MSC2946: Spaces Summary

proposals/2946-spaces-summary.md

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+# MSC2946: Spaces Summary
+
+This MSC depends on [MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772), which
+describes why a Space is useful:
+
+> Collecting rooms together into groups is useful for a number of purposes. Examples include:
+>
+> * Allowing users to discover different rooms related to a particular topic: for example "official matrix.org rooms".
+> * Allowing administrators to manage permissions across a number of rooms: for example "a new employee has joined my company and needs access to all of our rooms".
+> * Letting users classify their rooms: for example, separating "work" from "personal" rooms.
+>
+> We refer to such collections of rooms as "spaces".
+
+This MSC attempts to solve how a member of a space discovers rooms in that space. This
+is useful for quickly exposing a user to many aspects of an entire community, using the
+examples above, joining the "official matrix.org rooms" space might suggest joining a few
+rooms:
+
+* A room to discuss development of the Matrix Spec.
+* An announcements room for news related to matrix.org.
+* An off-topic room for members of the space.
+
+## Proposal
+
+A new client-server API (and corresponding server-server API) is added which allows
+for querying for the rooms and spaces contained within a space. This allows a client
+to efficiently display a hierarchy of rooms to a user (i.e. without having
+to walk the full state of each room).
+
+### Client-server API
+
+An endpoint is provided to walk the space tree, starting at the provided room ID
+("the root room"), and visiting other rooms/spaces found via `m.space.child`
+events. It recurses into the children and into their children, etc.
+
+Any child room that the user is joined or is potentially joinable (per
+[MSC3173](https://github.com/matrix-org/matrix-doc/pull/3173)) is included in
+the response. When a room with a `type` of `m.space` is found, it is searched
+for valid `m.space.child` events to recurse into.
+
+In order to provide a consistent experience, the space tree should be walked in
+a depth-first manner, e.g. whenever a space is found it should be recursed into
+by sorting the children rooms and iterating through them.
+
+There could be loops in the returned child events; clients and servers should
+handle this gracefully. Similarly, note that a child room might appear multiple
+times (e.g. also be a grandchild). Clients and servers should handle this
+appropriately.
+
+This endpoint requires authentication and is not subject to rate-limiting.
+
+#### Request format
+
+```text
+GET /_matrix/client/r0/rooms/{roomID}/hierarchy
+```
+
+Query Parameters:
+
+* **`suggested_only`**: Optional. If `true`, return only child events and rooms
+  where the `m.space.child` event has `suggested: true`.  Must be a  boolean,
+  defaults to `false`.
+
+  This applies transitively, i.e. if a `suggested_only` is `true` and a space is
+  not suggested then it should not be searched for children. The inverse is also
+  true, if a space is suggested, but a child of that space is not then the child
+  should not be included.
+* **`limit`**: Optional: a client-defined limit to the maximum
+  number of rooms to return per page. Must be a non-negative integer.
+
+  Server implementations should impose a maximum value to avoid resource
+  exhaustion.
+* **`max_depth`**: Optional: The maximum depth in the tree (from the root room)
+  to return. The deepest depth returned will not include children events. Defaults
+  to no-limit. Must be a non-negative integer.
+
+  Server implementations may wish to impose a maximum value to avoid resource
+  exhaustion.
+* **`from`**: Optional. Pagination token given to retrieve the next set of rooms.
+
+  Note that if a pagination token is provided, then the parameters given for
+  `suggested_only` and `max_depth` must be the same.
+
+#### Response Format
+
+* **`rooms`**: `[object]` For each room/space, starting with the root room, a
+  summary of that room. The fields are the same as those returned by
+  `/publicRooms` (see
+  [spec](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-publicrooms)),
+  with the addition of:
+  * **`room_type`**: the value of the `m.type` field from the room's
+    `m.room.create` event, if any.
+  * **`children_state`**: The stripped state of the `m.space.child` events of
+    the room per [MSC3173](https://github.com/matrix-org/matrix-doc/pull/3173).
+    In addition to the standard stripped state fields, the following is included:
+    * **`origin_server_ts`**: This is required for sorting of rooms as specified
+      below.
+* **`next_batch`**: Optional `string`. The token to supply in the `from` param
+  of the next `/hierarchy` request in order to request more rooms. If this is absent,
+  there are no more results.
+
+#### Example request:
+
+```text
+GET /_matrix/client/r0/rooms/%21ol19s%3Ableecker.street/hierarchy?
+    limit=30&
+    suggested_only=true&
+    max_depth=4
+```
+
+#### Example response:
+
+```jsonc
+{
+    "rooms": [
+        {
+            "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",
+            "children_state": [
+                {
+                    "type": "m.space.child",
+                    "state_key": "!efgh:example.com",
+                    "content": {
+                        "via": ["example.com"],
+                        "suggested": true
+                    },
+                    "room_id": "!ol19s:bleecker.street",
+                    "sender": "@alice:bleecker.street",
+                    "origin_server_ts": 1432735824653
+                },
+                { ... }
+            ]
+        },
+        { ... }
+    ],
+    "next_batch": "abcdef"
+}
+```
+
+#### Errors:
+
+An HTTP response with a status code of 403 and an error code of `M_FORBIDDEN`
+should be returned if the user doesn't have permission to view/peek the root room.
