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MSC3440 Threading via m.thread relation

Problem

Threading allows users to branch out a new conversation from the main timeline of a room to each other. This is particularly useful in high traffic rooms where multiple conversations can happen in parallel or when a single discussion might stretch over a very long period of time.

The main goal of implementing threads is to facilitate conversations that are easier to follow and smoother to read. Threading is very clearly a core requirement for any modern messaging solution, and Matrix uptake is suffering due to the lack of progress.

Proposal

Event format

A new relation type (see MSC2674) m.thread expresses that an event belongs to a thread.

"m.relates_to": {
  "rel_type": "m.thread",
  "event_id": "$thread_root"
}

Where $thread_root is the event ID of the root message in the thread.

When a thread root is aggregated (as in MSC2675), it returns a summary of the thread: the latest message, a list of participants and the total count of messages. I.e. in places which include bundled relations (per MSC2675), the thread root would include additional information in the unsigned field:

{
  "event_id": "$root_event",
  "unsigned": {
    "m.relations": {
      "m.thread": {
        "latest_event": {
          "event_id": "$thread_event",
          // ...
        },
        "count": 7,
        "current_user_participated": true
      }
    }
  }
}

  • latest_event: The most recent event (topologically) which relates to this event, with rel_type of m.thread. Events sent by ignored users are not considered for the latest event.

    The latest event should be serialised in the same form as the event itself; this includes adding any bundled aggregations for the event (and applying edits).1

  • count: An integer counting the number of m.thread events, excluding events sent by ignored users.

  • current_user_participated: A boolean flag, which is set to true if the current logged in user has participated in the thread. The user has participated if:

    • They created the current event.
    • They created an event with a m.thread relation targeting the current event.

Rich replies in a thread

Rich replies are still handled via the m.in_reply_to field of m.relates_to. However clients should specify that this is not a thread fallback by setting the is_falling_back property to false.

"m.relates_to": {
    "rel_type": "m.thread",
    "event_id": "$thread_root",
    "is_falling_back": false,
    "m.in_reply_to": {
        "event_id": "$event_target"
    }
}

It is possible that an m.in_reply_to event targets an event that is outside the related thread. Clients should always do their utmost to display the rich reply and when clicked, the event should be displayed and highlighted in its original context.

A rich reply without rel_type: m.thread targeting a thread relation must be rendered in the main timeline. This will allow users to advertise threaded messages in the room.

Backwards compatibility

A thread will be displayed as a chain of replies on clients unaware of threads.

Thread-ready clients should always include an m.in_reply_to property when sending a threaded event. Unless the user is explicitly replying to another event (see "Rich replies in a thread", above), the m.in_reply_to property should reference the latest message-like event in the thread, and clients should also specify that m.in_reply_to is a fallback mechanism (rather than a genuine reply) by setting the is_falling_back property to true.

(If omitted, is_falling_back defaults to false, and receiving clients will treat the m.in_reply_to part of the event as a genuine reply.)

"m.relates_to": {
    "rel_type": "m.thread",
    "event_id": "ev1",
    "is_falling_back": true,
    "m.in_reply_to": {
        "event_id": "last_event_id_in_thread",
    }
  }

Historically replies have been limited to text messages due to the legacy fallback prepended to formatted_body. This MSC is dependant on MSC3676 which strips that requirement to unlock use of any event type in this context.

Fetch all relations to a thread root

To fetch an entire thread, the /relations API can be used as defined in MSC2675

GET /_matrix/client/unstable/rooms/!room_id:domain/relations/$thread_root/m.thread

Where $thread_root is the event ID of the root message in the thread.

Any API which receives events should bundle relations (apart from non-gappy incremental syncs), for instance: initial sync, gappy incremental sync, /messages and /context.

Fetch all threads in a room

Event filters (as used by endpoints including /messages, /sync and /context) are extended with new options to allow filtering events by their relating events:

  • related_by_rel_types: A list of relation types to include. An event A is included in the filter only if there exists another event B which relates to A with a rel_type which is defined in the list
  • related_by_senders: A list of senders to include. An event A is included in the filter only if there exists another event B which relates to A, and which has a sender which is in the list.

This can also be combined with the sender field to search for threads which a user has participated in (or not participated in).

GET /_matrix/client/v3/rooms/!room_id:domain/messages?filter=...

The filter string includes the new fields, above. In this example, the URL encoded JSON is presented unencoded and formatted for legibility:

{
  "types": ["m.room.message"],
  "related_by_senders": [
    // ...
  ],
  "related_by_rel_types": ["m.thread"]
}

Note that the newly added filtering parameters return events based on information in related events. Consider the following events in a room:

  • A: a m.room.message event sent by alice
  • B: a m.room.message event sent by bob which relates to A with type m.thread

Using a filter of "related_by_rel_types": ["m.thread"] would return event A as it has another event which relates to it via m.thread.

Similarly, using a filter of "related_by_senders": ["bob"] would return event A as it has another event which relates to it sent by bob.

Server capabilities

Threads might have sporadic support across servers, to simplify feature detections for clients, a homeserver must advertise unstable support for threads as part of the /versions API:

{
  "unstable_features": {
    "org.matrix.msc3440": true,
    // ...
  }
}

Limitations

Read receipts

Read receipts and read markers assume a single chronological timeline. Threading changes that assumption making the current API not very practical.

Clients can synthesize read receipts but it is possible that some notifications get lost on a fresh start where the clients have to start off the m.read information received from the homeserver.

Synchronising the synthesized notification count across devices is out of scope and deferred to a later MSC.

Single-layer event aggregation

This MSC does not include support for nested threads.

