matrix-spec/proposals/1772-groups-as-rooms.md

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.

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:

1. 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",
           "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 (~), or consist of more than 50 characters, are forbidden and should be ignored if received.)

   If default is set to true, that indicates a "default child": see below.

2. Separately, rooms can claim parents via the m.room.parent state event:

   {
       "type": "m.room.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.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.

   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 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

TODO: much of this is orthogonal to the headline feature of "spaces", and should be moved to a separate MSC.

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.

The new auto_users key is maintained by a bot user, as described below.

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.

If mappings is not a list, the whole event is ignored. Any entries in the list which do not match the expected format are ignored.

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, updating the value of auto_users, 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.

Some sort of rate-limiting may be required to handle the case where the mapped space has a high rate of membership churn.

Alternatives

Things that were considered and dismissed:

• Extend the auth rules to include state from other rooms. Although this feels cleaner, a robust implementation would be a hugely complicated undertaking. In particular, room state resolution is closely linked to event authorisation, and is already highly complex and hard to reason about, and yet is fundamental to the security of Matrix.

  In short, we believe such a change would require significant research and modelling. A solution based on such a foundation could not practically be implemented in the near future.

• Rather than defining the mapping in the room, define a template power-levels event in a parent space, which will be inherited by all child rooms. For example:

  {
      "type": "m.space.child_power_levels",
      "state_key": "",
      "content": {
          // content as per regular power_levels event
      }
  }
  

  Problem 1: No automated mapping from space membership to user list, so the user list would have to be maintained manually. On the other hand, this could be fine in some situations, where we're just using the space to group together rooms, rather than as a user list.

  Problem 2: No scope for nuance, where different rooms have slightly different PLs.

  Problem 3: what happens to rooms where several spaces claim it as a child? They end up fighting?

  Problem 4: Doesn't allow for random room admins to delegate their PLs to a space without being admins in that space.

• To implemplement the mapping, we require any user who is an admin in the space (ie, anyone who has permission to change the access rights in the space) to also be admins and members of any child rooms.

  Say Bob is an admin in #doglovers and makes a change that should be propagated to all children of that space. His server is then responsible for generating a power-levels event on his behalf for each room.

  Problem 1: Bob may not want to be a member of all such rooms.

  Problem 2: It will feel odd that Bob's user is seen to be generating PL events every time someone comes and goes from the space.

  Problem 3: It doesn't allow users to set up their own rooms to mirror a space, without having any particular control in that space (though it is questionable if that is actually a useful feature, at least as far as PLs are concerned.)

• Another alternative for implementing the mapping: the user that created the relationship event (or rather, their homeserver, using the user's ID) is responsible for copying access controls into the room.

  Problem 1: What do you do if the admin who sets up the PL relationship disappears? The humans have to step in and create a new admin?

  Problem 2: Again it seems odd that these PL changes come from a single user.

• Is it possible to implement the mappings from multiple users, some of which may not have PL 100? After all it's possible to set rooms up so that you can change PL events without having PL 100.

  It gets horribly messy very quickly, where some admin users can make some changes. So some get supressed and then get made later anyway by a different admin user?

• Is it possble to apply finer-grained control to the m.room.power_level_mappings event than "you must be max(PL)"? Applying restrictions post-hoc (ie, having the admin bot ignore settings which were set by underpriviledged users) is an absolute minefield. It might be possible to apply restrictions at the point that the event is set, but it sounds fiddly and it's not clear there is a real use-case.

• This solution smells a bit funny because of the expansions (causing all the redundant mxids everywhere as the groups constantly get expanded every time something happens).

  • Could we could put a hash of the space membership in the PL instead of expanding the whole list, so that servers have a way to check if they are applying the same list as everyone else?

    Feels like it will have bad failure modes: what is a server supposed to do when the hash doesn't match?

  • Could version the space memberships, so you can compare with the source of the space membership data?

  • PL events just record the delta from the previous one? (So a new server would need to get all the PLs ever, but… is that a bad thing?) ... maybe

  These optimisations can all be punted down the road to a later room version.

• Other ways of handling the merge of automatic and manual PL settings:

  • Add hints to the automated mapper so that it can maintain manually-assigned PLs. This could either be another field in power_levels which plays no part in event auth:

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

    ... or stored in a separate event. Clients would be responsible for updating both copies of the manually-assigned PLs on change.

    Problem: Requiring clients to make two changes feels fragile. What if they get it wrong? what if they don't know about the second copy because they haven't been designed to work in rooms in spaces?

  • Require that even regular PLs go through the automated mapper, by making them an explicit input to that mapper, for example with entries in the m.room.power_level_mappings event suggested above.

