matrix-spec/content/client-server-api/modules/spaces.md

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Spaces

{{% added-in v="1.2" %}}

Often used to group rooms of similar subject matter (such as a public "Official matrix.org rooms" space or personal "Work stuff" space), spaces are a way to organise rooms while being represented as rooms themselves.

A space is defined by the m.space room type, making it known as a "space-room". The space's name, topic, avatar, aliases, etc are all defined through the existing relevant state events within the space-room.

Sending normal m.room.message events within the space-room is discouraged - clients are not generally expected to have a way to render the timeline of the room. As such, space-rooms should be created with m.room.power_levels which prohibit normal events by setting events_default to a suitably high number. In the default power level structure, this would be 100. Clients might wish to go a step further and explicitly ignore notification counts on space-rooms.

Membership of a space is defined and controlled by the existing mechanisms which govern a room: m.room.member, m.room.history_visibility, and m.room.join_rules. Public spaces are encouraged to have a similar setup to public rooms: world_readable history visibility, published canonical alias, and suitably public join_rule. Invites, including third-party invites, still work just as they do in normal rooms as well.

All other aspects of regular rooms are additionally carried over, such as the ability to set arbitrary state events, hold room account data, etc. Spaces are just rooms with extra functionality on top.

Managing rooms/spaces included in a space

Spaces form a hierarchy of rooms which clients can use to structure their room list into a tree-like view. The parent/child relationship can be defined in two ways: with m.space.child state events in the space-room, or with m.space.parent state events in the child room.

In most cases, both the child and parent relationship should be defined to aid discovery of the space and its rooms. When only a m.space.child is used, the space is effectively a curated list of rooms which the rooms themselves might not be aware of. When only a m.space.parent is used, the rooms are "secretly" added to spaces with the effect of not being advertised directly by the space.

{{% boxes/warning %}} Considering spaces are rooms themselves, it is possible to nest spaces within spaces, and it is possible to create a loop. Though the creation of loops is explicitly disallowed, implementations might still encounter them and must be careful not to loop infinitely when this happens.

Clients and servers should additionally be aware of excessively long trees which may cause performance issues. {{% /boxes/warning %}}

m.space.child relationship

When using this approach, the state events get sent into the space-room which is the parent to the room. The state_key for the event is the child room's ID.

For example, to achieve the following:

#space:example.org
    #general:example.org (!abcdefg:example.org)
    !private:example.org

the state of #space:example.org would consist of:

Unimportant fields trimmed for brevity.

{
    "type": "m.space.child",
    "state_key": "!abcdefg:example.org",
    "content": {
        "via": ["example.org"]
    }
}

{
    "type": "m.space.child",
    "state_key": "!private:example.org",
    "content": {
        "via": ["example.org"]
    }
}

No state events in the child rooms themselves would be required (though they can also be present). This allows for users to define personal/private spaces to organise their own rooms without needing explicit permission from the room moderators/admins.

Child rooms can be removed from a space by omitting the via key of content on the relevant state event, such as through redaction or otherwise clearing the content.

{{% event event="m.space.child" %}}

Ordering of children within a space

When the client is displaying the children of a space, the children should be ordered using the algorithm below. In some cases, like a traditional left side room list, the client may override the ordering to provide better user experience. A theoretical space summary view would however show the children ordered.

Taking the set of space children, first order the children with a valid order key lexicographically by Unicode code-points such that \x20 (space) is sorted before \x7E (~). Then, take the remaining children and order them by the origin_server_ts of their m.space.child event in ascending numeric order, placing them after the children with a valid order key in the resulting set.

In cases where the order values are the same, the children are ordered by their timestamps. If the timestamps are the same, the children are ordered lexicographically by their room IDs (state keys) in ascending order.

Noting the careful use of ASCII spaces here, the following demonstrates a set of space children being ordered appropriately:

Unimportant fields trimmed for brevity.

[
    {
        "type": "m.space.child",
        "state_key": "!b:example.org",
        "origin_server_ts": 1640341000000,
        "content": {
            "order": " ",
            "via": ["example.org"]
        }
    },
    {
        "type": "m.space.child",
        "state_key": "!a:example.org",
        "origin_server_ts": 1640141000000,
        "content": {
            "order": "aaaa",
            "via": ["example.org"]
        }
    },
    {
        "type": "m.space.child",
        "state_key": "!c:example.org",
        "origin_server_ts": 1640841000000,
        "content": {
            "order": "first",
            "via": ["example.org"]
        }
    },
    {
        "type": "m.space.child",
        "state_key": "!e:example.org",
        "origin_server_ts": 1640641000000,
        "content": {
            "via": ["example.org"]
        }
    },
    {
        "type": "m.space.child",
        "state_key": "!d:example.org",
        "origin_server_ts": 1640741000000,
        "content": {
            "via": ["example.org"]
        }
    }
]

1. !b:example.org is first because \x20 is before aaaa lexically.
2. !a:example.org is next because aaaa is before first lexically.
3. !c:example.org is next because first is the last order value.
4. !e:example.org is next because the event timestamp is smallest.
5. !d:example.org is last because the event timestamp is largest.

m.space.parent relationships

Rooms can additionally claim to be part of a space by populating their own state with a parent event. Similar to child events within spaces, the parent event's state_key is the room ID of the parent space, and they have a similar via list within their content to denote both whether or not the link is valid and which servers might be possible to join through.

To avoid situations where a room falsely claims it is part of a given space, m.space.parent events should be ignored unless one of the following is true:

• A corresponding m.space.child event can be found in the supposed parent space.
• The sender of the m.space.parent event has sufficient power level in the supposed parent space to send m.space.child state events (there doesn't need to be a matching child event).

{{% boxes/note %}} Clients might need to peek into a parent space to inspect the room state if they aren't already joined. If the client is unable to peek the state, the link should be assumed to be invalid. {{% /boxes/note %}}

{{% boxes/note %}} A consequence of the second condition is that a room admin being demoted in the parent space, leaving the parent space, or otherwise being removed from the parent space can mean that a previously valid m.space.parent event becomes invalid. {{% /boxes/note %}}

m.space.parent events can additionally include a canonical boolean key in their content to denote that the parent space is the main/primary space for the room. This can be used to, for example, have the client find other rooms by peeking into that space and suggesting them to the user. Only one canonical parent should exist, though this is not enforced. To tiebreak, use the lowest room ID sorted lexicographically by Unicode code-points.

{{% event event="m.space.parent" %}}

Discovering rooms within spaces

Often the client will want to assist the user in exploring what rooms/spaces are part of a space. This can be done with crawling m.space.child state events in the client and peeking into the rooms to get information like the room name, though this is impractical for most cases.

Instead, a hierarchy API is provided to walk the space tree and discover the rooms with their aesthetic details.

The GET /hierarchy API works in a depth-first manner: when it encounters another space as a child it recurses into that space before returning non-space children.

{{% boxes/warning %}} Though prohibited, it is still possible for loops to occur. Servers should gracefully break loops.

Additionally, a given child room might appear multiple times in the response as a grandchild (for example). {{% /boxes/warning %}}

{{% http-api spec="client-server" api="space_hierarchy" %}}

Server behaviour

In the case where the server does not have access to the state of a child room, it can request the information over federation with the GET /hierarchy API. The response to this endpoint should be cached for a period of time. The response might additionally contain information about rooms the requesting user is already a member of, or that the server is aware of - the local data should be used instead of the remote server's data.

Note that the response to the client endpoint is contextual based on the user. Servers are encouraged to cache the data for a period of time, though permission checks may need to be performed to ensure the response is accurate for that user.