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Update backward extremity docs to make it clear that it does not indicate whether we have fetched an events' prev_events
(#11469)
Spawning from https://github.com/matrix-org/synapse/pull/9445#discussion_r758958181 Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
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Update section about backward extremities in the room DAG concepts doc to correct the misconception about backward extremities indicating whether we have fetched an events' `prev_events`.
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@ -38,16 +38,15 @@ Most-recent-in-time events in the DAG which are not referenced by any other even
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The forward extremities of a room are used as the `prev_events` when the next event is sent.
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The forward extremities of a room are used as the `prev_events` when the next event is sent.
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## Backwards extremity
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## Backward extremity
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The current marker of where we have backfilled up to and will generally be the
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The current marker of where we have backfilled up to and will generally be the
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oldest-in-time events we know of in the DAG.
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`prev_events` of the oldest-in-time events we have in the DAG. This gives a starting point when
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backfilling history.
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This is an event where we haven't fetched all of the `prev_events` for.
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When we persist a non-outlier event, we clear it as a backward extremity and set
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all of its `prev_events` as the new backward extremities if they aren't already
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Once we have fetched all of its `prev_events`, it's unmarked as a backwards
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persisted in the `events` table.
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extremity (although we may have formed new backwards extremities from the prev
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events during the backfilling process).
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## Outliers
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## Outliers
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@ -56,8 +55,7 @@ We mark an event as an `outlier` when we haven't figured out the state for the
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room at that point in the DAG yet.
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room at that point in the DAG yet.
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We won't *necessarily* have the `prev_events` of an `outlier` in the database,
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We won't *necessarily* have the `prev_events` of an `outlier` in the database,
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but it's entirely possible that we *might*. The status of whether we have all of
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but it's entirely possible that we *might*.
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the `prev_events` is marked as a [backwards extremity](#backwards-extremity).
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For example, when we fetch the event auth chain or state for a given event, we
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For example, when we fetch the event auth chain or state for a given event, we
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mark all of those claimed auth events as outliers because we haven't done the
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mark all of those claimed auth events as outliers because we haven't done the
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