First iteration of specifying Spaces

MSCs:
* #3288
* #2946
* #1772

Note that this makes modifications to the underlying MSCs as well. These are intended to be minor edits to aid clarity/accuracy of the MSCs, as per the proposal process. Functionally, clients and servers might need to change their behaviour slightly as is expected of implementing this stuff early. Synapse has these changes (alongside backwards compatibility) here: matrix-org/synapse#11667

Author: turt2live

content/client-server-api/_index.md

@@ -1854,6 +1854,27 @@ the topic to be removed from the room.
 
 ## Rooms
 
+### Types
+
+{{% added-in v="1.2" %}}
+
+Optionally, rooms can have types to denote their intended function. A room
+without a type does not necessarily mean it has a specific default function,
+though commonly these rooms will be for conversational purposes.
+
+Room types are best applied when a client might need to differentiate between
+two different rooms, such as conversation-holding and data-holding. If a room
+has a type, it is specified in the `type` key of an [`m.room.create`](#mroomcreate)
+event. To specify a room's type, provide it as part of `creation_content` on
+the create room request.
+
+In this specification the following room types are specified:
+
+* [`m.space`](#spaces)
+
+Unspecified room types are permitted through the use of
+[Namespaced Identifiers](/appendices/#common-namespaced-identifier-grammar).
+
 ### Creation
 
 The homeserver will create an `m.room.create` event when a room is
@@ -2190,6 +2211,7 @@ that profile.
 | [Server ACLs](#server-access-control-lists-acls-for-rooms) | Optional  | Optional | Optional | Optional | Optional |
 | [Server Notices](#server-notices)                          | Optional  | Optional | Optional | Optional | Optional |
 | [Moderation policies](#moderation-policy-lists)            | Optional  | Optional | Optional | Optional | Optional |
+| [Spaces](#spaces)                                          | Optional  | Optional | Optional | Optional | Optional |
 
