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

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

This obsoletes [MSC1215](https://github.com/matrix-org/matrix-doc/issues/1215).

## 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 which
attempts to provide this functionality (see
[matrix-doc#1513](https://github.com/matrix-org/matrix-doc/issues/1513)). This
API has some serious issues:
 * 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.

In this document, the existing implementation will be referred to as
"`/r0/groups`".

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](https://github.com/matrix-org/matrix-doc/pull/1840)).

We introduce an `m.space.child` state event type which defines the rooms within
the group: A `present: true` key is included to distinguish from a deleted
state event. Something like:

```json
{
    "type": "m.space.child",
    "state_key": "#room1:example.com",
    "contents": {
        "present": true
    }
}

{
    "type": "m.space.child",
    "state_key": "#room2:example.com",
    "contents": {
        "present": true,
        "autojoin": true   # TODO: what does this mean?
    }
}

{
    "type": "m.space.child",
    "state_key": "#oldroom:example.com",
    "contents": {}
}
```

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). Depending on the configuration of the space (in particular whether
`m.room.history_visibility` is set to `world_readable` or otherwise),
membership of the space may be 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.

### Long description

We would like to allow groups 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.

### Inheritance of power-levels

TODO

### Automated joins/leaves

TODO

## 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.

### Sub-spaces

Questions to be answered here include:

 * Should membership of a sub-space grant any particular access to the parent
   space, or vice-versa? We might need to extend `m.room.history_visibility` to
   support more flexibility; fortunately this is not involved in event auth so
   does not require new room versions.

 * What happens if somebody defines a cycle? (It's probably fine, but anything
   interpreting the relationships needs to be careful to limit recursion.)

### 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](https://github.com/matrix-org/matrix-doc/issues/1778).

One way this might be implemented is:

 * User publishes the spaces they wish to announce on their profile
   ([MSC1769](https://github.com/matrix-org/matrix-doc/issues/1769)
   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](https://github.com/matrix-org/matrix-doc/pull/1840) for room
   types.

 * [MSC1776](https://github.com/matrix-org/matrix-doc/issues/1776) for
   effective peeking over the C/S API.

 * [MSC1777](https://github.com/matrix-org/matrix-doc/issues/1777) (or similar)
   for effective peeking over Federation.

These dependencies are shared with profiles-as-rooms
([MSC1769](https://github.com/matrix-org/matrix-doc/issues/1769)).

## 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.

  This is particularly problematic if that membership list is based on an
  external service such as LDAP, since there is no way to keep the space
  membership in sync with the LDAP directory.

* 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).

## Issues

How does this work with
[MSC1228](https://github.com/matrix-org/matrix-doc/issues/1228) (removing MXIDs)?

## Unstable prefix

While this proposal is not in a published version of the specification,
implementations should use `org.matrix.msc1772` to represent the `m`
namespace. For example, `m.space.child` becomes
`org.matrix.msc1772.space.child`.


## 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


## Footnotes

[1] It's a