Optimise async get event lookups

changelog.d/13435.misc

@@ -1 +1 @@
-Prevent unnecessary lookups to any external get event cache. Contributed by Nick @ Beeper (@fizzadar).
+Prevent unnecessary lookups to any external `get_event` cache. Contributed by Nick @ Beeper (@fizzadar).

synapse/storage/databases/main/events_worker.py

@@ -602,7 +602,8 @@ async def _get_events_from_cache_or_db(
         """
         # Shortcut: check if we have any events in the *in memory* cache - this function
         # may be called repeatedly for the same event so at this point we cannot reach
-        # out to any external cache for performance reasons.
+        # out to any external cache for performance reasons, this is done later on in
+        # the `get_missing_events_from_cache_or_db` function below.
         event_entry_map = self._get_events_from_local_cache(
             event_ids,
         )
@@ -664,14 +665,13 @@ async def get_missing_events_from_cache_or_db() -> Dict[
                 #
                 missing_events = {}
                 try:
-                    # First fetch from the cache - including any external caches
-                    cache_missing_events = await self._get_events_from_cache(
+                    # Try to fetch from any external cache, in-memory cache checked above
+                    missing_events = await self._get_events_from_external_cache(
                         missing_events_ids,
                     )
-                    missing_events.update(cache_missing_events)
                     # Now actually fetch any remaining events from the DB
                     db_missing_events = await self._get_events_from_db(
-                        missing_events_ids - set(cache_missing_events.keys()),
+                        missing_events_ids - missing_events.keys(),
                     )
                     missing_events.update(db_missing_events)
                 except Exception as e:
@@ -780,9 +780,30 @@ async def _get_events_from_cache(
         )
 
         missing_event_ids = {e for e in events if e not in event_map}
+        event_map.update(
+            await self._get_events_from_external_cache(
+                events=missing_event_ids,
+                update_metrics=update_metrics,
+            )
+        )
+
+        return event_map
+
+    async def _get_events_from_external_cache(
+        self, events: Iterable[str], update_metrics: bool = True
+    ) -> Dict[str, EventCacheEntry]:
+        """Fetch events from any configured external cache.
+
+        May return rejected events.
+
+        Args:
+            events: list of event_ids to fetch
+            update_metrics: Whether to update the cache hit ratio metrics
+        """
+        event_map = {}
 
-        for event_id in missing_event_ids:
-            ret = await self._get_event_cache.get(
+        for event_id in events:
+            ret = await self._get_event_cache.get_external(
                 (event_id,), None, update_metrics=update_metrics
             )
             if ret:

synapse/util/caches/lrucache.py

@@ -834,6 +834,15 @@ async def get(
     ) -> Optional[VT]:
         return self._lru_cache.get(key, update_metrics=update_metrics)
 
+    async def get_external(
+        self,
+        key: KT,
+        default: Optional[T] = None,
+        update_metrics: bool = True,
+    ) -> Optional[VT]:
+        # This method should fetch from any configured external cache, in this case noop.
+        return None
+
     def get_local(
         self, key: KT, default: Optional[T] = None, update_metrics: bool = True
     ) -> Optional[VT]: