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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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Eric Eastwood 2021-12-03 18:25:04 -06:00 committed by GitHub
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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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docs/development/room-dag-concepts.md
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@ -38,16 +38,15 @@ Most-recent-in-time events in the DAG which are not referenced by any other even
The forward extremities of a room are used as the `prev_events` when the next event is sent.
## Backwards extremity
## Backward extremity
The current marker of where we have backfilled up to and will generally be the
oldest-in-time events we know of in the DAG.
`prev_events` of the oldest-in-time events we have in the DAG. This gives a starting point when
backfilling history.
This is an event where we haven't fetched all of the `prev_events` for.
Once we have fetched all of its `prev_events`, it's unmarked as a backwards
extremity (although we may have formed new backwards extremities from the prev
events during the backfilling process).
When we persist a non-outlier event, we clear it as a backward extremity and set
all of its `prev_events` as the new backward extremities if they aren't already
persisted in the `events` table.
## Outliers
@ -56,8 +55,7 @@ We mark an event as an `outlier` when we haven't figured out the state for the
room at that point in the DAG yet.
We won't *necessarily* have the `prev_events` of an `outlier` in the database,
but it's entirely possible that we *might*. The status of whether we have all of
the `prev_events` is marked as a [backwards extremity](#backwards-extremity).
but it's entirely possible that we *might*.
For example, when we fetch the event auth chain or state for a given event, we
mark all of those claimed auth events as outliers because we haven't done the