MSC3266: Room summary API

proposals/3266-room-summary.md

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+# MSC3266: Room Summary API
+
+Quite a few clients and tools have a need preview a room:
+
+- A client may want to show the room in the roomlist, when showing a space.
+- matrix.to may want to show avatar and name of a room.
+- Nextcloud may want to list the names and avatars of your `/joined_rooms` when
+  asking where to share the media.
+- A client may want to preview a room, when hovering a room alias, id or after
+  clicking on it.
+- A client may want to preview a room, when the user is trying to knock on it or
+  to show pending knocks.
+- A traveller bot may use that to show a room summary on demand without actually
+  keeping the whole room state around and having to subscribe to /sync (or
+  using the appservice API).
+
+There are a few ways to request a room summary, but they only support some of
+the use cases. The spaces summary API
+([MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946)) only provides
+limited control over what rooms to summarize and only works for rooms in spaces.
+`{roomid}/initialSync` and `{roomid}/state/{event_type}` don't work over
+federation and are much heavier than necessary or need a lot of http calls for
+each room.
+
+## Proposal
+
+A new client-server API, which allows you to fetch a summary of a room by id or
+alias and a corresponding server-server API, to fetch a summary over federation.
+
+### Client-Server API
+
+The API returns the summary of the specified room, if the room could be found
+and the client should be able to view its contents according to the join_rules,
+history visibility, space membership and similar rules outlined in
+[MSC3173](https://github.com/matrix-org/matrix-doc/pull/3173) as well as if the
+user is already a member of that room.
+
+A request could look like this:
+
+```
+GET /_matrix/client/r0/rooms/{roomidOrAlias}/summary?
+    via=matrix.org&
+    via=neko.dev
+```
+
+- `roomidOrAlias` can be the roomid or an alias to a room.
+- `via` are servers, that should be tried to request a summary from, if it can't
+  be generated locally. These can be from a matrix URI, matrix.to link or a
+  `m.space.child` event for example.
+
+A response includes the stripped state in the following format:
+
+```json5
+{
+  room_id: "!ol19s:bleecker.street",
+  avatar_url: "mxc://bleecker.street/CHEDDARandBRIE",
+  guest_can_join: false,
+  name: "CHEESE",
+  num_joined_members: 37,
+  topic: "Tasty tasty cheese",
+  world_readable: true,
+  join_rules: "public",
+  room_type: "m.space",
+  is_direct: true,
+  membership: "invite",
+  is_encrypted: true,
+}
+```
+
+These are the same fields as those returned by `/publicRooms`, with a few
+additions: `room_type`, `is_direct`, `membership` and `is_encrypted`.
+
+All those fields are already accessible as the stripped state according to
+[MSC3173](https://github.com/matrix-org/matrix-doc/pull/3173), with the
+exception of `membership` and potentially `is_direct`.
+
+#### Rationale and description of reponse fields
+
+| fieldname          | description                                                                                                                                           | rationale                                                                                                                             |
+| ------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------- |
+| room_id            | Id of the room                                                                                                                                        | Useful, when the API is called with an alias or to disambiguate multiple responses clientside.                                        |
+| avatar_url         | Avatar of the room                                                                                                                                    | Copied from `publicRooms`.                                                                                                            |
+| guest_can_join     | If guests can join the room.                                                                                                                          | Copied from `publicRooms`.                                                                                                            |
+| name               | Name of the room                                                                                                                                      | Copied from `publicRooms`.                                                                                                            |
+| num_joined_members | Member count of the room                                                                                                                              | Copied from `publicRooms`.                                                                                                            |
+| topic              | Topic of the room                                                                                                                                     | Copied from `publicRooms`.                                                                                                            |
+| world_readable     | If the room history can be read without joining.                                                                                                      | Copied from `publicRooms`.                                                                                                            |
+| join_rules         | Join rules of the room                                                                                                                                | Copied from `publicRooms`.                                                                                                            |
+| room_type          | Optional. Type of the room, if any, i.e. `m.space`                                                                                                    | Used to distinguish rooms from spaces.                                                                                                |
+| is_direct          | Optional. If this is a direct chat. The server should use the usual rules to figure out, if this is a direct chat, not just look in the create event. | May be used in previews to distinguish normal rooms from DMs.                                                                         |
+| membership         | The current membership of this user in the room. Usually `leave` if the room is fetched over federation.                                              | Useful to distinguish invites and knocks from joined rooms.                                                                           |
+| is_encrypted       | Optional. If the room is encrypted. This is already accessible as stripped state. Currently a bool, but maybe the algorithm makes more sense?         | Some users may only want to join encrypted rooms or clients may want to filter out encrypted rooms, if they don't support encryption. |
+
+It should be possible to call this API without authentication, but servers may
+rate limit how often they fetch information over federation more heavily, if the
+user is unauthenticated. Also the fields `membership` and `is_direct` will be
+missing.
+
+### Server-Server API
+
+The Server-Server API mirrors the Client-Server API, with a few exceptions. The
+`membership` and `is_direct` fields are never present. No `via` field is
+necessary on the request, since servers should not forward the request to other
+servers.
+
+The server can't know, which user is requesting the summary. As such it should
+apply visibility rules to check if any user on the requesting server would have
+access to the summary.
+
+A request would be made as follows:
+
+```
+GET /_matrix/federation/v1/summary/{roomid}
+```
+
+The requesting server should cache the response to this request.
+
+Note that the federation API only allows roomids and should use the usual
+protocols to resolve the alias first, since it makes no sense to let anything
+but the authoritive server for that alias resolve it.
+
+## Potential issues
+
+### Perfomance
+
+Clients may start calling this API very often instead of using the batched
+summary API (MSC2946) for spaces or caching the state received via `/sync`.
+Looking up all the state events required for this API may cause performance
+issues in that case.
+
+To mitigate that, servers are recommended to cache the response for this API and
+apply rate limiting if necessary.
+
+## Alternatives
+
+- The spaces summary API could be used, but it doesn't work for arbitrary rooms
+    and you always need to pass the parent space, without any control over the
+    rooms being returned.
+- The `/state` API could be used, but the response is much bigger than needed,
+    can't be cached as easily and may need more requests. This also doesn't work
+    over federation (yet).
+- Peeking could solve this too, but with additional overhead and
+    [MSC2753](https://github.com/matrix-org/matrix-doc/pull/2753) is much more
+    complex.
+
+## Security considerations
+
+This API may leak data, if implemented incorrectly or malicious servers could
+return wrong results for a summary.
+
+Those are the same concerns as on [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946)
+or [MSC3173](https://github.com/matrix-org/matrix-doc/pull/3173).
+
+This API could also used for denial of service type attacks. Appropriate
+ratelimiting and caching should be able to mitigate that.
+
+## Unstable prefix
+
+This uses the `im.nheko.summary` unstable prefix instead of the `summary` key in
+the request paths during the MSC phase. As a result:
+
+- the client API will be
+    `/_matrix/client/unstable/rooms/{roomidOrAlias}/im.nheko.summary`
+- the federation API will be
+    `/_matrix/federation/unstable/im.nheko.summary/{roomid}`