644 lib/parrhesia/subscriptions/index.ex
 create mode 100644 test/parrhesia/subscriptions/index_test.exs

diff --git a/PROGRESS.md b/PROGRESS.md
index b75fa1c..ac6a5bc 100644
--- a/PROGRESS.md
+++ b/PROGRESS.md
@@ -28,8 +28,8 @@ Implementation checklist for Parrhesia relay.
 
 ## Phase 3 — fanout + performance primitives
 
-- [ ] Build ETS-backed subscription index
-- [ ] Implement candidate narrowing by kind/author/tag
+- [x] Build ETS-backed subscription index
+- [x] Implement candidate narrowing by kind/author/tag
 - [ ] Add bounded outbound queues/backpressure per connection
 - [ ] Add telemetry for ingest/query/fanout latency + queue depth
 
diff --git a/lib/parrhesia/subscriptions/index.ex b/lib/parrhesia/subscriptions/index.ex
new file mode 100644
index 0000000..ef3ad63
--- /dev/null
+++ b/lib/parrhesia/subscriptions/index.ex
@@ -0,0 +1,433 @@
+defmodule Parrhesia.Subscriptions.Index do
+  @moduledoc """
+  ETS-backed subscription index used for fanout candidate narrowing.
+
+  Subscriptions are keyed by `{owner_pid, subscription_id}` and indexed by kind,
+  author pubkey, and single-letter tag values.
+  """
+
+  use GenServer
+
+  alias Parrhesia.Protocol.Filter
+
+  @wildcard_key :all
+
+  @type subscription_id :: String.t()
+  @type owner :: pid()
+  @type subscription_key :: {owner(), subscription_id()}
+  @type filter :: map()
+
+  @spec start_link(keyword()) :: GenServer.on_start()
+  def start_link(opts \\ []) do
+    name = Keyword.get(opts, :name)
+
+    if is_nil(name) do
+      GenServer.start_link(__MODULE__, :ok)
+    else
+      GenServer.start_link(__MODULE__, :ok, name: name)
+    end
+  end
+
+  @spec upsert(owner(), subscription_id(), [filter()]) :: :ok | {:error, term()}
+  def upsert(owner_pid, subscription_id, filters) do
+    upsert(__MODULE__, owner_pid, subscription_id, filters)
+  end
+
+  @spec upsert(GenServer.server(), owner(), subscription_id(), [filter()]) ::
+          :ok | {:error, term()}
+  def upsert(server, owner_pid, subscription_id, filters) do
+    GenServer.call(server, {:upsert, owner_pid, subscription_id, filters})
+  end
+
+  @spec remove(owner(), subscription_id()) :: :ok
+  def remove(owner_pid, subscription_id) do
+    remove(__MODULE__, owner_pid, subscription_id)
+  end
+
+  @spec remove(GenServer.server(), owner(), subscription_id()) :: :ok
+  def remove(server, owner_pid, subscription_id) do
+    GenServer.call(server, {:remove, owner_pid, subscription_id})
+  end
+
+  @spec remove_owner(owner()) :: :ok
+  def remove_owner(owner_pid) do
+    remove_owner(__MODULE__, owner_pid)
+  end
+
+  @spec remove_owner(GenServer.server(), owner()) :: :ok
+  def remove_owner(server, owner_pid) do
+    GenServer.call(server, {:remove_owner, owner_pid})
+  end
+
+  @spec candidate_subscription_keys(map()) :: [subscription_key()]
+  def candidate_subscription_keys(event) do
+    candidate_subscription_keys(__MODULE__, event)
+  end
+
+  @spec candidate_subscription_keys(GenServer.server(), map()) :: [subscription_key()]
+  def candidate_subscription_keys(server, event) do
+    GenServer.call(server, {:candidate_subscription_keys, event})
+  end
+
+  @spec fetch_filters(GenServer.server(), owner(), subscription_id()) ::
+          {:ok, [filter()]} | :error
+  def fetch_filters(server \\ __MODULE__, owner_pid, subscription_id) do
+    GenServer.call(server, {:fetch_filters, owner_pid, subscription_id})
+  end
+
+  @impl true
+  def init(:ok) do
+    {:ok,
+     %{
+       subscriptions_table: :ets.new(:subscriptions_table, [:set, :protected]),
+       kind_index_table: :ets.new(:subscription_kind_index, [:bag, :protected]),
+       author_index_table: :ets.new(:subscription_author_index, [:bag, :protected]),
+       tag_index_table: :ets.new(:subscription_tag_index, [:bag, :protected]),
