MSC3869: Read event relations with the Widget API

proposals/3869-widgetapi-read-event-relations.md

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+# MSC3869: Read event relations with the Widget API
+
+[MSC2762](https://github.com/matrix-org/matrix-spec-proposals/pull/2762) specifies a Widget API that
+is able to receive events from the client. It supports reading state events from a room and room
+events from a room timeline. It is also possible to provide a filter to only receive selected events.
+
+While the existing APIs are a good fit for receiving live updates and getting state events, it has
+some limitations regarding room events. The client only provides events that it already loaded until
+a request. But for some use cases, the widget needs to have a reliable way to query _all relevant_
+events from a room (ex: have a certain type; belong to a certain application defined grouping).
+
+The polls feature ([MSC3381](https://github.com/matrix-org/matrix-spec-proposals/pull/3381)) uses
+serverside aggregation of message relationships
+([MSC2675](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/2675-aggregations-server.md))
+to solve this use case. Related events reference each other by a `rel_type` and the client-server API
+provides an endpoint to receive server-aggregated relations.
+
+Having this API available to widgets, it could (1) access existing events to e.g. show an enhanced view
+of the polls feature or (2) read own events for a custom use case such as a whiteboard application.
+
+## Proposal
+
+We will add a new interface to the widget API to get all relations of an event with a known event id.
+We won't introduce new capabilities but instead rely on the capabilities that were introduced by
+[MSC2762](https://github.com/matrix-org/matrix-spec-proposals/pull/2762).
+
+The following rules apply:
+
+1. If the requested event-id belongs to a event, and the widget didn't receive the respective
+   `m.receive.event:<event type>` or `m.receive.state_event:<event type>` capability, the widget
+   rejects the request.
+2. The list of related events only include events that the widget has the respective
+   `m.receive.event:<event type>` or `m.receive.state_event:<event type>` capability for. Other
+   events are silently ignored.
+
+To trigger the read, widgets will use a new `fromWidget` request with the action `read_relations`
+which takes the following shape:
+
+```json
+{
+  "api": "fromWidget",
+  "widgetId": "20200827_WidgetExample",
+  "requestid": "generated-id-1234",
+  "action": "read_relations",
+  "data": {
+    "room_id": "!room-id",
+    "event_id": "$event-id",
+    "rel_type": "m.reference",
+    "event_type": "m.room.message",
+    "limit": 50,
+    "from": "from_token",
+    "to": "to_token",
+    "direction": "b"
+  }
+}
+```
+
+Under `data`, all keys are a representation of the
+`_matrix/client/v1/rooms/{roomID}/relations/{event_id}[/{rel_type}[/{event_type}]]` API that was
+introduced by
+([MSC2675](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/2675-aggregations-server.md)).
+This also includes the paging parameters.
+
+The `event_id` parameter is a required parameter that represents the parent event to be read.
+
+The `room_id` parameter specifies the room to look within. When undefined, the client should look in
+the user's currently viewed room.
+
+The `rel_type` parameter specifies the relationship type of child events to search for. If not
+defined, all types will be returned.
+
+The `event_type` parameter specifies the type of child events to search for. If not defined, all
+types will be returned.
+
+The `limit`, `from`, and `to` parameters work like described in
+([MSC2675](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/2675-aggregations-server.md)).
+
+The `direction` parameter is used to specify the direction to search for relations. It has the same
+semantic as defined by ([MSC2675](https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/2675-aggregations-server.md)).
+
+This is an example of a minimal request to get an event from the current room:
+
+```json
+{
+  "api": "fromWidget",
+  "widgetId": "20200827_WidgetExample",
+  "requestid": "generated-id-1234",
+  "action": "read_relations",
+  "data": {
+    "event_id": "$event-id"
+  }
+}
+```
+
+The client SHOULD NOT modify the data of the request. The widget is responsible for producing valid
+events - the client MUST pass through any errors, such as permission errors, to the widget using the
+standard error response in the Widget API.
+
+If the request was successful, the client sends the following response:
+
+```json
+{
+  "api": "fromWidget",
+  "widgetId": "20200827_WidgetExample",
+  "requestid": "generated-id-1234",
+  "action": "create_room",
+  "data": {
+    "event_id": "$event-id"
+  },
+  "response": {
+    "original_event": {
+      "type": "m.room.message",
+      "..."
+    },
+    "chunk": [
+      {
+        "type": "m.room.message",
+        "..."
+      },
+      { "..." }
+    ],
+    "next_batch": "next_batch_token",
+    "prev_batch": "prev_batch_token"
+  }
+}
+```
+
+The `original_event` field contains the event that has the requested event id.
+
+The `chunk` field contains an array of events that are related to the parent event and matches the
+filters and the capabilities. This list might include less events than the specified `limit` due to
+the filter operations.
+
+The `next_batch` field is a cursor that can be used in the `from` or `to` fields to get the next page
+of events. If undefined, there are no more events to receive.
+
+The `prev_batch` field is a cursor that can be used in the `from` or `to` fields to get the previous
+page of events. If undefined, there are no more events to receive.
+
+## Potential issues
+
+In an e2ee room, all the events must be decrypted in the client prior to applying the filters or
+providing them to the widget. This can take a considerable amount of time. The widget should take
+care that it selects a reasonable `limit` to not run into timeouts in the widget transport layer.
+
+## Alternatives
+
+We could also add features to let the client "scroll up the room", i.e. trigger the pagination for a
+room timeline and stick with the original read interfaces. However, this would potentially load a
+lot of unrelated events which slows the read process down. In addition, the client must potentially
+decrypt all the messages in the room before being able to filter them accordingly. Using the relations
+feature, the decryption problem is still present, but the set of events that must be decrypted and
+searched is minimized.
+
+The same limitations would apply if we would consider to provide direct access to the
+`GET /_matrix/client/v3/rooms/{roomId}/messages` endpoint.
+
+## Security considerations
+
+The same considerations as in [MSC2762](https://github.com/matrix-org/matrix-spec-proposals/pull/2762)
+apply. The widget will be able to receive the same set of events, but can just use a different
+approach to request them.
+
+## Unstable prefix
+
+While this MSC is not present in the spec, clients and widgets should:
+
+- Use `org.matrix.msc3869.` in place of `m.` in all new identifiers of this MSC.
+- Use `org.matrix.msc3869.read_relations` in place of `read_relations` for the action type in the
+  `fromWidget` requests.
+- Only call/support the `action`s if an API version of `org.matrix.msc3869` is advertised.