Optimise async get event lookups

synapse/storage/databases/main/events_worker.py

@@ -600,7 +600,10 @@ async def _get_events_from_cache_or_db(
         Returns:
             map from event id to result
         """
-        event_entry_map = await self._get_events_from_cache(
+        # 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.
+        event_entry_map = self._get_events_from_local_cache(
             event_ids,
         )
 
@@ -632,7 +635,7 @@ async def _get_events_from_cache_or_db(
 
         if missing_events_ids:
 
-            async def get_missing_events_from_db() -> Dict[str, EventCacheEntry]:
+            async def get_missing_events_from_cache_or_db() -> Dict[str, EventCacheEntry]:
                 """Fetches the events in `missing_event_ids` from the database.
 
                 Also creates entries in `self._current_event_fetches` to allow
@@ -657,10 +660,18 @@ async def get_missing_events_from_db() -> Dict[str, EventCacheEntry]:
                 # the events have been redacted, and if so pulling the redaction event
                 # out of the database to check it.
                 #
+                missing_events = {}
                 try:
-                    missing_events = await self._get_events_from_db(
+                    # First fetch from the cache - including any external caches
+                    cache_missing_events = await self._get_events_from_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.update(db_missing_events)
                 except Exception as e:
                     with PreserveLoggingContext():
                         fetching_deferred.errback(e)
@@ -679,7 +690,7 @@ async def get_missing_events_from_db() -> Dict[str, EventCacheEntry]:
             # cancellations, since multiple `_get_events_from_cache_or_db` calls can
             # reuse the same fetch.
             missing_events: Dict[str, EventCacheEntry] = await delay_cancellation(
-                get_missing_events_from_db()
+                get_missing_events_from_cache_or_db()
             )
             event_entry_map.update(missing_events)