Adding rooms to a space is rate limited providing a poor user experience

[clokep]

It is possible to add many rooms to a space in one-go, unfortunately Synapse will rate limit this sometimes causing a poor user experience. This is particularly bad when creating "just me" rooms where the user might want to add a high number of rooms at once.

See element-hq/element-web#16891.

[clokep]

To clarify -- this is not about rate limiting joining of rooms, but seems to be about adding (state) events to a room.

[clokep]

I think a proposal of what could be done is to allow a room admin a higher rate limit for adding space events (m.space.child and m.space.parent) to a room than the standard rate limit, which defaults to 10.

Seems like this will touch some of the rate limiting code for messages:

synapse/synapse/handlers/message.py

Lines 1105 to 1126 in bb4b118

	if ratelimit:
	# We check if this is a room admin redacting an event so that we
	# can apply different ratelimiting. We do this by simply checking
	# it's not a self-redaction (to avoid having to look up whether the
	# user is actually admin or not).
	is_admin_redaction = False
	if event.type == EventTypes.Redaction:
	original_event = await self.store.get_event(
	event.redacts,
	redact_behaviour=EventRedactBehaviour.AS_IS,
	get_prev_content=False,
	allow_rejected=False,
	allow_none=True,
	)
	is_admin_redaction = bool(
	original_event and event.sender != original_event.sender
	)
	await self.base_handler.ratelimit(
	requester, is_admin_redaction=is_admin_redaction
	)

[ara4n]

alternatively, should we consider a batch API? trickling events at any rate from the client sounds a bit flimsy.

[clokep]

alternatively, should we consider a batch API? trickling events at any rate from the client sounds a bit flimsy.

That's a good thought. I wonder if we have any bulk endpoints already and if we rate limit per endpoint call or per message that way...

[clokep]

It seems both room create (and upgrade room) kind of do this. Implementation-wise they have a custom ratelimiter instead of the normal event ratelimiter, so this isn't fully without precedence. I guess we would have to consider if it was specific to spaces or a generic "dump a bunch of state into a room" endpoint.

[clokep]

I put together an MSC for batch state updates: matrix-org/matrix-spec-proposals#3192 which I hope would meet the needs of this.