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