mirror of
https://github.com/element-hq/synapse.git
synced 2024-12-21 03:42:55 +03:00
296 lines
29 KiB
HTML
296 lines
29 KiB
HTML
|
<!DOCTYPE HTML>
|
||
|
|
<html lang="en" class="sidebar-visible no-js light">
|
||
|
|
<head>
|
||
|
|
<!-- Book generated using mdBook -->
|
||
|
|
<meta charset="UTF-8">
|
||
|
|
<title>Room DAG concepts - Synapse</title>
|
||
|
|
<!-- Custom HTML head -->
|
||
|
|
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
|
||
|
|
<meta name="description" content="">
|
||
|
|
<meta name="viewport" content="width=device-width, initial-scale=1">
|
||
|
|
<meta name="theme-color" content="#ffffff" />
|
||
|
|
|
||
|
|
<link rel="icon" href="../favicon.svg">
|
||
|
|
<link rel="shortcut icon" href="../favicon.png">
|
||
|
|
<link rel="stylesheet" href="../css/variables.css">
|
||
|
|
<link rel="stylesheet" href="../css/general.css">
|
||
|
|
<link rel="stylesheet" href="../css/chrome.css">
|
||
|
|
<link rel="stylesheet" href="../css/print.css" media="print">
|
||
|
|
<!-- Fonts -->
|
||
|
|
<link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
|
||
|
|
<link rel="stylesheet" href="../fonts/fonts.css">
|
||
|
|
<!-- Highlight.js Stylesheets -->
|
||
|
|
<link rel="stylesheet" href="../highlight.css">
|
||
|
|
<link rel="stylesheet" href="../tomorrow-night.css">
|
||
|
|
<link rel="stylesheet" href="../ayu-highlight.css">
|
||
|
|
|
||
|
|
<!-- Custom theme stylesheets -->
|
||
|
|
<link rel="stylesheet" href="../docs/website_files/table-of-contents.css">
|
||
|
|
<link rel="stylesheet" href="../docs/website_files/remove-nav-buttons.css">
|
||
|
|
<link rel="stylesheet" href="../docs/website_files/indent-section-headers.css">
|
||
|
|
<link rel="stylesheet" href="../docs/website_files/version-picker.css">
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<!-- Provide site root to javascript -->
|
||
|
|
<script type="text/javascript">
|
||
|
|
var path_to_root = "../";
|
||
|
|
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
|
||
|
|
</script>
|
||
|
|
|
||
|
|
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
||
|
|
<script type="text/javascript">
|
||
|
|
try {
|
||
|
|
var theme = localStorage.getItem('mdbook-theme');
|
||
|
|
var sidebar = localStorage.getItem('mdbook-sidebar');
|
||
|
|
if (theme.startsWith('"') && theme.endsWith('"')) {
|
||
|
|
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
|
||
|
|
}
|
||
|
|
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
|
||
|
|
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
|
||
|
|
}
|
||
|
|
} catch (e) { }
|
||
|
|
</script>
|
||
|
|
|
||
|
|
<!-- Set the theme before any content is loaded, prevents flash -->
|
||
|
|
<script type="text/javascript">
|
||
|
|
var theme;
|
||
|
|
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
||
|
|
if (theme === null || theme === undefined) { theme = default_theme; }
|
||
|
|
var html = document.querySelector('html');
|
||
|
|
html.classList.remove('no-js')
|
||
|
|
html.classList.remove('light')
|
||
|
|
html.classList.add(theme);
|
||
|
|
html.classList.add('js');
|
||
|
|
</script>
|
||
|
|
|
||
|
|
<!-- Hide / unhide sidebar before it is displayed -->
|
||
|
|
<script type="text/javascript">
|
||
|
|
var html = document.querySelector('html');
|
||
|
|
var sidebar = 'hidden';
|
||
|
|
if (document.body.clientWidth >= 1080) {
|
||
|
|
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
||
|
|
sidebar = sidebar || 'visible';
|
||
|
|
}
|
||
|
|
html.classList.remove('sidebar-visible');
|
||
|
|
html.classList.add("sidebar-" + sidebar);
|
||
|
|
</script>
|
||
|
|
|
||
|
|
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
|
||
|
|
<div class="sidebar-scrollbox">
|
||
|
|
<ol class="chapter"><li class="chapter-item expanded affix "><li class="part-title">Introduction</li><li class="chapter-item expanded "><a href="../welcome_and_overview.html">Welcome and Overview</a></li><li class="chapter-item expanded affix "><li class="part-title">Setup</li><li class="chapter-item expanded "><a href="../setup/installation.html">Installation</a></li><li class="chapter-item expanded "><a href="../postgres.html">Using Postgres</a></li><li class="chapter-item expanded "><a href="../reverse_proxy.html">Configuring a Reverse Proxy</a></li><li class="chapter-item expanded "><a href="../setup/forward_proxy.html">Configuring a Forward/Outbound Proxy</a></li><li class="chapter-item expanded "><a