Room List Summary API

[ara4n]

Somehow we've got this far without having an API which gives a minimal summary of all the rooms you are in - i.e. Room Avatar, Room Name, possibly room topic, possibly most recent message, possibly number of users, whether room is encrypted...

One way to do this could be to use /sync with a very very heavy filter (i.e. no room messages) - especially if combined with paginated sync (as per matrix-org/synapse#893).

Alternatively we could have yet another API, but you'd kinda hope that /sync was performant enough to make this work.

Edit: the use case for this would be to rapidly allow clients to display a room list at login without having to do a full initial sync. This could then be used in conjunction with 'paginated sync' to poll subsets of rooms for updates, as per the 'hybrid sync' proposal from https://docs.google.com/document/d/1qlcXWWTccC-vleHajmzMZWrbhtwnWyzhA7f92TkfWYU/edit. On thinking further about it, we probably don't need a new API at all, and we can stitch the different syncs together via their since tokens. However, we would need the paginated /sync API extensions for this to work.

N.B. this is unrelated to lazy loading members

[ara4n]

One /sync filter would look something like:

{
  "room": {
    "state": {
      "types": [
        "m.room.name",
        "m.room.avatar_url"
      ]
    },
    "timeline": {
      "limit": 1
    },
    "ephemeral": {
      "not_types": [
        "*"
      ]
    }
  },
  "event_fields": [
    "type",
    "content"
  ]
}

...except it's then very unclear how we avoid a race between that /sync stream and the main app /sync stream...

[non-Jedi]

Would the race be any less of a problem with an independent api?

In general, if at all possible, I'm highly in favor of using a filter for this rather than a new endpoint.

[richvdh]

Fixed by matrix-org/matrix-spec-proposals#4186, I think