Cache the result of fetching the room hierarchy over federation. (#10647)

changelog.d/10647.misc

@@ -0,0 +1 @@
+Improve the performance of the `/hierarchy` API (from [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946)) by caching responses received over federation.

synapse/federation/federation_client.py

@@ -111,6 +111,23 @@ class FederationClient(FederationBase):
             reset_expiry_on_get=False,
         )
 
+        # A cache for fetching the room hierarchy over federation.
+        #
+        # Some stale data over federation is OK, but must be refreshed
+        # periodically since the local server is in the room.
+        #
+        # It is a map of (room ID, suggested-only) -> the response of
+        # get_room_hierarchy.
+        self._get_room_hierarchy_cache: ExpiringCache[
+            Tuple[str, bool], Tuple[JsonDict, Sequence[JsonDict], Sequence[str]]
+        ] = ExpiringCache(
+            cache_name="get_room_hierarchy_cache",
+            clock=self._clock,
+            max_len=1000,
+            expiry_ms=5 * 60 * 1000,
+            reset_expiry_on_get=False,
+        )
+
     def _clear_tried_cache(self):
         """Clear pdu_destination_tried cache"""
         now = self._clock.time_msec()
@@ -1324,6 +1341,10 @@ class FederationClient(FederationBase):
                remote servers
         """
 
+        cached_result = self._get_room_hierarchy_cache.get((room_id, suggested_only))
+        if cached_result:
+            return cached_result
+
         async def send_request(
             destination: str,
         ) -> Tuple[JsonDict, Sequence[JsonDict], Sequence[str]]:
@@ -1370,20 +1391,23 @@ class FederationClient(FederationBase):
             return room, children, inaccessible_children
 
         try:
-            return await self._try_destination_list(
+            result = await self._try_destination_list(
                 "fetch room hierarchy",
                 destinations,
                 send_request,
                 failover_on_unknown_endpoint=True,
             )
         except SynapseError as e:
+            # If an unexpected error occurred, re-raise it.
+            if e.code != 502:
+                raise
+
             # Fallback to the old federation API and translate the results if
             # no servers implement the new API.
             #
             # The algorithm below is a bit inefficient as it only attempts to
-            # get information for the requested room, but the legacy API may
+            # parse information for the requested room, but the legacy API may
             # return additional layers.
-            if e.code == 502:
             legacy_result = await self.get_space_summary(
                 destinations,
                 room_id,
@@ -1419,9 +1443,11 @@ class FederationClient(FederationBase):
 
             # It isn't clear from the response whether some of the rooms are
             # not accessible.
-                return requested_room, children, ()
+            result = (requested_room, children, ())
 
-            raise
+        # Cache the result to avoid fetching data over federation every time.
+        self._get_room_hierarchy_cache[(room_id, suggested_only)] = result
+        return result
 
 
 @attr.s(frozen=True, slots=True, auto_attribs=True)