+This should also be returned if that room does not exist, which matches the
+behavior of other room endpoints (e.g.
+[`/_matrix/client/r0/rooms/{roomID}/aliases`](https://matrix.org/docs/spec/client_server/latest#get-matrix-client-r0-rooms-roomid-aliases))
+to not divulge that a room exists which the user doesn't have permission to view.
+
+An HTTP response with a status code of 400 and an error code of `M_INVALID_PARAM`
+should be returned if the `from` token provided is unknown to the server or if
+the `suggested_only` or `max_depth` parameters are modified during pagination.
+
+#### Server behaviour
+
+The server should generate the response as discussed above, by doing a depth-first
+search (starting at the "root" room) for any `m.space.child` events. Any
+`m.space.child` with an invalid `via` are discarded (invalid is defined as in
+[MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772): missing, not an
+array or an empty array).
+
+In the case of the homeserver not having access to the state of a room, the
+server-server API (see below) can be used to query for this information over
+federation from one of the servers provided in the `via` key of the
+`m.space.child` event. It is recommended to cache the federation response for a
+period of time. The federation results may contain information on a room
+that the requesting server is already participating in; the requesting server
+should use its local data for such rooms rather than the data returned over
+federation.
+
+When the current response page is full, the current state should be persisted
+and a pagination token should be generated (if there is more data to return).
+To prevent resource exhaustion, the server may expire persisted data that it
+deems to be stale.
+
+The persisted state will include:
+
+* The processed rooms.
+* Rooms to process (in depth-first order with rooms at the same depth
+  ordered [according to MSC1772, as updated to below](#msc1772-ordering)).
+* Room information from federation responses for rooms which have yet to be
+  processed.
+
+### Server-server API
+
+The Server-Server API has a similar interface to the Client-Server API, but a
+simplified response. It is used when a homeserver is not participating in a room
+(and cannot summarize room due to not having the state).
+
+The main difference is that it does *not* recurse into spaces and does not support
+pagination. This is somewhat equivalent to a Client-Server request with a `max_depth=1`.
+
+Additional federation requests are made to recurse into sub-spaces. This allows
+for trivially caching responses for a short period of time (since it is not
+easily known the room summary might have changed).
+
+Since the server-server API does not know the requesting user, the response should
+divulge information based on if any member of the requesting server could join
+the room. The requesting server is trusted to properly filter this information
+using the `world_readable`, `join_rules`, and `allowed_room_ids` fields from the
+response.
+
+If the target server is not a member of some children rooms (so would have to send
+another request over federation to inspect them), no attempt is made to recurse
+into them. They are simply omitted from the `children` key of the response.
+(Although they will still appear in the `children_state`key of the `room`.)
+
+Similarly, if a server-set limit on the size of the response is reached, additional
+rooms are not added to the response and can be queried individually.
+
+#### Request format
+
+```text
+GET /_matrix/federation/v1/hierarchy/{roomID}
+```
+
+Query Parameters:
+
+* **`suggested_only`**: The same as the Client-Server API.
+
+#### Response format
+
+The response format is similar to the Client-Server API:
+
+* **`room`**: `[object]` The summary of the requested room, as given in the
+  Client-Server API response.
+* **`children`**: `[object]` For each room/space, a summary of that room. The fields
+  are the same as those returned by `/publicRooms` (see
+  [spec](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-publicrooms)),
+  with the addition of:
+  * **`room_type`**: the value of the `m.type` field from the room's
+    `m.room.create` event, if any.
+  * **`allowed_room_ids`**: A list of room IDs which give access to this room per
+    [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083).<sup id="a1">[1](#f1)</sup>
+* **`inaccessible_children`**: Optional `[string]`. A list of room IDs which are
+  children of the requested room, but are inaccessible to the requesting server.
+  Other servers are unlikely to have information about them as well, thus the
+  requesting server can consider the rooms inaccessible from everywhere.
+
+  This is used to differentiate between rooms which the requesting server does
+  not have access to from those that the target server cannot include in the
+  response (which will simply be missing in the response).
+
+#### Example request:
+
+```jsonc
+GET /_matrix/federation/v1/hierarchy/{roomID}?
+    suggested_only=true
+```
+
+#### Errors:
+
+An HTTP response with a status code of 404 and an error code of `M_NOT_FOUND` is
+returned if the target server is not a member of the requested room or the
+requesting server is not allowed to access the room.
