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>
1 files changed, 7 insertions, 9 deletions
diff --git a/docs/development/room-dag-concepts.md b/docs/development/room-dag-concepts.md
index 5eed72bec6..cbc7cf2949 100644
--- a/docs/development/room-dag-concepts.md
+++ b/docs/development/room-dag-concepts.md
@@ -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
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