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https://github.com/element-hq/synapse
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Optimize backfill receiving to have less missing prev_event thrashing
Pulled from scratch changes in, https://github.com/matrix-org/synapse/pull/13864
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@ -75,6 +75,7 @@ from synapse.state import StateResolutionStore
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from synapse.storage.databases.main.events import PartialStateConflictError
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from synapse.storage.databases.main.events_worker import EventRedactBehaviour
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from synapse.storage.state import StateFilter
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from synapse.storage.util.id_generators import AbstractStreamIdGenerator
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from synapse.types import (
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PersistedEventPosition,
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RoomStreamToken,
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@ -644,9 +645,71 @@ class FederationEventHandler:
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f"room {ev.room_id}, when we were backfilling in {room_id}"
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)
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# We expect the events from the `/backfill` response to start from
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# `?v` and include events that preceded it (so the list will be
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# newest -> oldest, reverse-chronological). It's described in the
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# spec this way so we can rely on people doing it the right way for
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# the historical messages to show up correctly.
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reverse_chronological_events = events
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# `[::-1]` is just syntax to reverse the list and give us a copy
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chronological_events = reverse_chronological_events[::-1]
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# We want to calculate the `stream_ordering` from newest -> oldest
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# (reverse-chronological) (so MSC2716 historical events end up
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# sorting in the correct order) and persist oldest -> newest
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# (chronological) to get the least missing `prev_event` fetch
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# thrashing.
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# ------------------------------------------------------------------
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# Since we have been configured to write, we ought to have id generators,
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# rather than id trackers.
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assert (
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self._instance_name in self._config.worker.writers.events
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), "Can only write stream IDs on master"
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assert isinstance(self._store._backfill_id_gen, AbstractStreamIdGenerator)
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stream_ordering_manager = self._store._backfill_id_gen.get_next_mult(
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len(reverse_chronological_events)
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)
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async with stream_ordering_manager as stream_orderings:
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# Calculate the `stream_ordering` from newest -> oldest
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# (reverse-chronological) (so historical events end up sorting
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# in the correct order).
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#
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# Backfilled events start with `stream_ordering=-1` and
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# decrement. For events, that we backfill at the same `depth`
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# (like chains of historical messages) in order for them to have
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# the best chance of ending up in the correct order, assign
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# `stream_ordering` to the assumed reverse-chronological list of
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# events to backfill (where the newest events get
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# stream_ordering assigned first)
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#
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# depth : stream_ordering : event
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# ----- : --------------- : -----------------------
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# 1 : 1 : Event before 1
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# 2 : 2 : Event before 2
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# 3 : -4 : Historical message 1
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# 3 : -4 : Historical message 2
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# 3 : -3 : Historical message 3
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# 3 : -2 : Historical message 4
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# 3 : -1 : Historical message 5
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# 3 : 3 : Event after 1
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# 4 : 4 : Event after 2
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#
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for event, stream in zip(
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reverse_chronological_events, stream_orderings
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):
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event.internal_metadata.stream_ordering = stream
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await self._process_pulled_events(
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dest,
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events,
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# Persist events from oldest -> newest (chronological) to get
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# the least missing `prev_event` fetch thrashing.
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# `_process_pulled_events` does some sorting of its own by
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# `depth` but if we let it sort the reverse-chronological list
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# of events, it naively orders events with the same depth in the
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# opposite order we want. If we pass it an already sorted by
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# depth list, then everything lines up.
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chronological_events,
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backfilled=True,
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)
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@ -209,7 +209,11 @@ class PersistEventsStore:
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async with stream_ordering_manager as stream_orderings:
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for (event, _), stream in zip(events_and_contexts, stream_orderings):
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event.internal_metadata.stream_ordering = stream
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# If someone has already decided the stream_ordering for the
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# event before, then just use that. This is done during backfill
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# to help ordering of MSC2716 historical messages.
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if event.internal_metadata.stream_ordering is None:
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event.internal_metadata.stream_ordering = stream
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await self.db_pool.runInteraction(
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"persist_events",
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