MSC4354: Sticky Events #18968

kegsay · 2025-09-24T13:30:21Z

This implements MSC4354: Sticky Events. To enable it, set this in homeserver.yaml:

experimental_features:
    msc4354_enabled: true

Complement tests: matrix-org/complement#806

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Pull request is based on the develop branch
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Code style is correct (run the linters)

This only works on postgres for now

Persist inside persist_events to guarantee it is done. After that txn, recheck soft failure.

MadLittleMods

Looked everything over but didn't full evaluate the sticky events part against the MSC or fully ponder weird corners.

synapse/events/builder.py

synapse/federation/federation_base.py

MadLittleMods · 2025-09-26T21:23:54Z

synapse/handlers/sliding_sync/extensions.py

+        if not sticky_events_request.enabled:
+            return None
+        now = self.clock.time_msec()
+        from_id = from_token.stream_token.sticky_events_key if from_token else 0


Should we copy this pattern?

synapse/synapse/handlers/sliding_sync/extensions.py

Lines 928 to 931 in 2f2b854

if from_token:

from_stream_id = from_token.stream_token.thread_subscriptions_key

else:

from_stream_id = StreamToken.START.thread_subscriptions_key

synapse/handlers/sliding_sync/extensions.py

synapse/handlers/delayed_events.py

MadLittleMods · 2025-09-26T21:59:56Z

synapse/storage/databases/main/sticky_events.py

+# Consumers call 'get_sticky_events_in_rooms' which has `WHERE expires_at > ?`
+# to filter out expired sticky events that have yet to be deleted.
+DELETE_EXPIRED_STICKY_EVENTS_MS = 60 * 1000 * 60  # 1 hour
+MAX_STICKY_DURATION_MS = 3600000  # 1 hour


In the client event creation, we should be preventing creating events that have duration greater than MAX_STICKY_DURATION_MS

synapse/storage/databases/main/sticky_events.py

MadLittleMods · 2025-09-26T22:02:59Z

synapse/storage/databases/main/sticky_events.py

+        critical_auth_types = (
+            EventTypes.JoinRules,
+            EventTypes.PowerLevels,
+            EventTypes.Member,
+        )


Wondering how this plays with room versions. Do we need to be careful at all with forward compatibility, etc?

Should we be using auth_types_for_event instead?

I don't think we need to be that careful here. We will need to keep this updated if we make new room versions which break the current auth model though.

MadLittleMods · 2025-09-26T22:06:56Z

synapse/types/__init__.py

        9. `groups_key`: `1` (note that this key is now unused)
        10. `un_partial_stated_rooms_key`: `379`
        11. `thread_subscriptions_key`: 4242
+        12. `sticky_events_key`: 4141


Add it to the full example token above

tests/federation/test_federation_catch_up.py

Currently emits on joins and logs on receive, WIP.

…invalidate

reivilibre

I'm taking over this PR and the MSC.

This is my initial review, cross-corroborating with the MSC.
A good chunk of it is notes for myself.

synapse/events/__init__.py

reivilibre · 2025-12-12T17:39:34Z

synapse/federation/sender/__init__.py

+            await self._transaction_manager.send_new_transaction(
+                new_server, sticky_events, []
+            )


sticky_events is of unbounded length and can't be safely sent in a single federation /send request

reivilibre · 2025-12-12T17:44:08Z

synapse/federation/sender/per_destination_queue.py

+                        if sticky_events:
+                            # *prepend* these to the extrem list, so they are processed first.
+                            # This ensures they will show up before the forward extrem in stream order
+                            extrem_events = sticky_events + extrem_events


again, sticky_events could be of unbounded length and so it's not safe to send them all in a single federation /send request (maybe I'm missing something)

reivilibre · 2025-12-12T17:45:19Z

synapse/federation/sender/per_destination_queue.py

+
                    new_pdus = []
                    for p in extrem_events:
                        # We pulled this from the DB, so it'll be non-null


underneath this section, we filter out events that happened before the remote went offline. I'm not convinced this will reliably send sticky events to remotes that recently joined a room.

reivilibre · 2025-12-12T17:56:00Z

synapse/handlers/federation.py

+                        await self.store.upsert_room_on_join(
+                            room_id=room_id,
+                            room_version=room_version_obj,
+                            state_events=None,
+                        )


feels suspicious because we don't have a join event yet and we also haven't decided whether or not to mark the room as partial-state or not.
Will need to more closely review the impact of this.

reivilibre · 2025-12-15T17:14:18Z

synapse/streams/events.py

            groups_key=0,
            un_partial_stated_rooms_key=un_partial_stated_rooms_key,
            thread_subscriptions_key=thread_subscriptions_key,
+            sticky_events_key=sticky_events_key,


