matrix-spec-proposals

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MSC2836: Threading

This MSC probably supersedes https://github.com/matrix-org/matrix-doc/issues/1198

Matrix does not have arbitrarily nested threading for events. This is a desirable feature for implementing clones of social media websites like Twitter and Reddit. The aim of this MSC is to define the simplest possible API shape to implement threading in a useful way. This MSC does NOT attempt to consider use cases like editing or reactions, which have different requirements to simple threading (replacing event content and aggregating reactions respectively).

The API can be broken down into 2 sections:

Making relationships: specifying a relationship between two events.
Querying relationships: asking the server for relationships between events.

The rest of this proposal will outline the proposed API shape along with the considerations and justifications for it.

Making relationships

Relationships are made when sending or updating events. The proposed API shape is identical to MSC1849:

{
    "type": "m.room.message",
    "content": {
        "body": "i <3 shelties",
        "m.relationship": {
            "rel_type": "m.reference",
            "event_id": "$another_event_id"
        }
    }
}

Justifications for this were as follows:

Quicker iterations by having it in event content rather than at the top-level (at the event_id level).
Ability for relationships to be modified post-event creation (e.g by editing the event).
Doesn't require any additional server-side work (as opposed to adding the event ID as a query param e.g ?in-reply-to=$foo:bar).

Drawbacks include:

Additional work required for threading to work with E2EE. See MSC1849 for proposals, but they all boil down to having the m.relationship field unencrypted in the event content.

Edge cases:

Any event type can have an m.relationship field in its content.
Redacting an event with an m.relationship field DOES NOT remove the relationship. Instead, it is preserved similar to how membership is preserved for m.room.member events, with the following rules:
- Remove all fields except rel_type and event_id.
- If rel_type is not any of the three types m.reference, m.annotation or m.replace then remove it.
- If event_id is not a valid event ID ($ sigil, correct max length), then remove it. The decision to preserve this field is made so that users can delete offensive material without breaking the structure of a thread. This is different to MSC1849 which proposes to delete the relationship entirely.
It is an error to reference an event ID that the server is unaware of. Servers MUST check that they have the event in question: it need not be part of the connected DAG; it can be an outlier. This prevents erroneous relationships being made by abusing the CS API.
It is NOT an error to reference an event ID in another room. Cross-room threading is allowed and this proposal goes into more detail on how servers should handle this as a possible extension.
It is an error to reference yourself. Cyclical loops are still possible by using multiple events and servers should guard against this by only visiting events once.

Querying relationships

Relationships are queryed via a new CS API endpoint:

POST /_matrix/client/r0/event_relationships
{
    "event_id": "$abc123",         // the anchor point for the search, must be in a room you are allowed to see (normal history visibility checks apply)
    "max_depth": 4,                // if negative unbounded, default: 3.
    "max_breadth": 10,             // if negative unbounded, default: 10.
    "limit": 100,                  // the maximum number of events to return, server can override this, default: 100.
    "depth_first": true|false,     // how to walk the DAG, if false, breadth first, default: false.
    "recent_first": true|false,    // how to select nodes at the same level, if false oldest_first - servers compare against origin_server_ts, default: true.
    "include_parent": true|false,  // if event_id has a parent relation, include it in the response, default: false.
    "include_children": true|false // if there are events which reply to $event_id, include them all (depth:1) in the response: default: false.
    "direction": up|down           // if up, parent events (the events $event_id is replying to) are returned. If down, children events (events which reference $event_id) are returned, default: "down".
    "batch": "opaque_string"       // A token to use if this is a subsequent HTTP hit, default: "".
}

which returns:

{
    "events": [                    // the returned events, ordered by the 'closest' (by number of hops) to the anchor point.
        { ... }, { ... }, { ... },
    ],
    "next_batch": "opaque_string", // A token which can be used to retrieve the next batch of events, if the response is limited.
                                   // Optional: can be omitted if the server doesn't implement threaded pagination.
    "limited": true|false          // True if there are more events to return because the `limit` was reached. Servers are not obligated
                                   // to return more events, see if the next_batch token is provided or not.
}

Justifications for the request API shape are as follows:

