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MSC3999: Add causal parameter to /timestamp_to_event #3999

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@MadLittleMods MadLittleMods commented Apr 14, 2023

MSC3999: Add causal ?event_id=$abc query parameter to /timestamp_to_event to signal that we want to keep progressing from this event regardless of what timestamp shenanigans are going on.


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@MadLittleMods MadLittleMods changed the title MSCXXXX: Add causal parameter to /timestamp_to_event MSC3999: Add causal parameter to /timestamp_to_event Apr 14, 2023
@MadLittleMods MadLittleMods marked this pull request as ready for review April 14, 2023 11:13
@turt2live turt2live added proposal A matrix spec change proposal client-server Client-Server API kind:maintenance MSC which clarifies/updates existing spec needs-implementation This MSC does not have a qualifying implementation for the SCT to review. The MSC cannot enter FCP. labels Apr 14, 2023
Comment on lines +28 to +30
Instead of only defining a timestamp, we add an optional `event_id` query parameter
which represents a topological spot in the DAG that we can easily determine what comes
before or after from (a casual relationship).
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@MadLittleMods MadLittleMods Apr 17, 2023

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We should clarify that this should be added to the client and federation (server) /timestamp_to_event API's

Comment on lines +28 to +29
Instead of only defining a timestamp, we add an optional `event_id` query parameter
which represents a topological spot in the DAG that we can easily determine what comes
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@MadLittleMods MadLittleMods Apr 19, 2023

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Event ID's are globally unique but we're after a topological position in the DAG which is per-room so the event lookup should be constrained to the room.

Comment on lines +28 to +29
Instead of only defining a timestamp, we add an optional `event_id` query parameter
which represents a topological spot in the DAG that we can easily determine what comes
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What happens when event_id can't be found?

Should the homeserver try to backfill the event from other homeservers?

What error code should we throw if we can't find it in any case?


In less scrupulous scenarios or with bad intentioned actors, these timestamp loops can
occur throughout the room timeline.

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Another case why the causal parameter is useful: Events can occur at the same exact time and it's not possible to get the next one with /timestamp_to_event atm

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If the purpose is to allow Matrix Public Archive to go back through events in the room, why not use /messages and the from/to/start/end fields for pagination?

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