+       kind_wildcard_table: :ets.new(:subscription_kind_wildcard_index, [:bag, :protected]),
+       author_wildcard_table: :ets.new(:subscription_author_wildcard_index, [:bag, :protected]),
+       tag_wildcard_table: :ets.new(:subscription_tag_wildcard_index, [:bag, :protected]),
+       owner_subscriptions: %{},
+       owner_monitors: %{},
+       monitor_owners: %{}
+     }}
+  end
+
+  @impl true
+  def handle_call({:upsert, owner_pid, subscription_id, filters}, _from, state) do
+    with :ok <- validate_upsert_args(owner_pid, subscription_id, filters),
+         :ok <- Filter.validate_filters(filters) do
+      subscription_key = {owner_pid, subscription_id}
+      state = remove_existing_subscription(state, subscription_key)
+
+      index_entries = build_index_entries(filters)
+
+      true = :ets.insert(state.subscriptions_table, {subscription_key, filters, index_entries})
+      insert_index_entries(state, subscription_key, index_entries)
+
+      state =
+        state
+        |> ensure_owner_monitor(owner_pid)
+        |> track_owner_subscription(owner_pid, subscription_key)
+
+      {:reply, :ok, state}
+    else
+      {:error, reason} ->
+        {:reply, {:error, reason}, state}
+    end
+  end
+
+  def handle_call({:remove, owner_pid, subscription_id}, _from, state) do
+    subscription_key = {owner_pid, subscription_id}
+    state = remove_existing_subscription(state, subscription_key)
+    {:reply, :ok, state}
+  end
+
+  def handle_call({:remove_owner, owner_pid}, _from, state) do
+    state = remove_owner_subscriptions(state, owner_pid)
+    {:reply, :ok, state}
+  end
+
+  def handle_call({:candidate_subscription_keys, event}, _from, state) do
+    candidates =
+      state
+      |> kind_candidates(event)
+      |> MapSet.intersection(author_candidates(state, event))
+      |> MapSet.intersection(tag_candidates(state, event))
+      |> MapSet.to_list()
+
+    {:reply, candidates, state}
+  end
+
+  def handle_call({:fetch_filters, owner_pid, subscription_id}, _from, state) do
+    subscription_key = {owner_pid, subscription_id}
+
+    case :ets.lookup(state.subscriptions_table, subscription_key) do
+      [{^subscription_key, filters, _index_entries}] -> {:reply, {:ok, filters}, state}
+      [] -> {:reply, :error, state}
+    end
+  end
+
+  @impl true
+  def handle_info({:DOWN, monitor_ref, :process, owner_pid, _reason}, state) do
+    case Map.get(state.monitor_owners, monitor_ref) do
+      ^owner_pid ->
+        state = remove_owner_subscriptions(state, owner_pid)
+        {:noreply, state}
+
+      _other ->
+        {:noreply, state}
+    end
+  end
+
+  def handle_info(_message, state), do: {:noreply, state}
+
+  defp validate_upsert_args(owner_pid, subscription_id, filters)
+       when is_pid(owner_pid) and is_binary(subscription_id) and is_list(filters),
+       do: :ok
+
+  defp validate_upsert_args(_owner_pid, _subscription_id, _filters),
+    do: {:error, :invalid_subscription}
+
+  defp remove_existing_subscription(state, subscription_key) do
+    case :ets.lookup(state.subscriptions_table, subscription_key) do
+      [{^subscription_key, _filters, index_entries}] ->
+        true = :ets.delete(state.subscriptions_table, subscription_key)
+        delete_index_entries(state, subscription_key, index_entries)
+        untrack_owner_subscription(state, subscription_key)
+
+      [] ->
+        state
+    end
+  end
+
+  defp build_index_entries(filters) do
+    Enum.reduce(filters, empty_index_entries(), fn filter, acc ->
+      acc
+      |> index_kinds(filter)
+      |> index_authors(filter)
+      |> index_tags(filter)
+    end)
+  end
+
+  defp empty_index_entries do
+    %{
+      kinds: MapSet.new(),
+      kind_wildcard?: false,
+      authors: MapSet.new(),
+      author_wildcard?: false,
