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MSC4023: Thread ID for 2nd order-relation #4023

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f34e69e
Create proposal for m.thread_id unsigned field
May 31, 2023
d771c62
Update proposal wording
germain-gg Jun 2, 2023
8699c34
Rename 'm.thread_id' to 'thread_id'
Jun 2, 2023
036666c
Require all events to inform the unsigned thread_id property
Jun 2, 2023
fc9a8c4
Document edge case with missing first-order relation
Jun 2, 2023
541f210
Rewrite some parts of this MSC for clarity and address feedback.
dbkr Dec 6, 2023
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60 changes: 60 additions & 0 deletions proposals/4023-unsigned-thread-id.md
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# MSC4023: Thread ID for 2nd order-relation

[MSC3981](https://github.com/matrix-org/matrix-spec-proposals/pull/3981) defines
a way to recursively load relations in a thread context. However this does not
let clients determine with certainty in what timeline an event coming from a `/sync`
should end up in.

If the related event is unknown to the client, the only way to partition things
correctly is to fetch the related event and confirm whether this event belongs to
a thread or not.

This proposal wants to reduce the amount of work required for clients to partition
events with certainty in a time efficient manner.

## Proposal

All 2nd order-relation in a thread should add a `m.thread_id` property in their
`unsigned` field definition, referencing the thread root, as defined in MSC3440.

```jsonc
{
"m.thread_id": "$event_id"
}
```

Given the following diagram where `Reply1`, `Reply2` and `Reply3` are direct `m.thread`
relations. `Edit` and `Reaction` MUST include `m.thread_id` as defined in this
proposal.

```mermaid
graph TD;
Edit-->Reply3
Reaction-->Reply2
Reply3-->Root
Reply2-->Root
Reply1-->Root
```

## Potential issues

This could have performance implications on the server side, requiring more work
to be performed when fetching events in a room.

## Alternatives

If "[MSC3051: A scalable relation format](https://github.com/matrix-org/matrix-spec-proposals/pull/3051)"
was to be adopted, this MSC would be nulled.
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Why?

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These relations would also carry an m.thread relation, meaning they would already carry the piece of information we're after.
I guess it would still have some relevance for historical events.

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Hmm, maybe I don't think MSC3051 says they must include that information though? At least that wasn't how I read it.

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@tulir tulir May 31, 2023 •
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Then the alternative would be MSC3051 + a MSC that says relations to thread events must include the thread relation too (or adjusting 3051 to require that)

I'd certainly prefer fixing relations instead of building more hacks into the current non-scalable format (although fixing doesn't necessarily have to be MSC3051, it could also be switching to something more like what was originally planned, where each relation type is its own key like m.in_reply_to)

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That was one of the motivating usecases behind MSC3051:

You don't need to look up reply relations in multiple events for edits. The edited event is canonical and can be used standalone, without having to look up the original event to figure out, what was replied to. You can also remove a relation with an edit now. (Useful if you replied to the wrong message or didn't mean to reply to anyone.)

If you don't include the thread id, the event simply isn't in a thread with the relation format. This means you can edit events out of threads or move them between threads and you get rid of the second level lookup. (Also a lot of stuff just makes more sense)

It doesn't spell out reactions specifically, because currently those are assumed to not be possible to edit and threads weren't in the spec (neither were relations), but since the MSC is about including all current relations for an event inside of the event, that is the only thing that makes sense if you combine threads and reactions.

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Indeed, I meant 3051 instead of 4023. Have edited my comment.

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@andybalaam, I think you meant MSC3051 rather than MSC3501?

Thank you, fixed in my comment. Seems like you've had just enough coffee!

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@anoadragon453 anoadragon453 Jun 15, 2023 •
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@andybalaam It's even different from "What message am I a reaction to?" because, crucially, if we receive a receipt for a reaction, we need to act on it even if we don't have the event it is a reaction to (where by act on it I mean consider this thread to be read).

I'm confused why MSC3051 would not solve this use case. Would the reaction not have the following in its m.relations array property:

      {
        "event_id": "$root-of-some-thread",
        "rel_type": "m.thread"
      },

...which would allow you to know whether to mark the thread as read or not?

@uhoreg one key difference between the two is that MSC3051 relies on the sending client to populate the relations, so using that for this purpose would mean that the sending client would add the thread ID, which it could fail to do for whatever reason. On the other hand, this proposal relies on the server to add the thread ID, so would be more reliable. So I'm not entirely sure that MSC3051 is a replacement for this.

Consequently, if the server does not have the first-order event, it won't know to add the unsigned thread_id property to a second-order event.

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This somewhat randomly came up in discussions around stuck notifications. I'm more preferential to considering this MSC (4023) in the short term to fix the problem scope, as asking sending clients to do the right thing will take longer, potentially require the use of smart proxies (for constrained environments), and generally be harder to adopt in the short term.

This MSC will need to outline why MSC3051 is a good solution for a longer term, and that having the server calculate thread-ness in the meantime is best.

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Hopefully my edits clarify this. I haven't put in a great deal about why 3051 is more long term other than, well, it's a change that will literally take a long time to get rolled out. I don't think there's really much more to it?


## Security considerations

No security considerations

## Unstable prefix

While this MSC is not considered stable by the specification, implementations _must_ use
`org.matrix.msc4023.thread_id` in place of `m.thread_id`.

## Dependencies

This MSC does not have dependencies