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< h1 id = "room-dag-concepts" > < a class = "header" href = "#room-dag-concepts" > Room DAG concepts< / a > < / h1 >
< h2 id = "edges" > < a class = "header" href = "#edges" > Edges< / a > < / h2 >
< p > The word " edge" comes from graph theory lingo. An edge is just a connection
between two events. In Synapse, we connect events by specifying their
< code > prev_events< / code > . A subsequent event points back at a previous event.< / p >
< pre > < code > A (oldest) < ---- B < ---- C (most recent)
< / code > < / pre >
< h2 id = "depth-and-stream-ordering" > < a class = "header" href = "#depth-and-stream-ordering" > Depth and stream ordering< / a > < / h2 >
< p > Events are normally sorted by < code > (topological_ordering, stream_ordering)< / code > where
< code > topological_ordering< / code > is just < code > depth< / code > . In other words, we first sort by < code > depth< / code >
and then tie-break based on < code > stream_ordering< / code > . < code > depth< / code > is incremented as new
messages are added to the DAG. Normally, < code > stream_ordering< / code > is an auto
incrementing integer, but backfilled events start with < code > stream_ordering=-1< / code > and decrement.< / p >
< hr / >
< ul >
< li > < code > /sync< / code > returns things in the order they arrive at the server (< code > stream_ordering< / code > ).< / li >
< li > < code > /messages< / code > (and < code > /backfill< / code > in the federation API) return them in the order determined by the event graph < code > (topological_ordering, stream_ordering)< / code > .< / li >
< / ul >
< p > The general idea is that, if you're following a room in real-time (i.e.
< code > /sync< / code > ), you probably want to see the messages as they arrive at your server,
rather than skipping any that arrived late; whereas if you're looking at a
historical section of timeline (i.e. < code > /messages< / code > ), you want to see the best
representation of the state of the room as others were seeing it at the time.< / p >
< h2 id = "forward-extremity" > < a class = "header" href = "#forward-extremity" > Forward extremity< / a > < / h2 >
< p > Most-recent-in-time events in the DAG which are not referenced by any other events' < code > prev_events< / code > yet.< / p >
< p > The forward extremities of a room are used as the < code > prev_events< / code > when the next event is sent.< / p >
< h2 id = "backward-extremity" > < a class = "header" href = "#backward-extremity" > Backward extremity< / a > < / h2 >
< p > The current marker of where we have backfilled up to and will generally be the
< code > prev_events< / code > of the oldest-in-time events we have in the DAG. This gives a starting point when
backfilling history.< / p >
< p > When we persist a non-outlier event, we clear it as a backward extremity and set
all of its < code > prev_events< / code > as the new backward extremities if they aren't already
persisted in the < code > events< / code > table.< / p >
< h2 id = "outliers" > < a class = "header" href = "#outliers" > Outliers< / a > < / h2 >
< p > We mark an event as an < code > outlier< / code > when we haven't figured out the state for the
room at that point in the DAG yet.< / p >
< p > We won't < em > necessarily< / em > have the < code > prev_events< / code > of an < code > outlier< / code > in the database,
but it's entirely possible that we < em > might< / em > .< / p >
< p > 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
state calculation ourself.< / p >
< h2 id = "state-groups" > < a class = "header" href = "#state-groups" > State groups< / a > < / h2 >
< p > For every non-outlier event we need to know the state at that event. Instead of
storing the full state for each event in the DB (i.e. a < code > event_id -> state< / code >
mapping), which is < em > very< / em > space inefficient when state doesn't change, we
instead assign each different set of state a " state group" and then have
mappings of < code > event_id -> state_group< / code > and < code > state_group -> state< / code > .< / p >
< h3 id = "stage-group-edges" > < a class = "header" href = "#stage-group-edges" > Stage group edges< / a > < / h3 >
< p > TODO: < code > state_group_edges< / code > is a further optimization...
notes from @Azrenbeth, https://pastebin.com/seUGVGeT< / p >
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