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

This obsoletes 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 matrix-doc#971). 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 few server changes, other than better support for peeking (see Dependencies below).

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.

Spaces are referred to primarily by their alias, for example #foo:matrix.org.

Space-rooms are distinguished from regular messaging rooms by the m.room.type of m.space (see MSC1840). This allows clients to offer slightly customised user experience depending on the purpose of the room.

Space-rooms may have m.room.name 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.

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, with the exception that invite_state sent along with invites should be amended to include the event containing the type m.space, to allow clients to discern whether an invite is to a space-room or not.

XXX: Should we also include a MSC2946 summary of the space in the invite too?

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 should contain a via key which gives a list of candidate servers that can be used to join the room. present: true key is included to distinguish from a deleted state event. Something like:
```
{
    "type": "m.space.child",
    "state_key": "!abcd:example.com",
    "content": {
        "via": ["example.com", "test.org"],
        "present": true
    }
}

{
    "type": "m.space.child",
    "state_key": "!efgh:example.com",
    "content": {
        "via": ["example.com"],
        "present": true,
        "order": "abcd",
        "auto_join": true
    }
}

// no longer a child room
{
    "type": "m.space.child",
    "state_key": "!jklm:example.com",
    "content": {}
}
```
Children where present is not present or is not set to true 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 order values; rooms with no order come last. 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 should be ignored if received.)

If auto_join is set to true, that indicates that the child should be automatically joined by members of the space: see below.
Separately, rooms can claim a parent via the m.space.parent state event:
```
{
    "type": "m.space.parent",
    "state_key": "",
    "content": {
        "room_id": "!space:example.com",
        "via": ["example.com"]
    }
}
```
In this case, after a user joins such a room, the client could optionally start peeking into the parent space, enabling it to find other rooms in that space and group them together.

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 their sender has a sufficient power-level to send an m.space.child event in the parent. The rationale for checking the power level, rather than the actual presence of an m.space.child event in the parent, is to accommodate "secret" rooms (see below).

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

Note that each room can only declare a single parent. This could be extended in future to declare additional parents, but more investigation into appropriate semantics is needed.

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.

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 ignore the relationship rather than recursing infinitely.

Auto-joined children

The auto_join flag on a child listing allows a space admin to list the sub-spaces and rooms in that space which should be automatically joined by members of that space.

Joining should be performed by the client. This can optionally be sped up by using MSC2946 to get a summary of the spacetree to be joined, and then using a batch join API (when available) 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 autojoined children shown alongside it as siblings.

It may also be useful to have a way to "suggest" that members of a space should join certain children (but without actually autojoining them) - to advertise particular rooms more prominently than in the room directory. However, this can be added in a later MSC if it's found to be needed in practice.

Long description

We would like to allow spaces to have a long description using rich formatting. This will use a new state event type m.room.description (with empty state_key) whose content is the same format as m.room.message (ie, contains a msgtype and possibly formatted_body).

TODO: this could also be done via pinned messages. Failing that m.room.description should probably be a separate MSC.

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.

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 to 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.

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.)

Dependencies

MSC1840 for room types.
MSC2753 for effective peeking over the C/S API.
MSC2444 (or similar) for effective peeking over Federation.

These dependencies are shared with profiles-as-rooms (MSC1769).

Security considerations

The peek server has significant power. For example, a poorly chosen peek server could lie about the space membership and add an @evil_user:example.org.

Tradeoffs

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).

Unstable prefix

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

Proposed final identifier	Purpose	Development identifier
`m.space`	room type	`org.matrix.msc1772.space`
`m.space.child`	event type	`org.matrix.msc1772.space.child`
`m.space.parent`	event type	`org.matrix.msc1772.room.parent`

History

This replaces MSC1215: https://docs.google.com/document/d/1ZnAuA_zti-K2-RnheXII1F1-oyVziT4tJffdw1-SHrE
Other thoughts that led into this are at: https://docs.google.com/document/d/1hljmD-ytdCRL37t-D_LvGDA3a0_2MwowSPIiZRxcabs

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.

Footnotes

[1]: The converse, "anybody can join, provided they are not members of the '#catlovers' space" is less useful since (a) users in the banned space could simply leave it at any time; (b) this functionality is already somewhat provided by Moderation policy lists. ↩

[2]: Note that there is nothing stopping users sending and receiving invites in public rooms today, and they work as you might expect. The only difference is that you are not required to hold an invite when joining the room. ↩

[3]: This is a marginal decrease in security from the current situation with invite-only rooms. Currently, a misbehaving server can allow unauthorized users to join an invite-only room by first issuing an invite to that user. In theory that can be prevented by raising the PL required to send an invite, but in practice that is rarely done. ↩

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