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 returns a lot more data than
+necessary.  `{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/summary/{roomIdOrAlias}?
+    via=matrix.org&
+    via=neko.dev
+```
+
+(This is not under `/rooms`, because it can be used with an alias.
+
+- `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",
+  membership: "invite",
+  is_encrypted: true,
+}
+```
+
+These are the same fields as those returned by `/publicRooms`, with a few
+additions: `room_type`, `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`. These are the same fields as in
+[MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946) apart from the
+adition of the membership field. The membership can already be accessed by a
+client anyway, this API just provides it as a convenience.
+
+
+#### 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.                                                                                                |
+| 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` will be
+missing.
+
+### Server-Server API
+
+For the federation API
+[MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946) is reused. This
+provides all the information needed in this MSC, but it also provides a
+few additional fields and one level of children of this room.
+
+In theory one could also add the `max_depth` parameter with allowed values of 0
+and 1, so that child rooms are excluded, but this performance optimization does
+not seem necessary at this time and could be added at any later point while
+degrading gracefully.
+
+(Originally there was a separate federation API for this, but it was decided
+that lowering the duplication on the federation side is the way to go by the
+author.)
+
+## 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 returns more data than necessary
+    by default (but it can be limited to just 1 room) such as all the
+    `m.space.child` events in a space. (We do reuse the federation API now.)
+- For joined rooms, the `/sync` API can be used to get a summary for all joined
+    rooms. Apart from not working for unjoined rooms, like knocks, invites and
+    space children, `/sync` is very heavy for the server and the client needs to
+    cobble together information from the `state`, `timeline` and
+    [`summary`](https://github.com/matrix-org/matrix-doc/issues/688) sections to
+    calculate the room name, topic and other fields provided in this MSC.
+    Furthermore, the membership counts in the summary field are only included, if
+    the client is using lazy loading.
+    This MSC provides similar information as calling `/sync`, but it uses the
+    stripped state, which is needed to allow this to work for unjoined rooms and
+    it excludes `m.heroes` as well as membership events, since those are not
+    included in the stripped state of a room. (A client can call
+    `/joined_members` to receive those if needed. It may still make sense to
+    include heroes, but solving the security implications with that may better
+    be left to a separate MSC.)
+- 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). The variant of this API, which returns the full state
+    of a room, also does not return stripped events, which prevents it from
+    being used by non-members. The event for specific events DOES return
+    stripped events, but could not provide a member count for a room.
+- Peeking could solve this too, but with additional overhead and
+    [MSC2753](https://github.com/matrix-org/matrix-doc/pull/2753) is much more
+    complex.
+- This API could take a list of rooms with included `via`s for each room instead
+    of a single room (as a POST request). This may have performance benefits for
+    the federation API and a client could then easily request a summary of all
+    joined rooms. It could still request the summary of a single room by just
+    including only a single room in the POST or a convenience GET could be
+    provided by the server (that looks like this proposal).
+- [MSC3429](https://github.com/matrix-org/matrix-doc/pull/3429) is an
+    alternative implementation, but it chooses a different layout. While this
+    layout might make sense in the future, it is inconsistent with the APIs
+    already in use, harder to use for clients (iterate array over directly
+    including the interesting fields) and can't reuse the federation API. In my
+    opinion an MSC in the future, that bases all summary APIs on a list of
+    stripped events seems like the more reasonable approach to me and would make
+    the APIs more extensible.
+
+## 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 be 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. As such the paths are prefixed
+with `unstable/im.nheko.summary`.
+
+- the client API will be
+    `/_matrix/client/unstable/im.nheko.summary/summary/{roomIdOrAlias}`.
+
+Some implementations still use
+    `/_matrix/client/unstable/im.nheko.summary/rooms/{roomIdOrAlias}/summary`,
+    but this was a mistake in this MSC. Endpoints using aliases shouldn't be under /rooms.