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Proposal for Matrix "spaces" (formerly known as "groups as rooms (take 2)")

This MSC, and related proposals, supercede MSC1215.

Background and objectives

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".

Synapse and Element-Web currently implement an unspecced "groups" API (referred to as "/r0/groups" in this document) which attempts to provide this functionality (see MSC971). However, this is a complex API which has various problems (see appendix).

This proposal suggests a new approach where spaces are themselves represented by rooms, rather than a custom first-class entity. This requires minimal server changes.

The existing /r0/groups API would be deprecated in Synapse and remain unspecified.

Proposal

Each space is represented by its own room, known as a "space-room". The rooms within the space are determined by state events within the space-room.

Space-rooms are distinguished from regular messaging rooms by the presence of a 'type': 'm.space' property in the content of the m.room.create event. The value of the type property uses the Standardised Identifier Grammar from MSC2758. This allows clients to offer slightly customised user experience depending on the purpose of the room. Currently, no server-side behaviour is expected to depend on this property. A type property on the m.room.create event is used to ensure that a room cannot change between being a space-room and a non-space room. For more information, see the "Rejected Alternatives" section below.

As with regular rooms, public spaces are expected to have an alias, for example #foo:matrix.org, which can be used to refer to the space.

Space-rooms may have m.room.name, m.room.avatar and m.room.topic state events in the same way as a normal room.

Normal messages within a space-room are discouraged (but not blocked by the server): user interfaces are not expected to have a way to enter or display such messages. Space-rooms should be created with a power level for events_default of 100, to prevent the rooms accidentally/maliciously clogging up with messages from random members of the space.

Membership of spaces

Users can be members of spaces (represented by m.room.member state events as normal). The existing m.room.history_visibility mechanism controls whether membership of the space is required to view the room list, membership list, etc. "Public" or "community" spaces would be set to world_readable to allow clients to see the directory of rooms within the space by peeking into the space-room (thus avoiding the need to add m.room.member events to the event graph within the room).

Join rules, invites and 3PID invites work as for a normal room. In order for clients to distinguish space invites from room invites, all invites must now include the m.room.create event in their invite_state and knock_state.

Relationship between rooms and spaces

The intention is that rooms and spaces form a hierarchy, which clients can use to structure the user's room list into a tree view. The parent/child relationship can be expressed in one of two ways:

The admins of a space can advertise rooms and subspaces for their space by setting m.space.child state events. The state_key is the ID of a child room or space, and the content must contain a via key which gives a list of candidate servers that can be used to join the room. Something like:
```
// a child room
{
    "type": "m.space.child",
    "state_key": "!abcd:example.com",
    "content": {
        "via": ["example.com", "test.org"]
    }
}

// a child room with an ordering.
{
    "type": "m.space.child",
    "state_key": "!efgh:example.com",
    "content": {
        "via": ["example.com"],
        "order": "abcd"
    }
}

// no longer a child room
{
    "type": "m.space.child",
    "state_key": "!jklm:example.com",
    "content": {}
}
```
Children where via is not present or invalid (not an array) are ignored.

The order key is a string which is used to provide a default ordering of siblings in the room list. (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_ids in case of equal origin_server_ts. orders 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.)
Separately, rooms can claim parents via the m.space.parent state event.

Similar to m.space.child, the state_key is the ID of the parent space, and the content must contain a via key which gives a list of candidate servers that can be used to join the parent.
```
{
    "type": "m.space.parent",
    "state_key": "!space:example.com",
    "content": {
        "via": ["example.com"],
        "canonical": true
    }
}
```
Parents where via is not present or invalid (not an array) are ignored.

canonical determines whether this is the main parent for the space. When a user joins a room with a canonical parent, clients may switch to view the room in the context of that space, peeking into it in order to find other rooms and group them together. In practice, well behaved rooms should only have one canonical parent, but given this is not enforced: if multiple are present the client should select the one with the lowest room ID, as determined via a lexicographic ordering of the Unicode code-points.