 *Please see each module for more details on what clients need to
 implement.*

content/client-server-api/modules/spaces.md

@@ -0,0 +1,254 @@
+---
+type: module
+weight: 340
+---
+
+### Spaces
+
+{{% added-in v="1.2" %}}
+
+Often used to group rooms of similar subject matter (such as a public "Official
+matrix.org rooms" space or personal "Work stuff" space), spaces are a way to
+organise rooms while being represented as rooms themselves.
+
+A space is defined by the [`m.space` room type](#types), making it known as a
+"space-room". The space's name, topic, avatar, aliases, etc are all defined through
+the existing relevant state events within the space-room.
+
+Sending normal [`m.room.message`](#mroommessage) events within the space-room is
+discouraged - clients are not generally expected to have a way to render the timeline
+of the room. As such, space-rooms should be created with [`m.room.power_levels`](#mroompower_levels)
+which prohibit normal events by setting `events_default` to a suitably high number.
+In the default power level structure, this would be `100`. Clients might wish to
+go a step further and explicitly ignore notification counts on space-rooms.
+
+Membership of a space is defined and controlled by the existing mechanisms which
+govern a room: [`m.room.member`](#mroommember), [`m.room.history_visibility`](#mroomhistory_visibility),
+and [`m.room.join_rules`](#mroomjoin_rules). Public spaces are encouraged to have
+a similar setup to public rooms: `world_readable` history visibility, published
+canonical alias, and suitibly public `join_rule`. Invites, including third-party
+invites, still work just as they do in normal rooms as well.
+
+All other aspects of regular rooms are additionally carried over, such as the
+ability to set arbitrary state events, hold room account data, etc. Spaces are
+just rooms with extra functionality on top.
+
+#### Managing rooms/spaces included in a space
+
+Spaces form a hierarchy of rooms which clients can use to structure their room
+list into a tree-like view. The parent/child relationship can be defined in two
+ways: with [`m.space.child`](#mspacechild) state events in the space-room, or with
+[`m.space.parent`](#mspaceparent) state events in the child room.
+
+In most cases, both the child and parent relationship should be defined to aid
+discovery of the space and its rooms. When only a `m.space.child` is used, the space
+is effectively a curated list of rooms which the rooms themselves might not be aware
+of. When only a `m.space.parent` is used, the rooms are "secretly" added to spaces
+with the effect of not being advertised directly by the space.
+
+{{% boxes/warning %}}
+Considering spaces are rooms themselves, it is possible to nest spaces within spaces,
+infinitely. Though loops are explicitly disallowed, they are still possible. Loops
+must be broken rather than infinitely looping.
+
+Clients and servers should additionally be aware of excessively long trees which may
+cause performance issues.
+{{% /boxes/warning %}}
+
+##### `m.space.child` relationship
+
+When using this approach, the state events get sent into the space-room which is the
+parent to the room. The `state_key` for the event is the child room's ID.
+
+For example, to achieve the following:
+
+```
+#space:example.org
+    #general:example.org (!abcdefg:example.org)
+    !private:example.org
+```
+
+the state of `#space:example.org` would consist of:
+
+*Unimportant fields trimmed for brevity.*
+
+```json
+{
+    "type": "m.space.child",
+    "state_key": "!abcdefg:example.org",
+    "content": {
+        "via": ["example.org"]
+    }
+}
+```
+```json
+{
+    "type": "m.space.child",
+    "state_key": "!private:example.org",
+    "content": {
+        "via": ["example.org"]
+    }
+}
+```
+
+No state events in the child rooms themselves would be required. This allows for users
+to define personal/private spaces to organise their own rooms without needing explicit
+permission from the room moderators/admins.
+
+Child rooms can be removed from a space by omitting the `via` key of `content` on the
+relevant state event, such as through redaction or otherwise clearing the `content`.
+This is a temporary measure until it is possible to delete state events in Matrix.
+
+{{% event event="m.space.child" %}}
+
+###### Ordering
+
+When the client is displaying the children of a space, the children should be ordered
+using the algorithm below. In some cases, like a traditional left side room list, the
+client may override the ordering to provide better user experience. A theoretical
+space summary view would however show the children ordered.
+
+Taking the set of space children, first order the children with a valid `order` key
+lexicographically by Unicode code-points such that `\x20` (space) is sorted before
+`\x7E` (`~`). Then, take the remaining children and order them by the `origin_server_ts`
+of their `m.space.child` event in ascending numeric order, placing them after the
+children with a valid `order` key in the resulting set.
+
+In cases where the `order` or timestamps are the same, the children are ordered
+lexicographically by their room IDs (state keys) in ascending order.
+
+Noting the careful use of ASCII spaces here, the following demonstrates a set of space
+children being ordered appropriately:
+
+*Unimportant fields trimmed for brevity.*
+
+```json
+[
+    {
+        "type": "m.space.child",
+        "state_key": "!a:example.org",
+        "origin_server_ts": 1640141000000,
+        "content": {
+            "order": "aaaa",
+            "via": ["example.org"]
+        }
+    },
+    {
+        "type": "m.space.child",
+        "state_key": "!b:example.org",
+        "origin_server_ts": 1640341000000,
+        "content": {
+            "order": " ",
+            "via": ["example.org"]
+        }
+    },
+    {
+        "type": "m.space.child",
+        "state_key": "!c:example.org",
+        "origin_server_ts": 1640841000000,
+        "content": {
+            "order": "first",
+            "via": ["example.org"]
+        }
+    },
+    {
+        "type": "m.space.child",
+        "state_key": "!d:example.org",
+        "origin_server_ts": 1640741000000,
+        "content": {
+            "via": ["example.org"]
+        }
+    },
+    {
+        "type": "m.space.child",
+        "state_key": "!e:example.org",
+        "origin_server_ts": 1640641000000,
+        "content": {
+            "via": ["example.org"]
+        }
+    }
+]
+```
+
+The above would result in the following order:
+
+1. `!b:example.org` first because `\x20` is before `aaaa` lexically.
+2. `!a:example.org` next because `aaaa` is before `first` lexically.
+3. `!c:example.org` next because `first` is the last `order` value.
+4. `!e:example.org` next because the event timestamp is smallest.
+5. `!d:example.org` last because the event timestamp is largest.
+
+##### `m.space.parent` relationships
+
+Rooms can additionally claim to be part of a space by populating their own state
+with a parent event. Similar to child events within spaces, the parent event's
+`state_key` is the room ID of the parent space and have a similar `via` list
+within their `content` to denote both whether or not the link is valid and which
+servers might be possible to join through.
+
+To avoid situations where a room falsely claims it is part of a given space,
+`m.space.parent` events should be ignored unless one of the following is true:
+
+* A corresponding `m.space.child` event can be found in the supposed parent space.
+* The sender of the `m.space.parent` event has sufficient power level in the
+  supposed parent space to send `m.space.child` state events (there doesn't need
+  to be a matching child event).
+
+{{% boxes/note %}}
+Clients might need to peek into a parent space to inspect the room state if they
+aren't already joined. If the client is unable to peek the state, the link should
+be assumed to be invalid.
+{{% /boxes/note %}}
+
+{{% boxes/note %}}
+A consequence of the second condition is that a room admin being demoted in the
+parent space, leaving the parent space, or otherwise being removed from the parent
+space can mean that a previously valid `m.space.parent` event becomes invalid.
+{{% /boxes/note %}}
+
+`m.space.parent` events can additionally include a `canonical` boolean key in their
+`content` to denote that the parent space is the main/primary space for the room.
+This can be used to, for example, have the client find other rooms by peeking into
+that space and suggesting them to the user. Only one canonical parent should exist,
+though this is not enforced. To tiebreak, use the lowest room ID sorted lexicographically
+by Unicode code-points.
+
+{{% event event="m.space.parent" %}}
+
+#### Discovering rooms within spaces
+
+Often the client will want to assist the user in exploring what rooms/spaces are part
+of a space. This can be done with crawling [`m.space.child`](#mspacechild) state events
+in the client and peeking into the rooms to get information like the room name, though
+this is impractical for most cases.
+
+Instead, a hierarchy API is provided to walk the space tree and discover the rooms with
+their aesthetic details.
+
+The [`GET /hierarchy`](#get_matrixclientv1roomsroomidhierarchy) API works in a depth-first
+manner: when it encounters another space as a child it recurses into that space before
+returning non-space children.
+
+{{% boxes/warning %}}
+Though prohibited, it is still possible for loops to occur. Servers should gracefully
+break loops.
+
+Additionally, a given child room might appear multiple times in the response as a
+grandchild (for example).
+{{% /boxes/warning %}}
+
+{{% http-api spec="client-server" api="space_hierarchy" %}}
+
+##### Server behaviour
+
+In the case where the server does not have access to the state of a child room, it can
+request the information over federation with the
+[`GET /hierarchy`](/server-server-api/#get_matrixfederationv1hierarchyroomid) API. The
+response to this endpoint should be cached for a period of time. The response might
+additionally contain information about rooms the requesting user is already a member
+of, or that the server is aware of - the local data should be used instead of the remote
+server's data.
+
+Note that the response to this endpoint is contextual based on the user. Servers are
+encouraged to cache the data for a period of time, though permission checks may need to
+be performed to ensure the response is accurate for that user.

