Implement a "room complexity" API to signal to other homeservers how complex a room is

[hawkowl]

_matrix/federation/unstable/rooms/<room_id>/complexity, or so. Returns a "complexity metric", which is right now state_events / 500.

[jcgruenhage]

It has been suggested in #test:zemos.net to instead extend GET /_matrix/federation/v1/query/{queryType} with a new queryType called room_summary. I think this would be more consistent with the rest of the spec.

[Half-Shot]

(this probably needs a companion matrix-doc issue at the very least, and ideally a spec proposal)

EDIT: This is https://github.com/matrix-org/matrix-doc/issues/467

[jcgruenhage]

@Half-Shot I don't think this is exactly the matrix-doc issue you linked, that is more a generic thing.

[Half-Shot]

Hmm probably right.

[hawkowl]

Right now I'm putting this in the unstable namespace, as an experiment to see if it gets the desirable user experience. If it works, I think moving it into that spec'd API, post-r0, is where that'll end up.

[ara4n]

https://docs.google.com/document/d/1kZngFUTXfPEpx6qvxBwQeQEntZZsENdFX69XSVmDZ54/edit is the proto-MSC for this