wip use an index approach to avoid O(db_size) complexity in the presence cache

src/backend/core/services/presence.py

@@ -1,25 +1,11 @@
-"""Presence cache.
-
-Redis-backed memo of "this identity is currently connected to this room".
-
-This module is intentionally a *pure cache store* with no dependency on other
-services, so that `participants_management` (which talks to LiveKit) can
-import it without creating an import cycle. The composition of "check cache,
-fall back to LiveKit" lives in
-`ParticipantsManagement.check_if_in_meeting_cached`.
-
-Only positive answers are stored: a sticky negative would lock out someone
-who joins right after a miss for the whole TTL. The TTL is a safety net in
-case an invalidation webhook is lost.
-"""
+"""Presence cache."""
 
+from typing import List
 from uuid import UUID
 
 from django.conf import settings
 from django.core.cache import cache
 
-from core.utils import CACHE_SCAN_ITERSIZE
-
 
 class PresenceCache:
     """Store and invalidate (room, identity) presence entries."""
@@ -29,24 +15,65 @@ class PresenceCache:
         """Cache key for a (room, identity) presence entry."""
         return f"{settings.PRESENCE_KEY_PREFIX}_{room_id!s}_{identity}"
 
+    @staticmethod
+    def _get_index_key(room_id: UUID | str) -> str:
+        """Raw Redis key of the per-room identity index (a native SET).
+
+        Built through django-redis' make_key so it lives under the same
+        KEY_PREFIX/version namespace as the presence entries.
+        """
+        return cache.client.make_key(
+            f"{settings.PRESENCE_KEY_PREFIX}-index_{room_id!s}"
+        )
+
+    @staticmethod
+    def _redis(write: bool = True):
+        """Raw redis-py client.
+
+        SADD/SREM/SMEMBERS are not exposed by the Django cache API; this is
+        the documented django-redis escape hatch.
+        """
+        return cache.client.get_client(write=write)
+
+    def _index_members(self, room_id: UUID | str) -> List[str]:
+        """All identities currently indexed for the room."""
+        members = self._redis(write=False).smembers(self._get_index_key(room_id))
+        return [
+            member.decode() if isinstance(member, bytes) else member
+            for member in members
+        ]
+
     def is_marked_present(self, room_id: UUID | str, identity: str) -> bool:
         """Return True if a positive presence entry exists in cache."""
         return bool(cache.get(self._get_cache_key(room_id, identity)))
 
     def mark_present(self, room_id: UUID | str, identity: str) -> None:
-        """Record that `identity` is in `room_id`."""
+        """Record that `identity` is in `room_id` and index it for the room."""
         cache.set(
             self._get_cache_key(room_id, identity),
             True,
             timeout=settings.PRESENCE_CACHE_TIMEOUT,
         )
+        index_key = self._get_index_key(room_id)
+        pipe = self._redis().pipeline(transaction=False)
+        pipe.sadd(index_key, identity)
+        pipe.expire(index_key, settings.PRESENCE_CACHE_TIMEOUT)
+        pipe.execute()
 
     def clear(self, room_id: UUID | str, identity: str) -> None:
         """Forget presence for one participant (e.g. on participant_left)."""
         cache.delete(self._get_cache_key(room_id, identity))
+        self._redis().srem(self._get_index_key(room_id), identity)
 
     def clear_room(self, room_id: UUID | str) -> None:
-        """Forget presence for every participant of a room (on room_finished)."""
-        cache.delete_pattern(
-            self._get_cache_key(room_id, "*"), itersize=CACHE_SCAN_ITERSIZE
-        )
+        """Forget presence for every participant of a room (on room_finished).
+
+        Deletes the indexed entries and the index itself with targeted
+        commands instead of a full-keyspace pattern scan.
+        """
+        identities = self._index_members(room_id)
+        if identities:
+            cache.delete_many(
+                [self._get_cache_key(room_id, identity) for identity in identities]
+            )
+        self._redis().delete(self._get_index_key(room_id))

src/backend/core/tests/services/test_presence.py

@@ -90,19 +90,18 @@ def test_presence_clear_and_clear_room():
     assert presence.is_marked_present(other_room, "a") is True
 
 
-def test_presence_clear_room_scans_in_pages():
-    """clear_room removes every match, even across several SCAN pages,
-    and only within the room."""
+def test_presence_clear_room_removes_many_entries_and_the_index():
+    """clear_room removes every entry of the room through the index — never
+    a keyspace scan — and leaves other rooms untouched."""
     room_id, other_room = str(uuid4()), str(uuid4())
     presence = PresenceCache()
     for i in range(7):
         presence.mark_present(room_id, f"user-{i}")
     presence.mark_present(other_room, "user-0")
 
-    # An itersize smaller than the match count forces delete_pattern to
-    # page through several SCAN cursors rather than finish in one pass.
-    with mock.patch("core.utils.CACHE_SCAN_ITERSIZE", 3):
-        presence.clear_room(room_id)
+    presence.clear_room(room_id)
 
     assert all(not presence.is_marked_present(room_id, f"user-{i}") for i in range(7))
     assert presence.is_marked_present(other_room, "user-0") is True
+    assert presence._index_members(room_id) == []
+    assert presence._index_members(other_room) == ["user-0"]

src/backend/core/utils.py

@@ -499,6 +499,3 @@ def build_telephony_config():
         "default_country": country,
         "international_phone_number": international,
     }
-
-
-CACHE_SCAN_ITERSIZE = 500