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Avoid backfill when we already have messages to return (#15737)

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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 2023-06-13 12:31:08 -05:00 committed by GitHub
parent df945e0d7c
commit 0757d59ec4
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2 changed files with 116 additions and 22 deletions
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changelog.d/15737.feature Normal file
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@ -0,0 +1 @@
Improve `/messages` response time by avoiding backfill when we already have messages to return.

113
synapse/handlers/pagination.py
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@ -40,6 +40,11 @@ if TYPE_CHECKING:
logger = logging.getLogger(__name__)
# How many single event gaps we tolerate returning in a `/messages` response before we
# backfill and try to fill in the history. This is an arbitrarily picked number so feel
# free to tune it in the future.
BACKFILL_BECAUSE_TOO_MANY_GAPS_THRESHOLD = 3
@attr.s(slots=True, auto_attribs=True)
class PurgeStatus:
@ -486,13 +491,17 @@ class PaginationHandler:
room_id, room_token.stream
)
if not use_admin_priviledge and membership == Membership.LEAVE:
# If they have left the room then clamp the token to be before
# they left the room, to save the effort of loading from the
# database.
# This is only None if the room is world_readable, in which
# case "JOIN" would have been returned.
if (
pagin_config.direction == Direction.BACKWARDS
and not use_admin_priviledge
and membership == Membership.LEAVE
):
# This is only None if the room is world_readable, in which case
# "Membership.JOIN" would have been returned and we should never hit
# this branch.
assert member_event_id
leave_token = await self.store.get_topological_token_for_event(
@ -505,16 +514,12 @@ class PaginationHandler:
StreamKeyType.ROOM, leave_token
)
await self.hs.get_federation_handler().maybe_backfill(
room_id,
curr_topo,
limit=pagin_config.limit,
)
to_room_key = None
if pagin_config.to_token:
to_room_key = pagin_config.to_token.room_key
# Initially fetch the events from the database. With any luck, we can return
# these without blocking on backfill (handled below).
events, next_key = await self.store.paginate_room_events(
room_id=room_id,
from_key=from_token.room_key,
@ -524,6 +529,94 @@ class PaginationHandler:
event_filter=event_filter,
)
if pagin_config.direction == Direction.BACKWARDS:
# We use a `Set` because there can be multiple events at a given depth
# and we only care about looking at the unique continum of depths to
# find gaps.
event_depths: Set[int] = {event.depth for event in events}
sorted_event_depths = sorted(event_depths)
# Inspect the depths of the returned events to see if there are any gaps
found_big_gap = False
number_of_gaps = 0
previous_event_depth = (
sorted_event_depths[0] if len(sorted_event_depths) > 0 else 0
)
for event_depth in sorted_event_depths:
# We don't expect a negative depth but we'll just deal with it in
# any case by taking the absolute value to get the true gap between
# any two integers.
depth_gap = abs(event_depth - previous_event_depth)
# A `depth_gap` of 1 is a normal continuous chain to the next event
# (1 <-- 2 <-- 3) so anything larger indicates a missing event (it's
# also possible there is no event at a given depth but we can't ever
# know that for sure)
if depth_gap > 1:
number_of_gaps += 1
# We only tolerate a small number single-event long gaps in the
# returned events because those are most likely just events we've
# failed to pull in the past. Anything longer than that is probably
# a sign that we're missing a decent chunk of history and we should
# try to backfill it.
#
# XXX: It's possible we could tolerate longer gaps if we checked
# that a given events `prev_events` is one that has failed pull
# attempts and we could just treat it like a dead branch of history
# for now or at least something that we don't need the block the
# client on to try pulling.
#
# XXX: If we had something like MSC3871 to indicate gaps in the
# timeline to the client, we could also get away with any sized gap
# and just have the client refetch the holes as they see fit.
if depth_gap > 2:
found_big_gap = True
break
previous_event_depth = event_depth
# Backfill in the foreground if we found a big gap, have too many holes,
# or we don't have enough events to fill the limit that the client asked
# for.
missing_too_many_events = (
number_of_gaps > BACKFILL_BECAUSE_TOO_MANY_GAPS_THRESHOLD
)
not_enough_events_to_fill_response = len(events) < pagin_config.limit
if (
found_big_gap
or missing_too_many_events
or not_enough_events_to_fill_response
):
did_backfill = (
await self.hs.get_federation_handler().maybe_backfill(
room_id,
curr_topo,
limit=pagin_config.limit,
)
)
# If we did backfill something, refetch the events from the database to
# catch anything new that might have been added since we last fetched.
if did_backfill:
events, next_key = await self.store.paginate_room_events(
room_id=room_id,
from_key=from_token.room_key,
to_key=to_room_key,
direction=pagin_config.direction,
limit=pagin_config.limit,
event_filter=event_filter,
)
else:
# Otherwise, we can backfill in the background for eventual
# consistency's sake but we don't need to block the client waiting
# for a costly federation call and processing.
run_as_background_process(
"maybe_backfill_in_the_background",
self.hs.get_federation_handler().maybe_backfill,
room_id,
curr_topo,
limit=pagin_config.limit,
)
next_token = from_token.copy_and_replace(StreamKeyType.ROOM, next_key)
# if no events are returned from pagination, that implies