To avoid abuse where a room admin falsely claims that a room is part of a space that it should not be, clients could ignore such m.space.parent events unless either (a) there is a corresponding m.space.child event in the claimed parent, or (b) the sender of the m.space.child event has a sufficient power-level to send such an m.space.child event in the parent. (It is not necessarily required that that user currently be a member of the parent room - only the m.room.power_levels event is inspected.) [Checking the power-level rather than requiring an actual m.space.child event in the parent allows for "secret" rooms (see below).]

Where the parent space also claims a parent, clients can recursively peek into the grandparent space, and so on.

This structure means that rooms can end up appearing multiple times in the room list hierarchy, given they can be children of multiple different spaces (or have multiple parents in different spaces).

In a typical hierarchy, we expect both parent->child and child->parent relationships to exist, so that the space can be discovered from the room, and vice versa. Occasions when the relationship only exists in one direction include:

User-curated lists of rooms: in this case the space will not be listed as a parent of the room.
"Secret" rooms: rooms where the admin does not want the room to be advertised as part of a given space, but does want the room to form part of the hierarchy of that space for those in the know.

Cycles in the parent->child and child->parent relationships are not permitted, but clients (and servers) should be aware that they may be encountered, and MUST spot and break cycles rather than infinitely looping.

Suggested children

Space admins can mark particular children of a space as "suggested". This mainly serves as a hint to clients that that they can be displayed differently (for example by showing them eagerly in the room list), though future server-side interfaces (such as the summary API proposed in MSC2946) might also make use of it.

A suggested child is identified by a "suggested": true property in the m.space.child event:

{
    "type": "m.space.child",
    "state_key": "!abcd:example.com",
    "content": {
        "via": ["example.com", "test.org"],
        "suggested": true
    }
}

A child which is missing the suggested property is treated identically to a child with "suggested": false. A suggested child may be a room or a subspace.

Extended "room invite state"

The specification is currently vague about what room state should be available to users that have been invited to a room, though the Federation API spec does recommend that the invite_room_state sent over federation via PUT /_matrix/federation/v2/invite should include "the join rules, canonical alias, avatar, and name of the room".

This MSC proposes adding m.room.create to that list, so that the recipient of an invite can distinguish invites to spaces from other invites.

Future extensions

The following sections are not blocking parts of this proposal, but are included as a useful reference for how we imagine it will be extended in future.

Auto-joined children

We could add an auto_join flag to m.space.child events to allow a space admin to list the sub-spaces and rooms in that space which should be automatically joined by members of that space.

This would be distinct from a force-join: the user could subsequently part any auto-joined room if they desire.

Joining would be performed by the client. This could possibly be sped up by using a summary API (such as that proposed in MSC2946) to get a summary of the spacetree to be joined, and then using a batch join API to join whichever subset of it makes most sense for the client's UX.

Obviously auto-joining can be a DoS vector, and we consider it to be antisocial for a space to try to autojoin its members to more than 100 children (in total).

Clients could display the auto-joined children in the room list whenever the space appears in the list - thus helping users discover other rooms in a space even if they're not joined to that space. For instance, if you join #matrix:matrix.org, your client could show that room in the context of its parent space, with that space's auto-joined children shown alongside it as siblings.

Restricting access to the spaces membership list

In the existing /r0/groups API, the group server has total control over the visibility of group membership, as seen by a given querying user. In other words, arbitrary users can see entirely different views of a group at the server's discretion.

Whilst this is very powerful for mapping arbitrary organisational structures into Matrix, it may be overengineered. Instead, the common case is (we believe) a space where some users are publicly visible as members, and others are not.

One way of achieving this would be to create a separate space for the private members - e.g. have #foo:matrix.org and #foo-private:matrix.org. #foo-private:matrix.org is set up with m.room.history_visibility to not to allow peeking; you have to be joined to see the members.

It's worth noting that any member of a space can currently see who else is a member of that space, which might pose privacy problems for sensitive spaces. While the server itself will inevitably track the space membership in state events, a future MSC could restrict the membership from being sent to clients. This is essentially the same as matrix-doc#1653.

Flair

("Flair" is a term we use to describe a small badge which appears next to a user's displayname to advertise their membership of a space.)

The flair image for a group is given by the room avatar. (In future it might preferable to use hand-crafted small resolution images: see matrix-doc#1778.