    Problem: Requires clients to distinguish between rooms where there is an automated mapper, and those where the client should manipulate the PLs directly. (Maybe that's not so bad? The presence of the mappings event should be enough? But still sucks that there are two ways to do the same thing, and clients which don't support spaces will get it wrong.)

Restricting room membership based on space membership

A desirable feature is to give room admins the power to restrict membership of their room based on the membership of spaces (for example, "members of the #doglovers space can join this room without an invitation"¹).

We could represent the allowed spaces with additional content in the m.room.join_rules event. For example:

{
    "type": "m.room.join_rules",
    "state_key": "",
    "content": {
        "join_rule": "public",
        "allow": [
            {
                "space": "!mods:example.org",
                "via": ["example.org"],
            },
            {
                "space": "!users:example.org",
                "via": ["example.org"],
            }
        ]
    }
}

The allow key applies a restriction to the public join rule, so that only users satisfying one or more of the requirements should be allowed to join. Additionally, users who have received an explicit invite event are allowed to join². If the allow key is an empty list (or not a list at all), no users are allowed to join without an invite.

Unlike the regular invite join rule, the restriction cannot be enforced over federation by event authorization, so servers in the room are trusted not to allow invalid users to join.³

When a server receives a /join request from a client or a /make_join//send_join request from a server, the request should only be permitted if the user has a valid invite or is in one of the listed spaces (established by peeking).

XXX: redacting the join_rules above will reset the room to public, which feels dangerous?

A new room version is not absolutely required here, but may be advisable to ensure that servers that do not support allow do not join the room (and would also allow us to tweak the redaction rules to avoid the foot-gun).

Kicking users out when they leave the allowed space

XXX: this will probably be a future extension, rather than part of the initial implementation of allow.

In the above example, suppose @bob:server.example leaves !users:example.org: they should be removed from the room. One option is to leave the departure up to Bob's server server.example, but this places a relatively high level of trust in that server. Additionally, if server.example were offline, other users in the room would still see Bob in the room (and their servers would attempt to send message traffic to it).

Instead, we make the removal the responsibility of the room's admin bot (see above): the bot is expected to peek into any spaces in allow and kick any users who are members of the room and leave the union of the allowed spaces.

(XXX: should users in a space be kicked when that space is removed from the allow list? We think not, by analogy with what happens when you switch the join rules from public to invite.)

One problem here is that it will lead to users who joined via an invite being kicked. For example:

• @bob:server.example creates an invite-only room.
• Later, the join_rules are switched to public, with an allow of !users:example.org, of which Bob happens to be a member.
• Later still, Bob leaves !users:example.org.
• Bob is kicked from his own room.

Fixing this is thorny. Some sort of annotation on the membership events might help. but it's unclear what the desired semantics are:

• Assuming that users in a given space are not kicked when that space is removed from allow, are those users then given a pass to remain in the room indefinitely? What happens if the space is added back to allow and then the user leaves it?

• Suppose a user joins a room via a space (SpaceA). Later, SpaceB is added to the allow list and SpaceA is removed. What should happen when the user leaves SpaceB? Are they exempt from the kick?

Alternatives

• Maintain some sort of pre-approved list as the space membership changes in a similar way to the PL mapping, possibly via a new membership state.

  Could lead to a lot of membership churn, from a centralised control point.

• Base it on invite-only rooms, and generate invite events on the fly. Kind-of ok, except that we'd want the invites to be seen as having a sender of a management bot rather than an arbitrary user, which would mean that all joins would have to go through that one server (even from servers that were already participating in the room), which feels a bit grim. We could have multiple admin bots to mitigate this, but it gets a bit messy.

• Change the way that allow and invites interact, so that an invite does not exempt you from the allow requirements. This would be simpler to implement, but probably doesn't match the expected UX.

• Put the allow rules in a separate event? This is attractive because join_rules are involved in event auth and hence state resolution, and the fewer events that state res has to grapple with the better. However, doing this would probably require us to come up with a new join_rule state to tell servers to go and look for the allowed spaces.

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.

• 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.

• The allow feature for join_rules places increased trust in the servers in the room. We consider this acceptable: if you don't want evil servers randomly joining spurious users into your rooms, then a) don't let evil servers in your room in the first place, b) don't use allow lists, given the expansion increases the attack surface anyway by letting members in other rooms dictate who's allowed into your room.

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	room type	org.matrix.msc1772.space
m.space.child	event type	org.matrix.msc1772.space.child
m.room.parent	event type	org.matrix.msc1772.room.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
allow	key in m.room.join_rules event	org.matrix.msc1772.allow

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. ↩