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Update for new terminology and current thinking

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Richard van der Hoff 4 years ago
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proposals/1772-groups-as-rooms.md
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# Proposal for groups as rooms (take 2)
# Proposal for Matrix "spaces" (formerly known as "groups as rooms (take 2)")
This obsoletes [MSC1215](https://github.com/matrix-org/matrix-doc/issues/1215).
## Problem
## Background and objectives
The current groups API has some serious issues:
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
@ -22,26 +37,39 @@ The current groups API has some serious issues:
* 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.
* 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.
## Solution
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.
Represent groups by rooms rather than a custom first-class entity.
Spaces are referred to primarily by their alias, for example
`#foo:matrix.org`.
We reserve aliases which begin with a `+` to represent groups - e.g. the room
for group `+test:example.com` is `#+test:example.com`.
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 `m.room.group` and `m.room.subgroup` events which define the rooms
and subgroups within the group. A `present: true` key is included to
distinguish from a deleted state event. Something like:
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.room.group",
"type": "m.space.child",
"state_key": "#room1:example.com",
"contents": {
"present": true
@ -49,144 +77,151 @@ distinguish from a deleted state event. Something like:
}
{
"type": "m.room.group",
"type": "m.space.child",
"state_key": "#room2:example.com",
"contents": {
"present": true,
"autojoin": true
"autojoin": true # TODO: what does this mean?
}
}
{
"type": "m.room.subgroup",
"state_key": "+something:example.com",
"contents": {
"present": true
}
}
{
"type": "m.room.subgroup",
"state_key": "+otherthing:example.com",
"contents": {
"present": true
}
"type": "m.space.child",
"state_key": "#oldroom:example.com",
"contents": {}
}
```
Name, Topic, Membership etc share the same events as a normal room.
Space-rooms may have `m.room.name` and `m.room.topic` state events in the same
way as a normal room.
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).
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.
Long description requires a new event: `m.room.description`. This can also be
used for rooms as well as groups.
"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).
Groups may be nested, and membership of groups is defined as the union of the
members of the group room and its subgroups. If `+top:example.com` has two
subgroups, the user membership of `+top:example.com` is the union of the
subgroups and the group itself. This allows hierarchies of groups & users to be
defined.
Join rules, invites and 3PID invites work as for a normal room.
Clients peek in rooms (recursing into subgroups as needed) in order to determine
group membership.
### Long description
Invites, 3PID invites, Power Levels etc all work as for a normal room.
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`).
Normal messages within the room could be showed and used as a 'lobby' area for
the given group.
TODO: this could also be done via pinned messages. Failing that
`m.room.description` should probably be a separate MSC.
This requires no server changes at all, other than better support for peeking
(see Dependencies below), and could allow the existing /groups API to be
deprecated and removed outright.
### Inheritance of power-levels
## ACLs
TODO
Currently the group server has total control over specifying the list of users
who may be present in a group, 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.
### Automated joins/leaves
Whilst this is very powerful for mapping arbitrary group structures into Matrix,
it may be overengineered.
TODO
Instead, the common case is wanting to define a group where some users are
publicly visible as members, and others are not. This is what the current use
cases require today. A simple way of achieving would be to create a subgroup
for the private members - e.g. have `+sensitive:matrix.org` and
`+sensitive-private:matrix.org`. The membership of
`+sensitive-private:matrix.org` is set up with `m.room.join_rules` to not to
allow peeking; you have to be joined to see the members, and users who don't
want to be seen by the public to be member of the group are added to the
subgroup.
## Future extensions
In future, there may be a usecase for groups where members are unaware of the
other users' membership. This would also be useful for other rooms, and is left
for a future proposal.
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.
## Flair
### Sub-spaces
A proposal for how to safely determine user flair is:
Questions to be answered here include:
* User publishes the groups they wish to announce on their profile
* 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 a m.flair state event: it lists the groups which they are advertising.
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. However, we can't trust the flair which the
users advertise on the profile - it has to be cross-referenced from the
memberships of the groups in question.
To do this cross-referencing, options are:
1. The client checks the group membership (very inefficient, given the server
could/should do it for them), or...
2. The server checks the group membership by peeking the group and somehow
decorates the `m.flair` event as validated before sending it to the client.
This is also inefficient, as it forces the server to peek a potentially large
group (unless we extend federation to allow peeking specific state events)
3. The origin `m.flair` event includes the event_id of the user's
`m.room.membership` event in the group. The server performing the check can
then query this specific event from one of the servers hosting the group-room,
and we perhaps extend the S2S API to say whether a given state event is current
considered current_state or not. If the `m.room.membership` event is confirmed
as current, then the `m.flair` is decorated as being confirmed.
Of these, option 3 feels best?
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
This needs peeking to work effectively over the CS API
([MSC1776](https://github.com/matrix-org/matrix-doc/issues/1776)).
* [MSC1840](https://github.com/matrix-org/matrix-doc/pull/1840) for room
types.
This needs peeking to work effectively over federation (e.g. by having servers
join remote rooms as `@null:example.com` in order to participate in them for
peeking purposes - [MSC1777](https://github.com/matrix-org/matrix-doc/issues/1777)).
* [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
XXX: how do we stop cycles & recursion abuse of the subgroups?
* The peek server has significant power. TODO: expand.
## Tradeoffs
This consciously sacrifices the ability to delegate group lookups through
to a centralised group server. However, group data can already be stale as we
rely on cached attestations from federated servers to apply ACLs even if the
remote server is not available. So this isn’t much worse than eventually
consistent group membership as you’d find in a room.
* 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.
It also means that large groups have to be bridged in their entirety into the
room, rather than querying/searching incrementally. This is something we should
fix for bridged rooms in general too, however.
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.
This also consciously sacrifices the ability for a group server to provide
different 'views' of groups to different querying users, as being
overengineered. Instead, all common use cases should be modellable by modelling
group membership as room membership (nesting if required).
* 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
@ -197,11 +232,16 @@ How does this work with
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.room.subgroup` becomes
`org.matrix.msc1772.room.subgroup`.
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

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