One way this might be implemented is:

User publishes the spaces they wish to announce on their profile (MSC1769 as an m.flair state event: it lists the spaces which they are advertising.
When a client wants to know the current flair for a set of users (i.e. those which it is currently displaying in the timeline), it peeks the profile rooms of those users. (Ideally there would be an API to support peeking multiple rooms at once to facilitate this.)
The client must check that the user is actually a member of the advertised spaces. Nominally it can do this by peeking the membership list of the space; however for efficiency we could expose a dedicated Client-Server API to do this check (and both servers and clients can cache the results fairly aggressively.)

MSC2946: Spaces Summary API.
MSC2962: Managing power levels via Spaces.
MSC3083: Restricting room membership based on space membership.
MSC2753 for effective peeking over the C/S API.
MSC2444 (or similar) for effective peeking over Federation.

Security considerations

None at present.

Potential issues

If the membership of a space would be large (for example: an organisation of several thousand people), this membership has to be copied entirely into the room, rather than querying/searching incrementally.
If the membership list is based on an external service such as LDAP, it is hard to keep the space membership in sync with the LDAP directory. In practice, it might be possible to do so via a nightly "synchronisation" job which searches the LDAP directory, or via "AD auditing".
No allowance is made for exposing different 'views' of the membership list to different querying users. (It may be possible to simulate this behaviour using smaller spaces).
The requirement that m.space.parent links be ignored unless the sender has a high PL in the parent room could lead to suprising effects where a parent link suddenly ceases to take effect because a user loses their PL in the parent room. This is mitigated in the general case by honouring the parent link when there is a corresponding m.space.child event, however it remains a problem for "secret" rooms.
The via servers listed in the m.space.child and m.space.parent events could get out of date, and will need to be updated from time to time. This remains an unsolved problem.

Rejected alternatives

Use a separate state event for type of room

MSC1840 proposes the use of a separate m.room.type state event to distinguish different room types. This implies that rooms can dynamically switch between being a Space, and being a regular non-Space room. That is not a usecase we consider useful, and allowing it would impose significant complexity on client and server implementations. Specifically, client and server implementations who store spaces separately from rooms would have to support migrating back and forth between them and dealing with the ambiguities of room_ids no longer pointing to valid spaces, etc.

Use a different sigil/twigil for spaces

Groups used + as a sigil to differentiate them from rooms (e.g. +matrix:matrix.org). We considered doing similar for Spaces, e.g. a #+ twigil or reuse the + sigil, but concluded that the resulting complexity and exoticism is not worth it. This means that clients such as matrix.to have to peek into rooms to find out their type before being able to display an appropriate UI, and users will not know whether #matrix:matrix.org is a room or a space without using a client (e.g. if reading an advert). It also means that if the client UI requires a space alias the client will need to validate the entered data serverside.

Unstable prefix

The following mapping will be used for identifiers in this MSC during development:

Proposed final identifier	Purpose	Development identifier
`type`	property in `m.room.create`	`org.matrix.msc1772.type`
`m.space`	value of `type` in `m.room.create`	`org.matrix.msc1772.space`
`m.space.child`	event type	`org.matrix.msc1772.space.child`
`m.space.parent`	event type	`org.matrix.msc1772.space.parent`

History

This replaces MSC1215.
Other thoughts that led into this are documented.

Appendix: problems with the `/r0/groups` API

The existing /r0/groups API, as proposed in MSC971, has various problems, including:

It is a large API surface to implement, maintain and spec - particularly for all the different clients out there.
Much of the API overlaps significantly with mechanisms we already have for managing rooms:
- Tracking membership identity
- Tracking membership hierarchy
- Inviting/kicking/banning user
- Tracking key/value metadata
There are membership management features which could benefit rooms which would also benefit groups and vice versa (e.g. "auditorium mode")
The current implementations on Riot Web/iOS/Android all suffer bugs and issues which have been solved previously for rooms.
- no local-echo of invites
- failures to set group avatars
- ability to specify multiple admins
It doesn't support pushing updates to clients (particularly for flair membership): https://github.com/vector-im/riot-web/issues/5235
It doesn't support third-party invites.
Groups could benefit from other features which already exist today for rooms
- e.g. Room Directories
Groups are centralised, rather than being replicated across all participating servers.

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