The HTTP path: cross-room threading is allowed hence the path not being underneath /rooms. An alternative could be /events/$event_id/relationships but there's already an /events/$event_id deprecated endpoint and nesting this new MSC underneath a deprecated endpoint conveys the wrong meaning.
The HTTP method: there's a lot of data to provide to the server, and GET requests shouldn't have an HTTP body, hence opting for POST. The same request can produce different results over time so PUT isn't acceptable as an alternative.
The anchor point: pinning queries on an event is desirable as often websites have permalinks to events with replies underneath.
The max depth: Very few UIs show depths deeper than a few levels, so allowing this to be constrained in the API is desirable.
The max breadth: Very few UIs show breadths wider than a few levels, so allowing this to be constrained in the API is desirable.
The limit: For very large threads, a max depth/breadth can quickly result in huge numbers of events, so bounding the overall number of events is desirable. Furthermore, querying relationships is computationally expensive on the server, hence allowing it to arbitrarily override the client's limit (to avoid malicious clients setting a very high limit).
The depth first flag: Some UIs show a 'conversation thread' first which is depth-first (e.g Twitter), whereas others show immediate replies first with a little bit of depth (e.g Reddit).
The recent first flag: Some UIs show recent events first whereas others show the most up-voted or by some other metric. This MSC does not specify how to sort by up-votes, but it leaves it possible in a compatible way (e.g by adding a sort_by_reaction: 👍 which takes precedence which then uses recent_first to tie-break).
The include parent flag: Some UIs allow permalinks in the middle of a conversation, with a "Replying to [link to parent]" message. Allowing this parent to be retrieved in one API hit is desirable.
The include_children flag: Some UIs allow permalinks in the middle of a conversation, with immediate children responses visible. Allowing the children to be retrieved in one API hit is desirable.
The direction enum: The decision for literal up and down makes for easier reading than is_direction_up: false or equivalent. The direction is typically down - find all children from this event - but there is no reason why this cannot be inverted to walk up the DAG instead.
The batch token: This allows clients to retrieve additional results. It's contained inside the HTTP body rather than as a query param for simplicity - all the required data that the server needs is in the HTTP body. This token is optional as paginating is reasonably complex and should be opt-in to allow for ease of server implementation.

Justifications for the response API shape are as follows:

The events array: There are many possible ways to structure the thread, and the best way is known only to the client implementation. This API shape is unopinionated and simple.
The next batch token: Its presence indicates if there are more events and it is opaque to allow server implementations the flexibility for their own token format. There is no 'prev batch' token as it is intended for clients to request and persist the data on their side rather than page through results like traditional pagination.
The limited flag: Required in order to distinguish between "no more events" and "more events but I don't allow pagination". This additional state cannot be accurately represented by an empty next_batch token.

Server implementation:

Sanity check request and set defaults.
Can the user see (according to history visibility) event_id? If no, reject the request, else continue.
Retrieve the event. Add it to response array.
If include_parent: true and there is a valid m.relationship field in the event, retrieve the referenced event. Apply history visibility check to that event and if it passes, add it to the response array.
If include_children: true, lookup all events which have event_id as an m.relationship - this will almost certainly require servers to store this lookup in a dedicated table when events are created. Apply history visibility checks to all these events and add the ones which pass into the response array, honouring the recent_first flag and the limit.
Begin to walk the thread DAG in the direction specified, either depth or breadth first according to the depth_first flag, honouring the limit, max_depth and max_breadth values according to the following rules:
- If the response array is >= limit, stop.
- If already processed event, skip.
- Check how deep the event is compared to event_id, does it exceed (greater than) max_depth? If yes, skip.
- Check what number child this event is (ordered by recent_first) compared to its parent, does it exceed (greater than) max_breadth? If yes, skip.
- Process the event. If the event has been added to the response array already, do not include it a second time. If an event fails history visibiilty checks, do not add it to the response array and do not follow any references it may have. This algorithm bounds an infinite DAG into a "window" (governed by max_depth and max_breadth) and serves up to limit events at a time, until the entire window has been served. Critically, the limit has not been reached when the algorithm hits a max_depth or max_breadth, it is only reached when the response array is >= limit.
When the thread DAG has been fully visited or the limit is reached, return the response array (and a next_batch if the request was limited). If a request comes in with the next_batch set to a valid value, continue walking the thread DAG from where it was previously left, ensuring that no duplicate events are sent, and that any max_depth or max_breadth are honoured based on the original request - the max values always relate to the original event_id, NOT the event ID previously stopped at.

Querying relationships over federation

Relationships can be queried over federation using a new endpoint which is the same as the CS API format. See the CS API section for more info. The path used for this new federation endpoint is /_matrix/federation/v1/event_relationships. There is one additional response field: auth_chain which contains all the necessary auth events for the events in events, e.g:

{
    "events": [                    // the returned events, ordered by the 'closest' (by number of hops) to the anchor point.
        { ... }, { ... }, { ... },
    ],
    "next_batch": "opaque_string", // A token which can be used to retrieve the next batch of events, if the response is limited.
                                   // Optional: can be omitted if the server doesn't implement threaded pagination.
    "limited": true|false,         // True if there are more events to return because the `limit` was reached. Servers are not obligated
                                   // to return more events, see if the next_batch token is provided or not.
    "auth_chain": [                // The auth events required to authenticate events in `events`, in any order without duplicates.
        { ... }, { ... }, { ... },
    ]
}

Justification:

In an ideal world, every server would have the complete room DAG and would therefore be able to explore the full scope of a thread in a room. However, over federation, servers have an incomplete view of the room and will be missing many events. In absence of a specific API to explore threads over federation, joining a room with threads will result in an incomplete view.
The requirements here have a lot in common with the Event Context API. However, the context API has no federated equivalent. This means any requests for events the server is unaware of will incorrectly return 404 Not Found.
The same API shape is proposed to allow code reuse and because the same concerns and requirements are present for both federation and client-server.

