MSC4354: Sticky Events by kegsay · Pull Request #18968 · element-hq/synapse

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

…tchup_room_event_ids`. (#19320) A couple of tiny tweaks pulled out of #18968.

reivilibre · 2026-02-03T14:50:40Z

This PR is being superseded, the first one replacing it will be #19365 (with more to follow)

Part of: MSC4354 whose experimental feature tracking issue is #19409 Follows: #19340 (a necessary bugfix for `/event/` to set this metadata) Partially supersedes: #18968 This PR implements the first batch of work to support MSC4354 Sticky Events. Sticky events are events that have been configured with a finite 'stickiness' duration, capped to 1 hour per current MSC draft. Whilst an event is sticky, we provide stronger delivery guarantees for the event, both to our clients and to remote homeservers, essentially making it reliable delivery as long as we have a functional connection to the client/server and until the stickiness expires. This PR merely supports creating sticky events and receiving the sticky TTL metadata in clients. It is not suitable for trialling sticky events since none of the other semantics are implemented. Contains a temporary SQLite workaround due to a bug in our supported version enforcement: #19452 --------- Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org> Co-authored-by: Eric Eastwood <erice@element.io>

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

reivilibre added 7 commits December 18, 2025 12:22

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

Merged

reivilibre mentioned this pull request Jan 9, 2026

Support sending and receiving MSC4354 Sticky Event metadata. #19365

Merged

reivilibre added a commit that referenced this pull request Jan 13, 2026

Tweak docstrings and signatures of auth_types_for_event and `get_ca…

8e2e814

…tchup_room_event_ids`. (#19320) A couple of tiny tweaks pulled out of #18968.

reivilibre closed this Feb 3, 2026

reivilibre mentioned this pull request Feb 11, 2026

Introduce spam_checker_spammy internal event metadata. #19453

Open

reivilibre mentioned this pull request Feb 20, 2026

Expose MSC4354 Sticky Events over the legacy (v3) /sync API. #19487

Open

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MSC4354: Sticky Events#18968

MSC4354: Sticky Events#18968
kegsay wants to merge 65 commits intodevelopfrom
kegan/sticky-events

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

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