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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 >
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< li > Incremental < code > /sync?since=xxx< / code > returns things in the order they arrive at the server
(< code > stream_ordering< / code > ).< / li >
< li > Initial < code > /sync< / code > , < 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 >
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< / 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 >
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< 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. They are " floating" events that we haven't
yet correlated to the DAG.< / p >
< p > Outliers typically arise when we fetch the auth chain or state for a given
event. When that happens, we just grab the events in the state/auth chain,
without calculating the state at those events, or backfilling their
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< code > prev_events< / code > . Since we don't have the state at any events fetched in that
way, we mark them as outliers.< / p >
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< p > So, typically, we won't have the < code > prev_events< / code > of an < code > outlier< / code > in the database,
(though it's entirely possible that we < em > might< / em > have them for some other
reason). Other things that make outliers different from regular events:< / p >
< ul >
< li >
< p > We don't have state for them, so there should be no entry in
< code > event_to_state_groups< / code > for an outlier. (In practice this isn't always
the case, though I'm not sure why: see https://github.com/matrix-org/synapse/issues/12201).< / p >
< / li >
< li >
< p > We don't record entries for them in the < code > event_edges< / code > ,
< code > event_forward_extremeties< / code > or < code > event_backward_extremities< / code > tables.< / p >
< / li >
< / ul >
< p > Since outliers are not tied into the DAG, they do not normally form part of the
timeline sent down to clients via < code > /sync< / code > or < code > /messages< / code > ; however there is an
exception:< / p >
< h3 id = "out-of-band-membership-events" > < a class = "header" href = "#out-of-band-membership-events" > Out-of-band membership events< / a > < / h3 >
< p > A special case of outlier events are some membership events for federated rooms
that we aren't full members of. For example:< / p >
< ul >
< li > invites received over federation, before we join the room< / li >
< li > < em > rejections< / em > for said invites< / li >
< li > knock events for rooms that we would like to join but have not yet joined.< / li >
< / ul >
< p > In all the above cases, we don't have the state for the room, which is why they
are treated as outliers. They are a bit special though, in that they are
proactively sent to clients via < code > /sync< / code > .< / p >
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< h2 id = "forward-extremity" > < a class = "header" href = "#forward-extremity" > Forward extremity< / a > < / h2 >
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< p > Most-recent-in-time events in the DAG which are not referenced by any other
events' < code > prev_events< / code > yet. (In this definition, outliers, rejected events, and
soft-failed events don't count.)< / p >
< p > The forward extremities of a room (or at least, a subset of them, if there are
more than ten) are used as the < code > prev_events< / code > when the next event is sent.< / p >
< p > The " current state" of a room (ie: the state which would be used if we
generated a new event) is, therefore, the resolution of the room states
at each of the forward extremities.< / p >
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< h2 id = "backward-extremity" > < a class = "header" href = "#backward-extremity" > Backward extremity< / a > < / h2 >
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< p > The current marker of where we have backfilled up to and will generally be the
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< code > prev_events< / code > of the oldest-in-time events we have in the DAG. This gives a starting point when
backfilling history.< / p >
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< p > Note that, unlike forward extremities, we typically don't have any backward
extremity events themselves in the database - or, if we do, they will be " outliers" (see
above). Either way, we don't expect to have the room state at a backward extremity.< / p >
< p > When we persist a non-outlier event, if it was previously a backward extremity,
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 as non-outliers. This
therefore keeps the backward extremities up-to-date.< / p >
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< 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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