Initial MSC3855 draft.

Author: clokep

proposals/3855-threads-list-api.md

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+# MSC3855: Threads List API
+
+An endpoint specific to listing the threads in a room is proposed to solve two
+issues:
+
+1. Clients wish to display threads by the most recently active.
+2. Clients wish to display a list of threads the user is interested in.
+
+It is currently difficult for clients to sort threads by the most recently
+responded to. Clients use the `/messages` API with a filter of
+`"related_by_rel_types": ["m.thread"]` to fetch the list of threads in a room. This
+returns the root thread events in topological order of those events (either
+forwards or backwards depending on the `dir` parameter).
+
+Each event also includes bundled aggregation, which will include the latest
+event in each thread.
+
+In order to sort threads by the latest event in that thread clients must
+paginate through all of the threads in the room, inspect the latest event from
+the bundled aggregations and attempt to sort them. This can require many round
+trips to the server and is wasteful for both the client and server.
+
+Additionally, even with all of the threads a client is not able to accurately
+sort the threads since they lack the proper topological ordering of events. (The
+closest they can get is by using `age` or `origin_server_ts`, but this is not
+correct.)
+
+For the second problem, it is currently not possible for a client to query for
+threads that the user has participated in (as defined in
+[MSC3440](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/3440-threading-via-relations.md#event-format)).
+A client could add the user's MXID to the filter, e.g. `"related_by_senders":["@alice:example.com"]`,
+but this misses threads where the user sent the root message and has not yet replied.
+
+
+## Proposal
+
+### Client-Server endpoint
+
+A new endpoint is proposed to query for threads in a room. This endpoint requires
+authentication and is subject to rate-limiting. This endpoint includes
+[bundled aggregations](https://spec.matrix.org/v1.3/client-server-api/#aggregations)
+in the response.
+
+The returned threads are ordered by the most recently updated thread.
+
+#### Request format
+
+```
+GET /_matrix/client/v1/rooms/{roomId}/threads
+```
+
+Query Parameters:
+
+* **`filter`**: `enum`
+
+  Whether to include all threads in the room or only threads which the user has
+  participated in, meaning that the user has created the root event of the thread
+  or have created an event with a `m.thread` relation targeting the root.
+
+  One of `[all participated]`. Defaults to `all`.
+
+  XXX Rename this to something besides `filter`.
+* **`from`**: `string`
+
+  The token to start returning events from. This token can be obtained from a
+  `prev_batch` or `next_batch` token returned by the `/sync` endpoint, or from
+  an `end` token returned by a previous request to this endpoint.
+
+  If it is not provided, the homeserver shall return a list of threads from the
+  last visible event in the room history for the requesting user.
+* **`limit`**: Optional: a client-defined limit to the maximum
+  number of rooms to return per page. Must an integer greater than zero.
+
+  Server implementations should impose a maximum value to avoid resource
+  exhaustion.
+* **`to`**: `string`
+
+  The token to stop returning events at. This token can be obtained from a
+  `prev_batch` or `next_batch` token returned by the `/sync` endpoint, or from
+  an `end` token returned by a previous request to this endpoint.
+
+#### Response format
+
+* **`chunk`**: [`[ClientEvent]`](https://spec.matrix.org/v1.3/client-server-api/#room-event-format) **Required**
+
+  A list of room of events which are the root event of threads. Each event includes
+  bundled aggregations. The order is chronological by the latest event in that thread.
+* **`end`**: `string`
+
+  A token corresponding to the end of `chunk`. This token can be passed back to
+  this endpoint to request further events.
+
+  If no further events are available (either because we have reached the start
+  of the timeline, or because the user does not have permission to see any more
+  events), this property is omitted from the response.
+* **`start`**: `string` **Required**
+
+  A token corresponding to the start of `chunk`. This will be the same as the
+  value given in `from`.
+
+#### Example request:
+
+```
+GET /_matrix/client/v1/rooms/%21ol19s%3Ableecker.street/threads?
+    limit=25&
+    filter=participated
+```
+
+#### Example response:
+
+```json
+{
+  "chunk": [ClientEvent],
+  "end": "...",
+  "start": "..."
+}
+```
+
+### MSC3440 Filtering
+
+This MSC deprecates the [event filters added in MSC3440](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/3440-threading-via-relations.md#fetch-all-threads-in-a-room)
+(`related_by_rel_types` and `related_by_senders`) as the only known use-case is
+more efficiently solved by this MSC.
+
+## Potential issues
+
+None forseen.
+
+## Alternatives
+
+Additional parameters could be added to the `/messages` API to control the ordering
+of the returned results. This would likely not be compatible with all the other
+options available on that endpoint.
+
+Keeping this a separate API also gives the possibility of additional threads-specific
+filtering in the future.
+
+## Security considerations
+
+As with other endpoints that accept a `limit`, homeservers should apply a hard
+server-side maximum.
+
+## Unstable prefix
+
+The client-server API will be: `/_matrix/client/unstable/org.matrix.msc3855/rooms/{roomId}/threads`
+
+## Dependencies
+
+N/A