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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) &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="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 -&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> - - </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