Comment about a better future where we can get the state diff between…

… two events (#13586) Split off from #13561 Part of #13356 Mentioned in [internal doc](https://docs.google.com/document/d/1lvUoVfYUiy6UaHB6Rb4HicjaJAU40-APue9Q4vzuW3c/edit#bookmark=id.2tvwz3yhcafh)
matrix-org · Aug 24, 2022 · 0bf180c · 0bf180c
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diff --git a/changelog.d/13586.misc b/changelog.d/13586.misc
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+Comment about a better future where we can get the state diff between two events.
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
@@ -1041,6 +1041,14 @@ async def _get_state_ids_after_missing_prev_event(
             InvalidResponseError: if the remote homeserver's response contains fields
                 of the wrong type.
         """
+
+        # It would be better if we could query the difference from our known
+        # state to the given `event_id` so the sending server doesn't have to
+        # send as much and we don't have to process as many events. For example
+        # in a room like #matrix:matrix.org, we get 200k events (77k state_events, 122k
+        # auth_events) from this call.
+        #
+        # Tracked by https://github.com/matrix-org/synapse/issues/13618
         (
             state_event_ids,
             auth_event_ids,
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		Comment about a better future where we can get the state diff between two events.