Threads: The base (#1254)
* Spec MSC3440: Threading (just the base) Other threading MSCs to follow * Spec MSC3856: Threads list API * Spec MSC3715: Add`dir` to `/relations` * changelog * Apply suggestions from code review Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com> * Update changelogs/client_server/newsfragments/1254.feature Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com> Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>release/v1.4
Travis Ralston
2 years ago
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Add threading via `m.thread` relations, as per [MSC3440](https://github.com/matrix-org/matrix-spec-proposals/pull/3440), [MSC3816](https://github.com/matrix-org/matrix-spec-proposals/pull/3816), [MSC3856](https://github.com/matrix-org/matrix-spec-proposals/pull/3856), and [MSC3715](https://github.com/matrix-org/matrix-spec-proposals/pull/3715).
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---
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type: module
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---
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### Threading
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{{% added-in v="1.4" %}}
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Threads allow users to visually branch their conversations in a room. Typically mostly used
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when a room is discussing multiple topics, threads provide more organisation of communication
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that traditional [rich replies](#rich-replies) can't always offer.
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Clients SHOULD render threads differently to regular messages or replies in the timeline, such
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as by providing some context to what is going on in the thread but keeping the full conversation
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history behind a disclosure.
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Threads are established using a `rel_type` of `m.thread` and reference the *thread root* (the
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first event in a thread). It is not possible to create a thread from an event with a `rel_type`,
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which includes not being able to nest threads. All conversation in a thread reference the thread
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root instead of the most recent message, unlike rich reply chains.
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As a worked example, the following represents a thread and how it'd be formed:
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```json
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{
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// irrelevant fields excluded
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"type": "m.room.message",
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"event_id": "$alice_hello",
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"sender": "@alice:example.org",
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"content": {
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"msgtype": "m.text",
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"body": "Hello world! How are you?"
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}
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}
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```
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```json
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{
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// irrelevant fields excluded
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"type": "m.room.message",
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"event_id": "$bob_hello",
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"sender": "@bob:example.org",
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"content": {
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"m.relates_to": {
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"rel_type": "m.thread",
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"event_id": "$alice_hello"
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},
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"msgtype": "m.text",
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"body": "I'm doing okay, thank you! How about yourself?"
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}
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}
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```
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```json
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{
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// irrelevant fields excluded
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"type": "m.room.message",
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"event_id": "$alice_reply",
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"sender": "@alice:example.org",
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"content": {
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"m.relates_to": {
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"rel_type": "m.thread",
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"event_id": "$alice_hello" // note: always references the *thread root*
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},
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"msgtype": "m.text",
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"body": "I'm doing great! Thanks for asking."
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}
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}
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```
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As shown, any event without a `rel_type` can become a thread root by simply referencing it
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using an `m.thread` relationship.
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#### Fallback for unthreaded clients
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Clients which understand how to work with threads should simply do so, however clients which
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might not be aware of threads (due to age or scope) might not be able to helpfully represent
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the conversation history to its users.
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To work around this, events sent by clients which understand threads include [rich reply](#rich-replies)
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metadata to attempt to form a reply chain representation of the conversation. This representation
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is not ideal for heavily threaded rooms, but allows for users to have context as to what is
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being discussed with respect to other messages in the room.
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This representation is achieved by merging the two relationships and setting a new `is_falling_back`
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flag to `true`.
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```json
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// within an event's content...
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"m.relates_to": {
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// The m.thread relationship structure
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"rel_type": "m.thread",
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"event_id": "$root",
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// The rich reply structure
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"m.in_reply_to": {
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// The most recent message known to the client in the thread.
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// This should be something with a high chance of being rendered by the other client,
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// such as an `m.room.message` event.
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"event_id": "$target"
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},
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// A flag to denote that this is a thread with reply fallback
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"is_falling_back": true
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}
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```
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For `m.room.message` events represented this way, no [reply fallback](#fallbacks-for-rich-replies)
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is specified. This allows thread-aware clients to discard the `m.in_reply_to` object entirely
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when `is_falling_back` is `true`.
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{{% boxes/note %}}
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Clients which are acutely aware of threads (they do not render threads, but are otherwise
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aware of the feature existing in the spec) can treat rich replies to an event with a `rel_type`
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of `m.thread` as a threaded reply, for conversation continuity on the threaded client's side.
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To do this, copy the `event_id` (thread root) from the event being replied to, add the
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`m.in_reply_to` metadata, and add `is_falling_back: true` to `m.relates_to`.
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{{% /boxes/note %}}
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#### Replies within threads
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In the [fallback for unthreaded clients](#fallback-for-unthreaded-clients) section, a new
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`is_falling_back` flag is added to `m.relates_to`. This flag defaults to `false` when not
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provided, which also allows a threaded message to contain a reply itself.
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Aside from `is_falling_back` being `false` (or not specified), the fallback for unthreaded
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clients is used to create a reply within a thread: clients should render the event accordingly.
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#### Server behaviour
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##### Validation of `m.thread` relationships
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Servers SHOULD reject client requests which attempt to start a thread off an event with a
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`rel_type`. If the client attempts to target an event which already has an `m.thread`,
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`m.reference`, or any other `rel_type` then it should receive a HTTP 400 error response
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with appropriate error message, as per the [standard error response](#standard-error-response)
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structure.
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{{% boxes/note %}}
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A specific error code is not currently available for this case: servers should use `M_UNKNOWN`
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alongside the HTTP 400 status code.
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{{% /boxes/note %}}
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##### Server-side aggregation of `m.thread` relationships
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Given threads always reference the thread root, an event can have multiple "child" events which
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then form the thread itself. These events should be [aggregated](#aggregations) by the server.
