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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="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 +<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 "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> +<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 "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> +<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> + + </main> + + <nav class="nav-wrapper" aria-label="Page navigation"> + <!-- Mobile navigation buttons --> + <a rel="prev" href="../development/cas.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + <a rel="next" href="../auth_chain_difference_algorithm.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + <div style="clear: both"></div> + </nav> + </div> + </div> + + <nav class="nav-wide-wrapper" aria-label="Page navigation"> + <a rel="prev" href="../development/cas.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + <a rel="next" href="../auth_chain_difference_algorithm.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + </nav> + + </div> + + <script type="text/javascript"> + window.playground_copyable = true; + </script> + <script src="../elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> + <script src="../mark.min.js" type="text/javascript" charset="utf-8"></script> + <script src="../searcher.js" type="text/javascript" charset="utf-8"></script> + <script src="../clipboard.min.js" type="text/javascript" charset="utf-8"></script> + <script src="../highlight.js" type="text/javascript" charset="utf-8"></script> + <script src="../book.js" type="text/javascript" charset="utf-8"></script> + + <!-- Custom JS scripts --> + <script type="text/javascript" src="../docs/website_files/table-of-contents.js"></script> + </body> +</html> \ No newline at end of file |