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If we send out an event which refers to `prev_events` which other servers in the federation are missing, then (after a round or two of backfill attempts), they will end up asking us for `/state_ids` at a particular point in the DAG. As per #7893, this is quite expensive, and we tend to see lots of very similar requests around the same time. We can therefore handle this much more efficiently by using a cache, which (a) ensures that if we see the same request from multiple servers (or even the same server, multiple times), then they share the result, and (b) any other servers that miss the initial excitement can also benefit from the work. [It's interesting to note that `/state` has a cache for exactly this reason. `/state` is now essentially unused and replaced with `/state_ids`, but evidently when we replaced it we forgot to add a cache to the new endpoint.]
clokep
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| resp = await self._state_ids_resp_cache.wrap( | ||
| (room_id, event_id), self._on_state_ids_request_compute, room_id, event_id, | ||
| ) |
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The call above to self._state_resp_cache gets wrapped by dict(), which we don't do here. It doesn't seem necessary, but curious about the difference.
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the dict there was added in https://github.com/matrix-org/synapse/pull/6176/files. It looks like it's so that we can add a room_version property after it comes out of the cache, without accidentally modifying the cached response.
It is unclear to me why that implementation was chosen instead of adding the room_id in _on_context_state_request_compute.
In any case, I think it's unnecessary in the new code.
babolivier
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* commit 'f88c48f3b': 1.18.0rc1 Fix error reporting when using `opentracing.trace` (#7961) Fix typing replication not being handled on master (#7959) Remove hacky error handling for inlineDeferreds. (#7950) Convert tests/rest/admin/test_room.py to unix file endings (#7953) Support oEmbed for media previews. (#7920) Convert state resolution to async/await (#7942) Fix up types and comments that refer to Deferreds. (#7945) Do not convert async functions to Deferreds in the interactive_auth_handler (#7944) Convert more of the media code to async/await (#7873) Return an empty body for OPTIONS requests. (#7886) Downgrade warning on client disconnect to INFO (#7928) Convert presence handler helpers to async/await. (#7939) Update the auth providers to be async. (#7935) Put a cache on `/state_ids` (#7931)
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If we send out an event which refers to
prev_eventswhich other servers inthe federation are missing, then (after a round or two of backfill attempts),
they will end up asking us for
/state_idsat a particular point in the DAG.As per #7893, this is quite
expensive, and we tend to see lots of very similar requests around the same
time.
We can therefore handle this much more efficiently by using a cache, which (a)
ensures that if we see the same request from multiple servers (or even the same
server, multiple times), then they share the result, and (b) any other servers
that miss the initial excitement can also benefit from the work.
[It's interesting to note that
/statehas a cache for exactly thisreason.
/stateis now essentially unused and replaced with/state_ids, butevidently when we replaced it we forgot to add a cache to the new endpoint.]