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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="backwards-extremity"><a class="header" href="#backwards-extremity">Backwards extremity</a></h2>
<p>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.</p>
<p>This is an event where we haven't fetched all of the <code>prev_events</code> for.</p>
<p>Once we have fetched all of its <code>prev_events</code>, it's unmarked as a backwards
extremity (although we may have formed new backwards extremities from the prev
events during the backfilling process).</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>. The status of whether we have all of
the <code>prev_events</code> is marked as a <a href="#backwards-extremity">backwards extremity</a>.</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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