Synapse modules lack a way to know whether a given event has been sent to a remote homeserver

[anoadragon453]

Element has a customer who has the following use-case: they'd like to be able to prove whether a given event was transmitted to a given remote homeserver. Whether it was pushed via /send or pulled by the remote using either /backfill, /get_missing_events or /event.

They would like to have a module which can listen for moment and notify an external service that the transmission has occurred. The relevant metadata is:

• Event ID
• Room ID
• potentially sender
• Destination homeserver

I'd personally add "method of delivery" to that list.

To facilitate this, I envision a new Synapse module API named on_event_delivered_over_federation (or perhaps on_event_federated?) with the following signature:

async def on_event_delivered_over_federation(event: EventBase, destination_server_name: str, method: FederatedEventDeliveryMethod /* enum */) -> None

The module could retrieve the room_id and sender from the event if desired.

---

Ideally this would work across workers, but technically it doesn't need to. One could install this module on all workers, and then despite the federation sender that sent out the event, or the federation reader which received a relevant request, an attached module could report it. The reports could then be de-duplicated on the external service side.

[anoadragon453]

Ideally state events could also be tracked by this mechanism, which of course can be delivered in additional ways to a remote homeserver:

• In the response to /send_join
• Stripped state sent as part of an invite, knock, etc.
• If a remote server asks for /hierarchy over federation
• GET /_matrix/federation/v1/state/{roomId}

I also missed GET /_matrix/federation/v1/timestamp_to_event/{roomId} in the original report. Lots to instrument!