+
+### MSC1772 Ordering
+
+[MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772) defines the ordering
+of "default ordering of siblings in the room list" using the `order` key:
+
+> Rooms are sorted based on a lexicographic ordering of the Unicode codepoints
+> of the characters in `order` values. Rooms with no `order` come last, in
+> ascending numeric order of the `origin_server_ts` of their `m.room.create`
+> events, or ascending lexicographic order of their `room_id`s in case of equal
+> `origin_server_ts`. `order`s which are not strings, or do not consist solely
+> of ascii characters in the range `\x20` (space) to `\x7F` (~), or consist of
+> more than 50 characters, are forbidden and the field should be ignored if
+> received.
+
+Unfortunately there are situations when a homeserver comes across a reference to
+a child room that is unknown to it and must decide the ordering. Without being
+able to see the `m.room.create` event (which it might not have permission to see)
+no proper ordering can be given.
+
+Consider the following case of a space with 3 child rooms:
+
+```
+         Space A
+           |
+  +--------+--------+
+  |        |        |
+Room B   Room C   Room D
+```
+
+HS1 has users in Space A, Room B, and Room C, while HS2 has users in Room D. HS1 has no users
+in Room D (and thus has no state from it). Room B, C, and D do not have an
+`order` field set (and default to using the ordering rules above).
+
+When a user asks HS1 for the space summary with a `limit` equal to `2` it cannot
+fulfill this request since it is unsure how to order Room B, Room C, and Room D,
+but it can only return 2 of them. It *can* reach out over federation to HS2 and
+request a space summary for Room D, but this is undesirable:
+
+* HS1 might not have the permissions to know any  of the state of Room D, so might
+  receive a 404 error.
+* If we expand the example above to many rooms than this becomes expensive to
+  query a remote server simply for ordering.
+
+This proposes changing the ordering rules from MSC1772 to the following:
+
+> Rooms are sorted based on a lexicographic ordering of the Unicode codepoints
+> of the characters in `order` values. Rooms with no `order` come last, in
+> ascending numeric order of the `origin_server_ts` of their `m.space.child`
+> events, or ascending lexicographic order of their `room_id`s in case of equal
+> `origin_server_ts`. `order`s which are not strings, or do not consist solely
+> of ascii characters in the range `\x20` (space) to `\x7E` (~), or consist of
+> more than 50 characters, are forbidden and the field should be ignored if
+> received.
+
+This modifies the clause for calculating the order to use the `origin_server_ts`
+of the `m.space.child` event instead of the `m.room.create` event. This allows
+for a defined sorting of siblings based purely on the information available in
+the state of the space while still allowing for a natural ordering due to the
+age of the relationship.
+
+## Potential issues
+
+A large flat space (a single room with many `m.space.child` events) could cause
+a large federation response.
+
+## Alternatives
+
+Peeking to explore the room state could be used to build the tree of rooms/spaces,
+but this would be significantly more expensive for both clients and servers. It
+would also require peeking over federation (which is explored in
+[MSC2444](https://github.com/matrix-org/matrix-doc/pull/2444)).
+
+## Security considerations
+
+A space with many sub-spaces and rooms on different homeservers could cause
+a large number of federation requests. A carefully crafted space with inadequate
+server enforced limits could be used in a denial of service attack. Generally
+this is mitigated by enforcing server limits and caching of responses.
+
+The requesting server over federation is trusted to filter the response for the
+requesting user. The alternative, where the requesting server sends the requesting
+`user_id`, and the target server does the filtering, is unattractive because it
+rules out a caching of the result. This does not decrease security since a server
+could lie and make a request on behalf of a user in the proper space to see the
+given information. I.e. the calling server must be trusted anyway.
+
+## Unstable prefix
+
+During development of this feature it will be available at unstable endpoints.
+
+The client-server API will be:
+`/_matrix/client/unstable/org.matrix.msc2946/rooms/{roomID}/hierarchy`
+
+The server-server API will be:
+`/_matrix/federation/unstable/org.matrix.msc2946/hierarchy/{roomID}`
+
+## Footnotes
+
+<a id="f1"/>[1]: As a worked example, in the context of
+[MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083), consider that Alice
+and Bob share a server; Alice is a member of a space, but Bob is not. A remote
+server will not know whether the request is on behalf of Alice or Bob (and hence
+whether it should share details of restricted rooms within that space).
+
+Consider if the space is modified to include a restricted room on a different server
+which allows access from the space. When summarizing the space, the homeserver must make
+a request over federation for information on the room. The response should include
+the room (since Alice is able to join it). Without additional information the
+calling server does not know *why* they received the room and cannot properly
+filter the returned results.
+
+Note that there are still potential situations where each server individually
+doesn't have enough information to properly return the full summary, but these
+do not seem reasonable in what is considered a normal structure of spaces. (E.g.
+in the above example, if the remote server is not in the space and does not know
+whether the server is in the space or not it cannot return the room.)[↩](#a1)