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Sliding Sync: Use stream_ordering based timeline pagination for incremental sync #17510

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merged 19 commits into from
Aug 7, 2024
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Sliding Sync: Use stream_ordering based timeline pagination for incremental sync #17510

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75e464b
Working in Sliding sync
MadLittleMods Jul 31, 2024
c5966e5
Refine function
MadLittleMods Jul 31, 2024
3423f83
Adapt the other places
MadLittleMods Jul 31, 2024
f781e5b
Update comment to reflect what we're doing
MadLittleMods Jul 31, 2024
4aee1ab
Use `_assertTimelineEqual(...)` on all of the timeline tests
MadLittleMods Jul 31, 2024
cba4664
Add changelog
MadLittleMods Jul 31, 2024
d6dd34f
Fix lints:
MadLittleMods Jul 31, 2024
3075a15
Revert test changes in favor of separate PR
MadLittleMods Jul 31, 2024
d67c9b5
Fix tests
MadLittleMods Jul 31, 2024
efcc915
Update comment to describe when `stream_ordering` vs `topological_ord…
MadLittleMods Aug 5, 2024
0e12dde
Add `PaginateFunction` type
MadLittleMods Aug 5, 2024
6231fb0
Standardize way we fetch events for sync
MadLittleMods Aug 5, 2024
c5d0998
Rename `paginate_room_events(...)` -> `paginate_room_events_by_topolo…
MadLittleMods Aug 5, 2024
3540ac7
Standardize docstring
MadLittleMods Aug 5, 2024
f27e145
Rename `get_room_events_stream_for_room(...)` -> `paginate_room_event…
MadLittleMods Aug 5, 2024
a67b573
Merge branch 'develop' into madlittlemods/use-stream-ordering-based-t…
MadLittleMods Aug 5, 2024
0874a2e
Add checks to outside
MadLittleMods Aug 6, 2024
c98e8b1
Merge branch 'develop' into madlittlemods/use-stream-ordering-based-t…
MadLittleMods Aug 6, 2024
4888d44
Add future FIXME's to enforce to_key on FORWARDS
MadLittleMods Aug 6, 2024
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1 change: 1 addition & 0 deletions changelog.d/17510.bugfix
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix timeline ordering (using `stream_ordering` instead of topological ordering) in experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint.
2 changes: 1 addition & 1 deletion synapse/handlers/room.py
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Expand Up @@ -1750,7 +1750,7 @@ async def get_new_events(
from_key=from_key,
to_key=to_key,
limit=limit or 10,
order="ASC",
direction=Direction.FORWARDS,
)

events = list(room_events)
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25 changes: 13 additions & 12 deletions synapse/handlers/sliding_sync.py
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Expand Up @@ -1855,18 +1855,19 @@ async def get_room_sync_data(
room_membership_for_user_at_to_token.event_pos.to_room_stream_token()
)

timeline_events, new_room_key = await self.store.paginate_room_events(
room_id=room_id,
# The bounds are reversed so we can paginate backwards
# (from newer to older events) starting at to_bound.
# This ensures we fill the `limit` with the newest events first,
from_key=to_bound,
to_key=from_bound,
direction=Direction.BACKWARDS,
# We add one so we can determine if there are enough events to saturate
# the limit or not (see `limited`)
limit=room_sync_config.timeline_limit + 1,
event_filter=None,
timeline_events, new_room_key = (
await self.store.get_room_events_stream_for_room(
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@MadLittleMods MadLittleMods Jul 31, 2024 •
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As an alternative, we could adapt paginate_room_events(...) to conditionally use topological_ordering vs stream_ordering

room_id=room_id,
# The bounds are reversed so we can paginate backwards
# (from newer to older events) starting at to_bound.
# This ensures we fill the `limit` with the newest events first,
from_key=to_bound,
to_key=from_bound,
direction=Direction.BACKWARDS,
# We add one so we can determine if there are enough events to saturate
# the limit or not (see `limited`)
limit=room_sync_config.timeline_limit + 1,
)
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@MadLittleMods MadLittleMods Jul 31, 2024 •
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There is a possible question of, do we even want to do this?

Using stream_ordering makes sense in terms of /sync returning new items that the server received. it's also what the spec says we should do. We also make sure that if a new event is received, it's returned to the client. Otherwise, it seems like there is an edge case if we use topological_ordering that a new event might not be returned because it's topological position is calculated to be older.

Using topological_ordering makes sense in terms of matching /messages so people don't have to worry about different orders of things between API's.

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Yeah, so the way that sync v2 works in synapse currently is:

  1. The first time a room is sent down /sync we use paginate_room_events to get a chunk of history for the client, this basically results in the same as if no history was sent down /messages was used.
  2. We use get_room_events_stream_for_rooms to get all updates for rooms since the last /sync, which is in stream ordering.

I think this is probably the right way of looking at it, we use topological ordering when we paginate backwards, but stream ordering to fetch new updates to the room (even if those updates happened a while ago in the past).

What we probably want to do for sliding sync is to more or less replicate that behaviour: when we have an incremental fetch all updates (i.e. get_room_events_stream_for_rooms for joined rooms and fetch all membership changes), and then only use paginate_room_events for the new rooms.

(This also has the advantage we'd only need to sort and filter rooms we know have updates)

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Sounds pretty reasonable to me 👍

I've created a spec issue to track this: matrix-org/matrix-spec#1917

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)

# We want to return the events in ascending order (the last event is the
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20 changes: 15 additions & 5 deletions synapse/handlers/sync.py
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Original file line number Diff line number Diff line change
Expand Up @@ -43,6 +43,7 @@

from synapse.api.constants import (
AccountDataTypes,
Direction,
EventContentFields,
EventTypes,
JoinRules,
Expand Down Expand Up @@ -883,15 +884,20 @@ async def _load_filtered_recents(
# that have happened since `since_key` up to `end_key`, so we
# can just use `get_room_events_stream_for_room`.
# Otherwise, we want to return the last N events in the room
# in topological ordering.
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# in `stream_ordering`.
if since_key:
events, end_key = await self.store.get_room_events_stream_for_room(
room_id,
limit=load_limit + 1,
from_key=since_key,
to_key=end_key,
from_key=end_key,
to_key=since_key,
direction=Direction.BACKWARDS,
)
# We want to return the events in ascending order (the last event is the
# most recent).
events.reverse()
else:
# TODO: This should return events in `stream_ordering` order
events, end_key = await self.store.get_recent_events_for_room(
room_id, limit=load_limit + 1, end_token=end_key
)
Expand Down Expand Up @@ -2641,9 +2647,10 @@ async def _get_room_changes_for_incremental_sync(
# a "gap" in the timeline, as described by the spec for /sync.
room_to_events = await self.store.get_room_events_stream_for_rooms(
room_ids=sync_result_builder.joined_room_ids,
from_key=since_token.room_key,
to_key=now_token.room_key,
from_key=now_token.room_key,
to_key=since_token.room_key,
limit=timeline_limit + 1,
direction=Direction.BACKWARDS,
)

# We loop through all room ids, even if there are no new events, in case
Expand All @@ -2654,6 +2661,9 @@ async def _get_room_changes_for_incremental_sync(
newly_joined = room_id in newly_joined_rooms
if room_entry:
events, start_key = room_entry
# We want to return the events in ascending order (the last event is the
# most recent).
events.reverse()

prev_batch_token = now_token.copy_and_replace(
StreamKeyType.ROOM, start_key
Expand Down
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