diff --git a/proposals/3567-optional-from-on-messages.md b/proposals/3567-optional-from-on-messages.md new file mode 100644 index 00000000..31b2d247 --- /dev/null +++ b/proposals/3567-optional-from-on-messages.md @@ -0,0 +1,67 @@ +# MSC3567: Allow requesting events from the start/end of the room history + +It can be useful to request the latest events in a room directly without calling +`/_matrix/client/v3/sync` first to fetch the room state. Some use-cases include: + +* Requesting events using a different filter after receiving a `/sync` response. +* A client which does not need to fully sync an account, but wishes to inspect a + specific room's history (perhaps for exporting or auditing). + + +## Proposal + +The `from` field on the [`/_matrix/client/v3/rooms/{roomId}/messages`](https://spec.matrix.org/v1.1/client-server-api/#get_matrixclientv3roomsroomidmessages) +becomes optional. If it is not provided, the homeserver shall return a list of +messages from the first or last (per the value of the `dir` parameter) visible +event in the room history for the requesting user. + +Note that Synapse already implements this, but it is not spec-compliant. It is +known to be used by Element Android [^1] and Element Web, and there are other +use-cases involving threads [^2], which shows real-world usage that this would +be valuable. + +Ideally this would not be necessary and the `prev_batch` token received from +calling `/sync` could be provided as the pagination token to `/messages`, but this +will not work if you `/sync` with a filter that excludes a given class of event +(such as threaded replies), and all the events taking place in a room are of that +class. This will result in your `/sync` not returning an update for that room, +which means that your most recent `prev_batch` token precedes all the excluded +events. Trying to back-paginate from `prev_batch` using `/messages` will not +result in seeing the excluded events. + + +## Potential issues + +None. + + +## Alternatives + +The alternative is today's status quo: a client must first make a request to +`/_matrix/client/v3/sync` and then follow-up that request with queries to +`/_matrix/client/v3/rooms/{roomId}/messages`. This is clunky if the client is +going to throw away most of the information received from the `/sync` request. + +The behavior also seems undefined if a different `filter` parameter is provided +for the call to `/sync` compared to the one used for `/messages`. + + +## Security considerations + +None. + + +## Unstable prefix + +Since this is modifying the endpoint to support not including a field, no unstable +prefix is necessary. + + +## Dependencies + +N/A + +[^1]: https://github.com/matrix-org/synapse/issues/5538 + +[^2]: In order to list all threads in a room without pulling the history locally +it uses `/messages` to push the filtering onto the homeserver. See https://github.com/matrix-org/matrix-js-sdk/pull/2065