+      tags: MapSet.new(),
+      tag_wildcard?: false
+    }
+  end
+
+  defp index_kinds(acc, filter) do
+    case Map.get(filter, "kinds") do
+      kinds when is_list(kinds) ->
+        Enum.reduce(kinds, acc, fn kind, acc_inner ->
+          %{acc_inner | kinds: MapSet.put(acc_inner.kinds, kind)}
+        end)
+
+      _other ->
+        %{acc | kind_wildcard?: true}
+    end
+  end
+
+  defp index_authors(acc, filter) do
+    case Map.get(filter, "authors") do
+      authors when is_list(authors) ->
+        Enum.reduce(authors, acc, fn author, acc_inner ->
+          %{acc_inner | authors: MapSet.put(acc_inner.authors, author)}
+        end)
+
+      _other ->
+        %{acc | author_wildcard?: true}
+    end
+  end
+
+  defp index_tags(acc, filter) do
+    case tag_filters(filter) do
+      [] ->
+        %{acc | tag_wildcard?: true}
+
+      extracted_tag_filters ->
+        tags =
+          Enum.reduce(extracted_tag_filters, acc.tags, fn {tag_name, values}, tags_acc ->
+            put_tag_values(tags_acc, tag_name, values)
+          end)
+
+        %{acc | tags: tags}
+    end
+  end
+
+  defp tag_filters(filter) do
+    Enum.reduce(filter, [], fn
+      {<<"#", tag_name::binary-size(1)>>, values}, collected when is_list(values) ->
+        [{tag_name, values} | collected]
+
+      _entry, collected ->
+        collected
+    end)
+  end
+
+  defp put_tag_values(tags, tag_name, values) do
+    Enum.reduce(values, tags, fn value, tags_acc ->
+      MapSet.put(tags_acc, {tag_name, value})
+    end)
+  end
+
+  defp insert_index_entries(state, subscription_key, index_entries) do
+    Enum.each(index_entries.kinds, fn kind ->
+      true = :ets.insert(state.kind_index_table, {kind, subscription_key})
+    end)
+
+    if index_entries.kind_wildcard? do
+      true = :ets.insert(state.kind_wildcard_table, {@wildcard_key, subscription_key})
+    end
+
+    Enum.each(index_entries.authors, fn author ->
+      true = :ets.insert(state.author_index_table, {author, subscription_key})
+    end)
+
+    if index_entries.author_wildcard? do
+      true = :ets.insert(state.author_wildcard_table, {@wildcard_key, subscription_key})
+    end
+
+    Enum.each(index_entries.tags, fn {tag_name, value} ->
+      true = :ets.insert(state.tag_index_table, {{tag_name, value}, subscription_key})
+    end)
+
+    if index_entries.tag_wildcard? do
+      true = :ets.insert(state.tag_wildcard_table, {@wildcard_key, subscription_key})
+    end
+
+    :ok
+  end
+
+  defp delete_index_entries(state, subscription_key, index_entries) do
+    Enum.each(index_entries.kinds, fn kind ->
+      true = :ets.delete_object(state.kind_index_table, {kind, subscription_key})
+    end)
+
+    if index_entries.kind_wildcard? do
+      true = :ets.delete_object(state.kind_wildcard_table, {@wildcard_key, subscription_key})
+    end
+
+    Enum.each(index_entries.authors, fn author ->
+      true = :ets.delete_object(state.author_index_table, {author, subscription_key})
+    end)
+
+    if index_entries.author_wildcard? do
+      true = :ets.delete_object(state.author_wildcard_table, {@wildcard_key, subscription_key})
+    end
+
+    Enum.each(index_entries.tags, fn {tag_name, value} ->
+      true = :ets.delete_object(state.tag_index_table, {{tag_name, value}, subscription_key})
+    end)
+
+    if index_entries.tag_wildcard? do
+      true = :ets.delete_object(state.tag_wildcard_table, {@wildcard_key, subscription_key})
+    end
+
+    :ok
+  end
+
+  defp ensure_owner_monitor(state, owner_pid) do
+    case Map.fetch(state.owner_monitors, owner_pid) do
+      {:ok, _monitor_ref} ->
+        state
+
+      :error ->
+        monitor_ref = Process.monitor(owner_pid)
+
+        state
+        |> put_in([:owner_monitors, owner_pid], monitor_ref)