content/server-server-api.md

@@ -902,6 +902,13 @@ the server the room directory should be retrieved for.
 
 {{% http-api spec="server-server" api="public_rooms" %}}
 
+## Spaces
+
+To complement the [Client-Server API's Spaces module](/client-server-api/#spaces),
+homeservers need a way to query information about spaces from other servers.
+
+{{% http-api spec="server-server" api="space_hierarchy" %}}
+
 ## Typing Notifications
 
 When a server's users send typing notifications, those notifications

data/api/client-server/definitions/public_rooms_chunk.yaml

@@ -0,0 +1,70 @@
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+type: object
+title: "PublicRoomsChunk"
+properties:
+  aliases:
+    type: array
+    description: Aliases of the room. May be empty.
+    items:
+      type: string
+    example: ["#general:example.org"]
+  canonical_alias:
+    type: string
+    description: The canonical alias of the room, if any.
+    example: "#general:example.org"
+  name:
+    type: string
+    description: The name of the room, if any.
+    example: "General Chat"
+  num_joined_members:
+    type: integer
+    description: The number of members joined to the room.
+    example: 42
+  room_id:
+    type: string
+    description: The ID of the room.
+    example: "!abcdefg:example.org"
+  topic:
+    type: string
+    description: The topic of the room, if any.
+    example: "All things general"
+  world_readable:
+    type: boolean
+    description: Whether the room may be viewed by guest users without joining.
+    example: false
+  guest_can_join:
+    type: boolean
+    description: |-
+      Whether guest users may join the room and participate in it.
+      If they can, they will be subject to ordinary power level
+      rules like any other user.
+    example: true
+  avatar_url:
+    type: string
+    format: uri
+    description: The URL for the room's avatar, if one is set.
+    example: "mxc://example.org/abcdef"
+  join_rule:
+    type: string
+    description: |-
+      The room's join rule. When not present, the room is assumed to
+      be `public`.
+    example: "public"
+required:
+  - room_id
+  - num_joined_members
+  - world_readable
+  - guest_can_join