Nested threading is out of scope for this proposal and would be the subject of a different MSC. A m.thread event can only reference events that do not have a rel_type

[
  {
    "event_id": "ev1",
    // ...
  },
  {
    "event_id": "ev2",
    // ...
    "m.relates_to": {
      "rel_type": "m.thread",
      "event_id": "ev1",
      "is_falling_back": true,
      "m.in_reply_to": {
          "event_id": "ev1"
      }
    }
  },
  {
    "event_id": "ev3",
    // ...
    "m.relates_to": {
      "rel_type": "m.annotation",
      "event_id": "ev1",
      "key": "✅"
    }
  }
]

Given the above list of events, only ev1 would be a valid target for an m.thread relation event.

Servers should reject attempts to send events with invalid thread relations via the Client-Server API with an HTTP 400 status code and a M_UNKNOWN error code. Events received over federation should always be accepted without checking the validity of the relations as it would break the extensibility of this proposal in a future MSC.

This means that events with invalid thread relations can make their way into the network, either due by malicious activity or buggy implementation. If a client receives such events, it should hide them as soon as it can determine for certain that the associated event is not a valid target.

Servers are expected to not filter out invalid m.thread relations from the results when serving endpoints that deal with message relations. Clients that call those endpoints should be aware that they may return events with invalid relations, and deal with them appropriately.

Client considerations

Sending m.thread before fully implementing threads

There will be clients that will not or can't support threads. Whether this is a deliberate choice or because the system bridges to a platform that does not support threads, there are a number of steps developer of those systems can take to ensure continuity of conversation in the ecosystem.

Clients that do not offer a threading UI should behave as follows when replying, for best interaction with those that do. They should set the m.in_reply_to part as usual, and then add on "rel_type": "m.thread" and "event_id": "$thread_root", copying $thread_root from the replied-to event.

If the m.thread relation type is not present in an incoming event, it should be treated as not being part of the thread. For example, if a client has a separate area for displaying threads, clients can render the event in the main room timeline as a rich reply that will open and highlight the event in the thread context when clicked.

When replying to the following event, a client that does not support threads should copy in rel_type and event_id properties in their reply mixin.

{
  // ...
  "m.relates_to": {
    "rel_type": "m.thread",
    "event_id": "ev1",
    "is_falling_back": false,
    "m.in_reply_to": {
      "event_id": "$event_target"
    }
  }
}

Alternatives

"Threading as rooms", building on m.in_reply_to, and MSC2836 are the main alternatives here.

It is also worth noting that relations in this MSC could be expressed using the scalable relation format described in MSC3051.

Threads as rooms

Threads as rooms could provide full server-side APIs for navigating trees of events, and could be considered an extension of this MSC for scenarios which require that capability

"Threads as rooms" is the idea that each thread could just get its own Matrix room.

Advantages to "Threads as rooms" include:

  • May be simpler for client implementations
  • Restricting events visibility as the room creator
  • Ability to create read-only threads

Disadvantages include:

  • Access control, membership, history visibility, room versions etc needs to be synced between the thread-room and the parent room
  • Harder to control lifetime of threads in the context of the parent room if they're completely split off
  • Clients which aren't aware of them are going to fill up with a lot of rooms.
  • Bridging to non-threaded chat systems is trickier as you may have to splice together rooms

Threads via m.in_reply_to

The rationale for using a new relation type instead of building on m.in_reply_to is to re-use the event relationship APIs provided by MSC2675. The MSC3267 definition of m.reference relationships could be updated to mention threads (perhaps by using the key field from MSC2677 as the thread ID), but it is clearer to define a new relation type. It is unclear what impact this would have on MSC3267, but that is unimplemented by clients.

A big advantage of relations over rich replies is that they can be server-side aggregated. It means that a client is not bound to download the entire history of a room to have a comprehensive list of events being part of a thread.

Threads via serverside traversal of relationships MSC2836

Advantages include:

  • Fits other use cases than instant messaging
  • Simple possible API shape to implement threading in a useful way

Disadvantages include:

  • Relationships are queried using /event_relationships which is outside the bounds of the /sync API so lacks the nice things /sync gives you (live updates). That being said, the event will come down /sync, you just may not have the context required to see parents/siblings/children.
  • Threads can be of arbitrary width (unlimited direct replies to a single message) and depth (unlimited chain of replies) which complicates UI design when you just want "simple" threading.
  • Does not consider use cases like editing or reactions

Security considerations

None

Unstable prefix

Clients and servers should use list of unstable prefixes listed below while this MSC has not been included in a spec release.

  • io.element.thread should be used in place of m.thread as relation type
  • io.element.thread should be used in place of m.thread as a capability entry
  • io.element.relation_senders should be used in place of related_by_senders in the RoomEventFilter
  • io.element.relation_types should be used in place of related_by_rel_types in the RoomEventFilter
  • io.element.show_reply should be used in place of is_falling_back

While this MSC is considered stable, but not in a released version of the specification, clients should look for org.matrix.msc3440.stable as an unstable feature flag as part of the /versions API response to determine server support for the stable identifiers.

Note from the SCT: This MSC was previously slated for v1.3, however ultimately it was decided to move it to a later release.

Dependencies

This MSC builds on MSC2674, MSC2675, MSC3567 and, MSC3676 (which at the time of writing have not yet been accepted into the spec).

Footnotes

  1. It might seem like this could cause a loop where each latest event then includes another bundled aggregation with latest events in it, but this not possible since nested threading is not supported. This MSC does not limit the allowed other relations to bundle; but, at the time of writing, the only known relations are: m.replace (from MSC2676), m.annotation (from MSC2677), and m.reference (from MSC3267). ↩