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Avoid backfill when we already have messages to return (#15737)
We now only block the client to backfill when we see a large gap in the events (more than 2 events missing in a row according to `depth`), more than 3 single-event holes, or not enough messages to fill the response. Otherwise, we return the messages directly to the client and backfill in the background for eventual consistency sake. Fix https://github.com/matrix-org/synapse/issues/15696
This commit is contained in:
Eric Eastwood
GitHub
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changelog.d/15737.feature
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changelog.d/15737.feature
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@ -0,0 +1 @@
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Improve `/messages` response time by avoiding backfill when we already have messages to return.
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@ -40,6 +40,11 @@ if TYPE_CHECKING:
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logger = logging.getLogger(__name__)
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# How many single event gaps we tolerate returning in a `/messages` response before we
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# backfill and try to fill in the history. This is an arbitrarily picked number so feel
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# free to tune it in the future.
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BACKFILL_BECAUSE_TOO_MANY_GAPS_THRESHOLD = 3
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@attr.s(slots=True, auto_attribs=True)
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class PurgeStatus:
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@ -486,35 +491,35 @@ class PaginationHandler:
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room_id, room_token.stream
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)
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if not use_admin_priviledge and membership == Membership.LEAVE:
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# If they have left the room then clamp the token to be before
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# they left the room, to save the effort of loading from the
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# database.
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# If they have left the room then clamp the token to be before
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# they left the room, to save the effort of loading from the
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# database.
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if (
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pagin_config.direction == Direction.BACKWARDS
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and not use_admin_priviledge
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and membership == Membership.LEAVE
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):
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# This is only None if the room is world_readable, in which case
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# "Membership.JOIN" would have been returned and we should never hit
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# this branch.
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assert member_event_id
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# This is only None if the room is world_readable, in which
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# case "JOIN" would have been returned.
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assert member_event_id
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leave_token = await self.store.get_topological_token_for_event(
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member_event_id
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)
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assert leave_token.topological is not None
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if leave_token.topological < curr_topo:
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from_token = from_token.copy_and_replace(
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StreamKeyType.ROOM, leave_token
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)
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await self.hs.get_federation_handler().maybe_backfill(
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room_id,
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curr_topo,
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limit=pagin_config.limit,
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leave_token = await self.store.get_topological_token_for_event(
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member_event_id
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)
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assert leave_token.topological is not None
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if leave_token.topological < curr_topo:
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from_token = from_token.copy_and_replace(
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StreamKeyType.ROOM, leave_token
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)
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to_room_key = None
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if pagin_config.to_token:
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to_room_key = pagin_config.to_token.room_key
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# Initially fetch the events from the database. With any luck, we can return
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# these without blocking on backfill (handled below).
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events, next_key = await self.store.paginate_room_events(
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room_id=room_id,
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from_key=from_token.room_key,
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@ -524,6 +529,94 @@ class PaginationHandler:
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event_filter=event_filter,
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)
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if pagin_config.direction == Direction.BACKWARDS:
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# We use a `Set` because there can be multiple events at a given depth
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# and we only care about looking at the unique continum of depths to
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# find gaps.
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event_depths: Set[int] = {event.depth for event in events}
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sorted_event_depths = sorted(event_depths)
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# Inspect the depths of the returned events to see if there are any gaps
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found_big_gap = False
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number_of_gaps = 0
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previous_event_depth = (
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sorted_event_depths[0] if len(sorted_event_depths) > 0 else 0
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)
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for event_depth in sorted_event_depths:
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# We don't expect a negative depth but we'll just deal with it in
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# any case by taking the absolute value to get the true gap between
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# any two integers.
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depth_gap = abs(event_depth - previous_event_depth)
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# A `depth_gap` of 1 is a normal continuous chain to the next event
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# (1 <-- 2 <-- 3) so anything larger indicates a missing event (it's
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# also possible there is no event at a given depth but we can't ever
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# know that for sure)
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if depth_gap > 1:
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number_of_gaps += 1
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# We only tolerate a small number single-event long gaps in the
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# returned events because those are most likely just events we've
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# failed to pull in the past. Anything longer than that is probably
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# a sign that we're missing a decent chunk of history and we should
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# try to backfill it.
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#
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# XXX: It's possible we could tolerate longer gaps if we checked
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# that a given events `prev_events` is one that has failed pull
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# attempts and we could just treat it like a dead branch of history
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# for now or at least something that we don't need the block the
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# client on to try pulling.
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#
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# XXX: If we had something like MSC3871 to indicate gaps in the
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# timeline to the client, we could also get away with any sized gap
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# and just have the client refetch the holes as they see fit.
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if depth_gap > 2:
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found_big_gap = True
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break
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previous_event_depth = event_depth
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# Backfill in the foreground if we found a big gap, have too many holes,
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# or we don't have enough events to fill the limit that the client asked
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# for.
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missing_too_many_events = (
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number_of_gaps > BACKFILL_BECAUSE_TOO_MANY_GAPS_THRESHOLD
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)
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not_enough_events_to_fill_response = len(events) < pagin_config.limit
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if (
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found_big_gap
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or missing_too_many_events
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or not_enough_events_to_fill_response
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):
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did_backfill = (
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await self.hs.get_federation_handler().maybe_backfill(
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room_id,
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curr_topo,
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limit=pagin_config.limit,
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)
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)
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# If we did backfill something, refetch the events from the database to
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# catch anything new that might have been added since we last fetched.
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if did_backfill:
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events, next_key = await self.store.paginate_room_events(
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room_id=room_id,
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from_key=from_token.room_key,
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to_key=to_room_key,
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direction=pagin_config.direction,
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limit=pagin_config.limit,
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event_filter=event_filter,
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)
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else:
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# Otherwise, we can backfill in the background for eventual
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# consistency's sake but we don't need to block the client waiting
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# for a costly federation call and processing.
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run_as_background_process(
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"maybe_backfill_in_the_background",
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self.hs.get_federation_handler().maybe_backfill,
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room_id,
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curr_topo,
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limit=pagin_config.limit,
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)
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next_token = from_token.copy_and_replace(StreamKeyType.ROOM, next_key)
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# if no events are returned from pagination, that implies
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