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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 &quot;edge&quot; 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) &lt;---- B &lt;---- 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="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 &quot;floating&quot; 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
<code>prev_events</code>. Since we don't have the state at any events fetched in that
way, we mark them as outliers.</p>
<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>
<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. (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 &quot;current state&quot; 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>
<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>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 &quot;outliers&quot; (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>
<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 -&gt; state</code>
mapping), which is <em>very</em> space inefficient when state doesn't change, we
instead assign each different set of state a &quot;state group&quot; and then have
mappings of <code>event_id -&gt; state_group</code> and <code>state_group -&gt; 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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