+        |> put_in([:monitor_owners, monitor_ref], owner_pid)
+    end
+  end
+
+  defp track_owner_subscription(state, owner_pid, subscription_key) do
+    current = Map.get(state.owner_subscriptions, owner_pid, MapSet.new())
+    next = MapSet.put(current, subscription_key)
+    put_in(state, [:owner_subscriptions, owner_pid], next)
+  end
+
+  defp untrack_owner_subscription(state, {owner_pid, _subscription_id} = subscription_key) do
+    current = Map.get(state.owner_subscriptions, owner_pid, MapSet.new())
+    remaining = MapSet.delete(current, subscription_key)
+
+    if MapSet.size(remaining) == 0 do
+      state
+      |> maybe_demonitor_owner(owner_pid)
+      |> update_in([:owner_subscriptions], &Map.delete(&1, owner_pid))
+    else
+      put_in(state, [:owner_subscriptions, owner_pid], remaining)
+    end
+  end
+
+  defp maybe_demonitor_owner(state, owner_pid) do
+    case Map.pop(state.owner_monitors, owner_pid) do
+      {nil, _owner_monitors} ->
+        state
+
+      {monitor_ref, owner_monitors} ->
+        true = Process.demonitor(monitor_ref, [:flush])
+
+        state
+        |> Map.put(:owner_monitors, owner_monitors)
+        |> update_in([:monitor_owners], &Map.delete(&1, monitor_ref))
+    end
+  end
+
+  defp remove_owner_subscriptions(state, owner_pid) do
+    subscription_keys = Map.get(state.owner_subscriptions, owner_pid, MapSet.new())
+
+    state =
+      Enum.reduce(subscription_keys, state, fn subscription_key, acc ->
+        remove_existing_subscription(acc, subscription_key)
+      end)
+
+    state
+    |> maybe_demonitor_owner(owner_pid)
+    |> update_in([:owner_subscriptions], &Map.delete(&1, owner_pid))
+  end
+
+  defp kind_candidates(state, event) do
+    event
+    |> Map.get("kind")
+    |> index_candidates_for_value(state.kind_index_table, state.kind_wildcard_table)
+  end
+
+  defp author_candidates(state, event) do
+    event
+    |> Map.get("pubkey")
+    |> index_candidates_for_value(state.author_index_table, state.author_wildcard_table)
+  end
+
+  defp tag_candidates(state, event) do
+    tag_pairs = event_tag_pairs(Map.get(event, "tags"))
+    wildcard_candidates = lookup_candidates(state.tag_wildcard_table, @wildcard_key)
+
+    if MapSet.size(tag_pairs) == 0 do
+      wildcard_candidates
+    else
+      matched_candidates =
+        Enum.reduce(tag_pairs, MapSet.new(), fn {tag_name, value}, acc ->
+          MapSet.union(acc, lookup_candidates(state.tag_index_table, {tag_name, value}))
+        end)
+
+      MapSet.union(matched_candidates, wildcard_candidates)
+    end
+  end
+
+  defp index_candidates_for_value(value, index_table, wildcard_table) do
+    wildcard_candidates = lookup_candidates(wildcard_table, @wildcard_key)
+
+    if is_nil(value) do
+      wildcard_candidates
+    else
+      value_candidates = lookup_candidates(index_table, value)
+      MapSet.union(value_candidates, wildcard_candidates)
+    end
+  end
+
+  defp lookup_candidates(table, key) do
+    table
+    |> :ets.lookup(key)
+    |> Enum.reduce(MapSet.new(), fn
+      {^key, subscription_key}, acc -> MapSet.put(acc, subscription_key)
+      _entry, acc -> acc
+    end)
+  end
+
+  defp event_tag_pairs(tags) when is_list(tags) do
+    Enum.reduce(tags, MapSet.new(), fn
+      [tag_name, value | _rest], acc when is_binary(tag_name) and is_binary(value) ->
+        MapSet.put(acc, {tag_name, value})
+
+      _tag, acc ->
+        acc
+    end)
+  end
+
+  defp event_tag_pairs(_tags), do: MapSet.new()
+end
diff --git a/lib/parrhesia/subscriptions/supervisor.ex b/lib/parrhesia/subscriptions/supervisor.ex
index b009a04..68d7d4d 100644
--- a/lib/parrhesia/subscriptions/supervisor.ex
+++ b/lib/parrhesia/subscriptions/supervisor.ex
@@ -11,6 +11,10 @@ defmodule Parrhesia.Subscriptions.Supervisor do
 
