matrix-spec-proposals/proposals/0-threading.md

### MSC: Threading

*This MSC 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](https://github.com/matrix-org/matrix-doc/blob/matthew/msc1849/proposals/1849-aggregations.md):
```
{
    "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`.

Additional concerns:
 - The name of the `rel_type` is prone to bike-shedding: "m.reference", "m.reply", "m.in_reply_to", etc. This proposal opts for the decisions made
   in MSC1849 which is "m.reference".

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.


#### 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.
}
```

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.

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. For events at the same level, honour the `recent_first` flag.
   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.
 - 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_breath` are honoured
   _based on the original request_ - the max values always relate to the original `event_id`, NOT the event ID previously stopped at.

#### 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 `m.relationship` object: `servers` and `room_id`:
```
{
    "type": "m.room.message",
    "content": {
        "body": "i <3 shelties",
        "m.relationship": {
            "rel_type": "m.reference",
            "event_id": "$another_event_id",
            "servers": [ "localhost", "anotherhost" ],
            "room_id": "!someroomid:anotherhost",
        }
    }
}
```

Only servers can set these fields. If clients attempt to set them they will be replaced. 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 `servers`.
 - Fetch the room ID that the event belongs to. Add it to `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 /relationships` endpoint includes a new field:
 - `auto_join: true|false`: if `true`, will automatically join the user calling `/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 `room_id` and `servers`. If
   they exist, peek into the room ([MSC2444](https://github.com/matrix-org/matrix-doc/pull/2444)), or if that is not supported,
   join the room as the user querying `/relationships` if and only if `"auto_join": true` in the `/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}](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-join-roomidoralias)
   with the `server_name` query parameters set to those in `servers`.

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.