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Room DAG concepts

Edges

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 +prev_events. A subsequent event points back at a previous event.

A (oldest) <---- B <---- C (most recent)
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Depth and stream ordering

Events are normally sorted by (topological_ordering, stream_ordering) where +topological_ordering is just depth. In other words, we first sort by depth +and then tie-break based on stream_ordering. depth is incremented as new +messages are added to the DAG. Normally, stream_ordering is an auto +incrementing integer, but backfilled events start with stream_ordering=-1 and decrement.

/sync returns things in the order they arrive at the server (stream_ordering).
/messages (and /backfill in the federation API) return them in the order determined by the event graph (topological_ordering, stream_ordering).

The general idea is that, if you're following a room in real-time (i.e. +/sync), 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. /messages), you want to see the best +representation of the state of the room as others were seeing it at the time.

Forward extremity

Most-recent-in-time events in the DAG which are not referenced by any other events' prev_events yet.

The forward extremities of a room are used as the prev_events when the next event is sent.

Backwards 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.

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).

Outliers

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.

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.

State groups

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 event_id -> state +mapping), which is very space inefficient when state doesn't change, we +instead assign each different set of state a "state group" and then have +mappings of event_id -> state_group and state_group -> state.

Stage group edges

TODO: state_group_edges is a further optimization... +notes from @Azrenbeth, https://pastebin.com/seUGVGeT

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