   @impl true
   def init(_init_arg) do
-    Supervisor.init([], strategy: :one_for_one)
+    children = [
+      {Parrhesia.Subscriptions.Index, name: Parrhesia.Subscriptions.Index}
+    ]
+
+    Supervisor.init(children, strategy: :one_for_one)
   end
 end
diff --git a/lib/parrhesia/web/connection.ex b/lib/parrhesia/web/connection.ex
index bafaba9..ccf26d0 100644
--- a/lib/parrhesia/web/connection.ex
+++ b/lib/parrhesia/web/connection.ex
@@ -7,12 +7,14 @@ defmodule Parrhesia.Web.Connection do
 
   alias Parrhesia.Protocol
   alias Parrhesia.Protocol.Filter
+  alias Parrhesia.Subscriptions.Index
 
   @default_max_subscriptions_per_connection 32
 
   defstruct subscriptions: %{},
             authenticated_pubkeys: MapSet.new(),
-            max_subscriptions_per_connection: @default_max_subscriptions_per_connection
+            max_subscriptions_per_connection: @default_max_subscriptions_per_connection,
+            subscription_index: Index
 
   @type subscription :: %{
           filters: [map()],
@@ -22,12 +24,17 @@ defmodule Parrhesia.Web.Connection do
   @type t :: %__MODULE__{
           subscriptions: %{String.t() => subscription()},
           authenticated_pubkeys: MapSet.t(String.t()),
-          max_subscriptions_per_connection: pos_integer()
+          max_subscriptions_per_connection: pos_integer(),
+          subscription_index: GenServer.server() | nil
         }
 
   @impl true
   def init(opts) do
-    state = %__MODULE__{max_subscriptions_per_connection: max_subscriptions_per_connection(opts)}
+    state = %__MODULE__{
+      max_subscriptions_per_connection: max_subscriptions_per_connection(opts),
+      subscription_index: subscription_index(opts)
+    }
+
     {:ok, state}
   end
 
@@ -53,6 +60,7 @@ defmodule Parrhesia.Web.Connection do
 
       {:ok, {:close, subscription_id}} ->
         next_state = drop_subscription(state, subscription_id)
+        :ok = maybe_remove_index_subscription(next_state, subscription_id)
 
         response =
           Protocol.encode_relay({:closed, subscription_id, "error: subscription closed"})
@@ -78,9 +86,17 @@ defmodule Parrhesia.Web.Connection do
     {:ok, state}
   end
 
+  @impl true
+  def terminate(_reason, %__MODULE__{} = state) do
+    :ok = maybe_remove_index_owner(state)
+    :ok
+  end
+
   defp handle_req(%__MODULE__{} = state, subscription_id, filters) do
     with :ok <- Filter.validate_filters(filters),
          {:ok, next_state} <- upsert_subscription(state, subscription_id, filters) do
+      :ok = maybe_upsert_index_subscription(next_state, subscription_id, filters)
+
       response = Protocol.encode_relay({:eose, subscription_id})
       {:push, {:text, response}, next_state}
     else
@@ -125,6 +141,71 @@ defmodule Parrhesia.Web.Connection do
     %__MODULE__{state | subscriptions: subscriptions}
   end
 