I think a change of plan is transpiring and we won't need to include this in StreamToken anymore

synapse/visibility.py

tests/federation/test_federation_catch_up.py

tests/rest/client/utils.py

kegsay added 14 commits September 18, 2025 14:33

WIP MSC4354; stub out soft-failure code for now

abf658c

Persist sticky events in sticky_events table

8699534

This only works on postgres for now

Use multi-writer streams for sticky events

7801e68

Hook up replication receiver, add sticky events to sync tokens

e01a22b

Insert sticky events into /sync responses

3e7a5a6

Don't include expired sticky events in /sync responses

7af7429

Delete from sticky_events periodically

0cfdd0d

Get sticky events working with Simplified Sliding Sync

7c8daf4

Support MSC4140 Delayed Events with sticky events

ac0f8c2

Fix sqlite

1e812e4

Re-evaluate soft-failure on sticky events

2728b21

Don't re-evaluate spam

666e94b

Rejig when we persist sticky events

771692a

Persist inside persist_events to guarantee it is done. After that txn, recheck soft failure.

Update docs to not lie

ad6a2b9

kegsay requested a review from a team as a code owner September 24, 2025 13:30

kegsay mentioned this pull request Sep 24, 2025

MSC4354: Sticky Events matrix-org/matrix-spec-proposals#4354

Open

4 tasks

kegsay marked this pull request as draft September 24, 2025 13:30

MadLittleMods mentioned this pull request Sep 25, 2025

Improve documentation around streams, particularly ID generators and adding new streams. #18943

Merged

kegsay added 2 commits September 26, 2025 09:29

Send sticky events when catching up over federation

33d80be

Fix trial tests

148caef

MadLittleMods reviewed Sep 26, 2025

View reviewed changes

kegsay added 9 commits September 29, 2025 16:26

Add msc4354_sticky_duration_ttl_ms support

de3e9b4

Merge branch 'develop' into kegan/sticky-events

105d2cd

Use standard unstable identifiers

651e829

Don't persist sticky outliers

4acc98d

SQLite specific soft-failure update code

78c4097

JSON false not str

15453d4

Add msc4354 to /versions response

aa45bf7

Add NewServerJoined replication command

888ab79

Currently emits on joins and logs on receive, WIP.

Use a tri-state for soft failed to communicate when we need to cache …

aac3c84

…invalidate

Docstring tweaks

e9773ff

reivilibre reviewed Dec 15, 2025

View reviewed changes

reivilibre added 12 commits December 18, 2025 11:35

test utils: use f-strings

67a1af4

Use clock as a time source

fd8d0dd

redundant condition

155ea24

Clean up manual UPDATE many

55727c7

Tidy, annotate, comment

df3d2aa

Generate multiple IDs in single call

38f5dad

Select onecol to avoid manual unwrap

c90dd64

Build set directly

8b08b1f

Comment

e1b6eab

Build set directly

0ea82c5

Tidy, inline conditions, reformat

6522c3e

Fix docstring, tidy

6e874a9

reivilibre force-pushed the kegan/sticky-events branch from 1b0bbdd to 58d8395 Compare December 18, 2025 14:13

reivilibre added 5 commits December 18, 2025 15:04

Tidy, don't concat SQL, simplify accumulation

f86d9f8

Remove redundant override

d7092a3

Forced kwargs, describe to unfootgun

375ebb5

Fix MSC divergence: duration should be clamped, not ignored if high

ad3d66f

but -> except for

11472bb

reivilibre force-pushed the kegan/sticky-events branch from 683ed8c to 11472bb Compare December 18, 2025 15:04

reivilibre added 4 commits December 19, 2025 15:06

Use labelled duration_ms in test helper

99cfd75

Docstring on sticky

59b9f97

Docstring on MAX_DURATION_MS

f056f2d

Move is_mine_hs into the database

36ccb90

reivilibre force-pushed the kegan/sticky-events branch from 52a03e6 to 36ccb90 Compare December 19, 2025 16:44

reivilibre added 3 commits December 19, 2025 17:40

Don't duplicate clamping logic

4fb1c5a

Batch generation of stream_ids

683ef01

XXX

bbb06bc

reivilibre mentioned this pull request Dec 22, 2025

Tweak docstrings and signatures of auth_types_for_event and get_catchup_room_event_ids. #19320

Open

	if from_token:
	from_stream_id = from_token.stream_token.thread_subscriptions_key
	else:
	from_stream_id = StreamToken.START.thread_subscriptions_key

MSC4354: Sticky Events #18968

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kegsay commented Sep 24, 2025 •

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