Server behaviour:

When receiving a request to /event_relationships: ensure the server is in the room then walk the thread in the same manner as the CS API form. Do not make outbound /event_relationships requests on behalf of this request to avoid routing loops where 2 servers infinitely call /event_relationships to each other.
For each event returned: include all auth_events for that event recursively to create an auth chain and add them to auth_chain.
Servers should make outbound /event_relationships requests for client requests when they encounter an event ID they do not have. The event may have happened much earlier in the room which another server in the room can satisfy.

Cross-room threading extension

This MSC expands on the basic form to allow cross-room threading by allowing 2 extra fields to be specified in the unsigned section of the event: relationship_servers and relationship_room_id:

{
    "type": "m.room.message",
    "content": {
        "body": "i <3 shelties",
        "m.relationship": {
            "rel_type": "m.reference",
            "event_id": "$another_event_id",
        }
    },
    "unsigned": {
      "relationship_room_id":"!someroomid:anotherhost",
      "relationship_servers": [ "localhost", "anotherhost" ]
    }
}

Only servers can set these fields. The server should set these fields when an event is sent according to the following rules:

Check the client can view the event ID in question. If they cannot, reject the request.
Check that the room's m.room.create event allows cross-room threading by the presence of "m.cross_room_threading": true in the content field. If absent, it is false.
Fetch the servers currently in the room for the event. Add them all to relationship_servers.
Fetch the room ID that the event belongs to. Add it to relationship_room_id.

This proposal does not require any changes to /createRoom as "m.cross_room_threading": true can be specified via the creation_content field in that API.

The POST /event_relationships endpoint includes a new field:

auto_join: true|false: if true, will automatically join the user calling /event_relationships to rooms they are not joined to in order to explore the thread. Default: false.

Server implementation:

When walking the thread DAG, if there is an event that is not known, check the relationship for relationship_room_id and relationship_servers. If they exist, peek into the room (MSC2444), or if that is not supported, join the room as the user querying /event_relationships if and only if "auto_join": true in the /event_relationships request. This is required in order to allow the event to be retrieved. Server implementations can treat the auto join as the same as if the client made a request to /join/{roomIdOrAlias} with the server_name query parameters set to those in relationship_servers.
After joining the room, the server should use the federated API /event_relationships to explore the thread in the newly joined room as the server will not have those events. These events can be stored as outliers or dropped depending on storage requirements.

Security considerations:

Allowing cross-room threading leaks the event IDs in a given room, as well as which servers are in the room at the point the reply is sent. It is for this reason that there is an opt-in flag at room creation time.

Justifications:

Because the client needs to be able to view the event being replied to, it is impossible for the replying client to respond to an event which the server is unaware of. However, it is entirely possible for queries to contain events which the server is unaware of if the sender was on another homeserver. For this reason, we need to contain routing information somewhere so servers can retrieve the event and continue navigating the thread. As the client and server already have the event which contains a relationship to another event inside an unknown room, the simplest option is to also contain the routing information with that relationship.
The fields are in the unsigned section so clients cannot artificially set them, and because unsigned is where a lot of other server-calculated metadata resides.

Backreferences

Using the threading model proposed above, events contain information about their parents but parents do not contain information about their children. This means that events can follow their path up to the root event but cannot explore siblings not on that root path. "Backreferences" means adding additional metadata to allow the discovery of child events from a given parent event. These backreferences are necessary over federation where each server has an incomplete view of the thread. Consider:

Letters = Servers
Numbers = Events

!room1 (A,B)    !room2 (B,C)
                     1 <- parent
                     |
   +-----------------+
   |                 |
   2                 3 <- children

When event 2 is created, only servers A,B will know this.
A backreference event needs to be created in !room2 in order for C to ever be aware of event 2.
When C is aware of event 2, it can then peek/join into !room1 to receive updates to that fork.
This means that in order to create a relationship, the user must be in parent event's room.

The backreference event MUST be sent by the server on behalf of the client and look like:

{
    "type": "m.room.backref",
    "content": {
        "event_id": "$parent_event_id",
        "m.relationship": {
            "rel_type": "m.child",
            "event_id": "$child_event_id",
        }
    },
    "sender": "@user_making_relation:localhost",
    "room_id": "!parent_event:room",
    "unsigned": {
      "relationship_room_id":"!someroomid:anotherhost",
      "relationship_servers": [ "localhost", "anotherhost" ]
    }
}