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The aggregation for threads includes some information about the user's participation in the thread,
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the approximate number of events in the thread (as known to the server), and the most recent event
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in the thread (topologically). This is then bundled into the event as `m.thread`:
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```json
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{
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"event_id": "$root_event",
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// irrelevant fields not shown
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"unsigned": {
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"m.relations": {
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"m.thread": {
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"latest_event": {
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// A serialized copy of the latest event in the thread.
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// Some fields are not shown here for brevity.
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"event_id": "$message",
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"sender": "@alice:example.org",
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"room_id": "!room:example.org",
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"type": "m.room.message",
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"content": {
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"msgtype": "m.text",
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"body": "Woo! Threads!"
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}
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},
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"count": 7,
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"current_user_participated": true
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}
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}
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}
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}
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```
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`latest_event` is the most recent event (topologically to the server) in the thread sent by an
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un-[ignored user](#ignoring-users).
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Note that any bundled aggregations on `latest_event` should also be present. The server should be
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careful to avoid loops, though loops are not currently possible due to `m.thread` not being possible
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to target an event with a `rel_type` already.
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`count` is simply the number of events using `m.thread` as a `rel_type` pointing to the target event.
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It does not include events sent by [ignored users](#ignoring-users).
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`current_user_participated` is `true` when the authenticated user is either:
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1. The `sender` of the event receiving the bundle (they sent the thread root).
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2. The `sender` of an event which references the thread root with a `rel_type` of `m.thread`.
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#### Querying threads in a room
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Clients looking to get all the events in a thread can use
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[`GET /relations/{threadRootId}/m.thread`](#get_matrixclientv1roomsroomidrelationseventidreltype),
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however getting all threads in a room is done through a dedicated API:
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{{% http-api spec="client-server" api="threads_list" %}}
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@ -0,0 +1,135 @@
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# Copyright 2022 The Matrix.org Foundation C.I.C.
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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swagger: '2.0'
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info:
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title: "Matrix Client-Server Threads List API"
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version: "1.0.0"
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host: localhost:8008
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schemes:
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- https
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- http
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basePath: /_matrix/client/v1
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consumes:
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- application/json
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produces:
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- application/json
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securityDefinitions:
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$ref: definitions/security.yaml
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paths:
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"/rooms/{roomId}/threads":
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get:
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x-addedInMatrixVersion: "1.4"
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summary: Retrieve a list of threads in a room, with optional filters.
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description: |-
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Paginates over the thread roots in a room, ordered by the `latest_event` of each thread root
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in its bundle.
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operationId: getThreadRoots
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security:
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- accessToken: []
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parameters:
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- in: path
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type: string
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name: roomId
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description: The room ID where the thread roots are located.
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required: true
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x-example: "!room:example.org"
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- in: query
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type: string
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name: include
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enum: [all, participated]
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description: |-
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Optional (default `all`) flag to denote which thread roots are of interest to the caller.
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When `all`, all thread roots found in the room are returned. When `participated`, only
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thread roots for threads the user has [participated in](/client-server-api/#server-side-aggreagtion-of-mthread-relationships)
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will be returned.
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x-example: "all"
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- in: query
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type: integer
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name: limit
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description: |-
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Optional limit for the maximum number of thread roots to include per response. Must be an integer
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greater than zero.
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Servers should apply a default value, and impose a maximum value to avoid resource exhaustion.
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x-example: 20
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- in: query
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type: string
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name: from
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description: |-
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A pagination token from a previous result. When not provided, the server starts paginating from
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the most recent event visible to the user (as per history visibility rules; topologically).
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x-example: "next_batch_token"
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responses:
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200:
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description: |-
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A portion of the available thread roots in the room, based on the filter criteria.
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examples:
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application/json: {
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"chunk": [{ "$ref": "../../event-schemas/examples/m.room.message$m.text.yaml" }],
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"next_batch": "next_batch_token"
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}
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schema:
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type: object
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properties:
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chunk:
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type: array
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description: |-
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The thread roots, ordered by the `latest_event` in each event's aggregation bundle. All events
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returned include bundled [aggregations](/client-server-api/#aggregations).
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If the thread root event was sent by an [ignored user](/client-server-api/#ignoring-users), the
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event is returned redacted to the caller. This is to simulate the same behaviour of a client doing
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aggregation locally on the thread.
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items:
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$ref: "definitions/client_event.yaml"
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next_batch:
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type: string
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description: |-
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A token to supply to `from` to keep paginating the responses. Not present when there are
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no further results.
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required: [chunk]
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403:
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description: |-
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The user cannot view or peek on the room. A meaningful `errcode`
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and description error text will be returned. Example reasons for rejection are:
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- The room is not set up for peeking.
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- The user has been banned from the room.
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- The room does not exist.
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examples:
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application/json: {
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"errcode": "M_FORBIDDEN",
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"error": "You are not allowed to view this room."
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}
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schema:
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"$ref": "definitions/errors/error.yaml"
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400:
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description: |-
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The request was invalid in some way. A meaningful `errcode`
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and description error text will be returned. Example reasons for rejection are:
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- The `from` token is unknown to the server.
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examples:
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application/json: {
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"errcode": "M_INVALID_PARAM",
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"error": "Unknown pagination token"
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}
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schema:
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"$ref": "definitions/errors/error.yaml"
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429:
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description: This request was rate-limited.
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schema:
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"$ref": "definitions/errors/rate_limited.yaml"
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tags:
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- Threads
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