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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). XXX nobody has convinced me this is actually required.

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.

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 ontain 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",
        "default": 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 (~) are forbidden and should be ignored if received.)

If default is set to true, that indicates a "default child": see below.
Separately, rooms can claim parents via m.room.parent state events, where the state_key is the room ID of the parent space:
```
{
    "type": "m.room.parent",
    "state_key": "!space:example.com",
    "content": {
        "via": ["example.com"]
        "present": true
    }
}
```
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.room.parent events unless their sender has a sufficient power-level to send an m.room.child event in the parent.

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 with multiple parents. This implies that the room will appear multiple times in the room list hierarchy.

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.

Default children

The default flag on a child listing allows a space admin to list the "default" sub-spaces and rooms in that space. This means that when a user joins the parent space, they will automatically be joined to those default children.

XXX implement this on the client or server?

Clients could display the default children in the room list whenever the space appears in the list.

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.

Managing power levels via spaces

XXX: this section still in progress

One use-case for spaces is to help manage power levels across a group of rooms. For example: "Jim has just joined the management team at my company. He should have moderator rights across all of the company rooms."

Since the event-authorisation rules cannot easily be extended to consider membership in other rooms, we must map any changes in space membership onto real m.room.power_levels events.

Extending the power_levels event

We now have a mix of manually- and automatically- maintained power-level data. To support this, we extend the existing m.room.power_levels event to add an auto_users key:

{
    "type": "m.room.power_levels",
    "content": {
        "users": {
            "@roomadmin:example.com": 100
        },
        "auto_users": {
            "@spaceuser1:example.org": 50
        }
    }
}

A user's power level is then specified by an entry in either users or auto_users. Where a user appears in both sections, users takes precedence.

auto_users is subject to all of the same authorization checks as the existing users key (see https://matrix.org/docs/spec/rooms/v1#authorization-rules, paragraphs 10a, 10d, 10e).

This change necessitates a new room version.

Representing the mapping from spaces to power levels

The desired mapping from spaces to power levels is defined in a new state event type, m.room.power_level_mappings. The content should contain a mappings key which is an ordered list, for example:

{
   "type": "m.room.power_level_mappings",
   "state_key": "",
   "content": {
       "mappings": [
           {
               "space": "!mods:example.org",
               "via": ["example.org"],
               "power_level": 50
           },
           {
               "space": "!users:example.org",
               "via": ["example.org"],
               "power_level": 1
           }
       ]
    }
}

This means that a new m.room.power_levels event would be generated whenever the membership of either !mods or !users changes. If a user is in both spaces, !mods takes priority because that is listed first.

Implementing the mapping

When a new room is created, the server implicitly adds a "room admin bot" to the room, with the maximum power-level of any of the initial users. (Homeservers should implement this "bot" internally, rather than requiring separate software to be installed.)

It is proposed that this "admin bot" use the special user ID with empty localpart @:example.com.

This bot is then responsible for monitoring the power_level_mappings state, and peeking into any spaces mentioned in the content. It can then issue new m.room.power_levels events whenever the membership of the spaces in question changes.

It is possible that the admin bot is unable to perform the mapping (for example, the space cannot be peeked; or the membership of the space is so large that it cannot be expanded into a single m.room.power_levels event). It is proposed that the bot could notify the room of any problems via m.room.message messages of type m.msgtype.

Clearly, updating this event type is extremely powerful. It is expected that access to it is itself restricted via power_levels. This could be enforced by the admin bot so that no m.room.power_levels events are generated unless power_level_mappings is appropriately restricted.

Membership restrictions

A desirable feature is to give room admins the power to restrict membership of their room based on the membership of spaces (for example, "only members of the #doglovers space can join this room"¹).

XXX can we maybe do this with invites generated on demand? If not, we probably need some sort of "silent invite" state for each user,

By implication, when a user leaves the required space, they should be ejected from the room.

XXX: how do we implement the ejection? We could leave it up to the ejectee's server, but what happens if it doesn't play the game? So we probably need to enact a ban... but then, which server has responisiblity, and which user is used?

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.)

Inheriting join rules

If you make a parent space invite-only, should that (optionally?) cascade into child rooms? Seems to have some of the same problems as inheriting PLs.

Dependencies

MSC1840 for room types.
MSC1776 for effective peeking over the C/S API.
MSC1777 (or similar) for effective peeking over Federation.

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

Security considerations

The peek server has significant power. TODO: expand.

Tradeoffs

If the membership of a space would be large (for example: an organisation of several thousand people), this membership has to 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.child`	event type	`org.matrix.msc1772.space.child`
`m.space.parent`	event type	`org.matrix.msc1772.space.parent`
`m.room.power_level_mappings`	event type	`org.matrix.msc1772.room.power_level_mappings`
`auto_users`	key in `m.room.power_levels` event	`org.matrix.msc1772.auto_users`

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. ↩

17 KiB Raw Blame History