MSC4023: Thread ID for 2nd order-relation

proposals/4023-unsigned-thread-id.md

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+# MSC4023: Thread ID for second-order relation
+
+Events that have a non-thread relation to other events (eg. reactions) are
+considered to be in the thread that their parent event is in.
+[MSC3981](https://github.com/matrix-org/matrix-spec-proposals/pull/3981)
+defines a way to recursively load such relations with the `/relations` API
+but for such events, it is impossible for a client to know what thread the
+event is in when it arrives from `/sync` unless it already has, or fetches,
+the parent event. This means performing another client-server API request to
+fetch the parent event which is inefficient.
+
+This proposal wants to reduce the amount of work required for clients to partition
+events with certainty.
+
+## Proposal
+
+Whenever a server returns events in the client-server API, it adds a `thread_id`
+property to the `unsigned` field of the event with the ID of the event's thread root
+as defined in MSC3440. This applies to any endpoint that returns events including
+`/sync` and `/messages`.
+
+```jsonc
+{
+  "unsigned": {
+    "thread_id": "$event_id"
+  }
+}
+```
+
+All events that are not part of a thread should fill the `thread_id` property with
+the special value `main` – as defined in MSC3771.
+
+If a server does not have the first-order event, the unsigned `thread_id` property
+should be filled with a `null` value.
+
+## Potential issues
+
+### Database query performances
+
+This could have performance implications on the server side, requiring more work
+to be performed when fetching events in a room.
+
+### Missing first-order relation
+
+It is possible that a server will have the second-order event, but not have the
+first-order event (eg, it has received a reaction over federation, but has not
+yet received the event being reacted to).
+
+## Alternatives
+
+If "[MSC3051: A scalable relation format](https://github.com/matrix-org/matrix-spec-proposals/pull/3051)"
+were to be adopted, second-order relations could specify their thread directly as one of the relationships,
+in which case this MSC would become unnecessary. However, that MSC is a longer term evolution of the protocol
+and will take time for the ecosystem to adopt. This MSC is intended as a much shorter term fix.
+
+## 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 `thread_id`.
+
+## Dependencies
+
+This MSC does not have dependencies