+  defp maybe_upsert_index_subscription(
+         %__MODULE__{subscription_index: nil},
+         _subscription_id,
+         _filters
+       ),
+       do: :ok
+
+  defp maybe_upsert_index_subscription(
+         %__MODULE__{subscription_index: subscription_index},
+         subscription_id,
+         filters
+       ) do
+    case Index.upsert(subscription_index, self(), subscription_id, filters) do
+      :ok -> :ok
+      {:error, _reason} -> :ok
+    end
+  catch
+    :exit, _reason -> :ok
+  end
+
+  defp maybe_remove_index_subscription(
+         %__MODULE__{subscription_index: nil},
+         _subscription_id
+       ),
+       do: :ok
+
+  defp maybe_remove_index_subscription(
+         %__MODULE__{subscription_index: subscription_index},
+         subscription_id
+       ) do
+    :ok = Index.remove(subscription_index, self(), subscription_id)
+    :ok
+  catch
+    :exit, _reason -> :ok
+  end
+
+  defp maybe_remove_index_owner(%__MODULE__{subscription_index: nil}), do: :ok
+
+  defp maybe_remove_index_owner(%__MODULE__{subscription_index: subscription_index}) do
+    :ok = Index.remove_owner(subscription_index, self())
+    :ok
+  catch
+    :exit, _reason -> :ok
+  end
+
+  defp subscription_index(opts) when is_list(opts) do
+    opts
+    |> Keyword.get(:subscription_index, Index)
+    |> normalize_subscription_index()
+  end
+
+  defp subscription_index(opts) when is_map(opts) do
+    opts
+    |> Map.get(:subscription_index, Index)
+    |> normalize_subscription_index()
+  end
+
+  defp subscription_index(_opts), do: Index
+
+  defp normalize_subscription_index(subscription_index)
+       when is_pid(subscription_index) or is_atom(subscription_index),
+       do: subscription_index
+
+  defp normalize_subscription_index(_subscription_index), do: nil
+
   defp max_subscriptions_per_connection(opts) when is_list(opts) do
     opts
     |> Keyword.get(:max_subscriptions_per_connection)
diff --git a/test/parrhesia/subscriptions/index_test.exs b/test/parrhesia/subscriptions/index_test.exs
new file mode 100644
index 0000000..b85bce8
--- /dev/null
+++ b/test/parrhesia/subscriptions/index_test.exs
@@ -0,0 +1,81 @@
+defmodule Parrhesia.Subscriptions.IndexTest do
+  use ExUnit.Case, async: true
+
+  alias Parrhesia.Subscriptions.Index
+
+  test "returns narrowed candidates by kind, author, and tag" do
+    index = start_supervised!(Index)
+    owner = self()
+
+    author = String.duplicate("a", 64)
+
+    filters = [
+      %{
+        "kinds" => [1],
+        "authors" => [author],
+        "#e" => ["event-1"]
+      }
+    ]
+
+    assert :ok = Index.upsert(index, owner, "sub-1", filters)
+
+    matching_event = %{"kind" => 1, "pubkey" => author, "tags" => [["e", "event-1"]]}
+    non_matching_event = %{"kind" => 2, "pubkey" => author, "tags" => [["e", "event-1"]]}
+
+    assert Index.candidate_subscription_keys(index, matching_event) == [{owner, "sub-1"}]
+    assert Index.candidate_subscription_keys(index, non_matching_event) == []
+  end
+
+  test "treats missing kind and tag constraints as wildcards" do
+    index = start_supervised!(Index)
+    owner = self()
+
+    author = String.duplicate("b", 64)
+    filters = [%{"authors" => [author]}]
+
+    assert :ok = Index.upsert(index, owner, "sub-1", filters)
+
+    event = %{"kind" => 7, "pubkey" => author, "tags" => []}
+
+    assert Index.candidate_subscription_keys(index, event) == [{owner, "sub-1"}]
+  end
+
+  test "replaces existing subscription index entries on upsert" do
+    index = start_supervised!(Index)
+    owner = self()
+
+    assert :ok = Index.upsert(index, owner, "sub-1", [%{"kinds" => [1]}])
+    assert :ok = Index.upsert(index, owner, "sub-1", [%{"kinds" => [2]}])
+
+    kind_one_event = %{"kind" => 1, "pubkey" => "any", "tags" => []}
+    kind_two_event = %{"kind" => 2, "pubkey" => "any", "tags" => []}
+
+    assert Index.candidate_subscription_keys(index, kind_one_event) == []
+    assert Index.candidate_subscription_keys(index, kind_two_event) == [{owner, "sub-1"}]
+  end
+
+  test "removes subscriptions when owner process exits" do
+    index = start_supervised!(Index)
+
+    owner =
+      spawn(fn ->
+        receive do
+          :stop -> :ok
+        end
+      end)
+
+    assert :ok = Index.upsert(index, owner, "sub-1", [%{"kinds" => [1]}])
+
+    event = %{"kind" => 1, "pubkey" => "any", "tags" => []}
+    assert Index.candidate_subscription_keys(index, event) == [{owner, "sub-1"}]
+
+    ref = Process.monitor(owner)
+    send(owner, :stop)
+
+    assert_receive {:DOWN, ^ref, :process, ^owner, :normal}
+
+    _state = :sys.get_state(index)
+
+    assert Index.candidate_subscription_keys(index, event) == []
+  end
+end