href="../turn-howto.html">Configuring a Turn Server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../setup/turn/coturn.html">coturn TURN server</a></li><li class="chapter-item expanded "><a href="../setup/turn/eturnal.html">eturnal TURN server</a></li></ol></li><li class="chapter-item expanded "><a href="../delegate.html">Delegation</a></li><li class="chapter-item expanded affix "><li class="part-title">Upgrading</li><li class="chapter-item expanded "><a href="../upgrade.html">Upgrading between Synapse Versions</a></li><li class="chapter-item expanded affix "><li class="part-title">Usage</li><li class="chapter-item expanded "><a href="../federate.html">Federation</a></li><li class="chapter-item expanded "><a href="../usage/configuration/index.html">Configuration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../usage/configuration/config_documentation.html">Configuration Manual</a></li><li class="chapter-item expanded "><a href="../usage/configuration/homeserver_sample_config.html">Homeserver Sample Config File</a></li><li class="chapter-item expanded "><a href="../usage/configuration/logging_sample_config.html">Logging Sample Config File</a></li><li class="chapter-item expanded "><a href="../structured_logging.html">Structured Logging</a></li><li class="chapter-item expanded "><a href="../templates.html">Templates</a></li><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/index.html">User Authentication</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/single_sign_on/index.html">Single-Sign On</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../openid.html">OpenID Connect</a></li><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/single_sign_on/saml.html">SAML</a></li><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/single_sign_on/cas.html">CAS</a></li><li class="chapter-item expanded "><a href="../sso_mapping_providers.html">SSO Mapping Providers</a></li></ol></li><li class="chapter-item expanded "><a href="../password_auth_providers.html">Password Auth Providers</a></li><li class="chapter-item expanded "><a href="../jwt.html">JSON Web Tokens</a></li><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/refresh_tokens.html">Refresh Tokens</a></li></ol></li><li class="chapter-item expanded "><a href="../CAPTCHA_SETUP.html">Registration Captcha</a></li><li class="chapter-item expanded "><a href="../application_services.html">Application Services</a></li><li class="chapter-item expanded "><a href="../server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="../consent_tracking.html">Consent Tracking</a></li><li class="chapter-item expanded "><a href="../user_directory.html">User Directory</a></li><li class="chapter-item expanded "><a href="../message_retention_policies.html">Message Retention Policies</a></li><li class="chapter-item expanded "><a href="../modules/index.html">Pluggable Modules</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../modules/writing_a_module.html">Writing a module</a></li><li><ol class="section"><li class="chapter-item expanded "
|
||
|
|
</div>
|
||
|
|
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
|
||
|
|
</nav>
|
||
|
|
|
||
|
|
<div id="page-wrapper" class="page-wrapper">
|
||
|
|
|
||
|
|
<div class="page">
|
||
|
|
<div id="menu-bar-hover-placeholder"></div>
|
||
|
|
<div id="menu-bar" class="menu-bar sticky bordered">
|
||
|
|
<div class="left-buttons">
|
||
|
|
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
|
||
|
|
<i class="fa fa-bars"></i>
|
||
|
|
</button>
|
||
|
|
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
|
||
|
|
<i class="fa fa-paint-brush"></i>
|
||
|
|
</button>
|
||
|
|
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
||
|
|
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
|
||
|
|
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
|
||
|
|
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
|
||
|
|
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
|
||
|
|
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
|
||
|
|
</ul>
|
||
|
|
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
|
||
|
|
<i class="fa fa-search"></i>
|
||
|
|
</button>
|
||
|
|
<div class="version-picker">
|
||
|
|
<div class="dropdown">
|
||
|
|
<div class="select">
|
||
|
|
<span></span>
|
||
|
|
<i class="fa fa-chevron-down"></i>
|
||
|
|
</div>
|
||
|
|
<input type="hidden" name="version">
|
||
|
|
<ul class="dropdown-menu">
|
||
|
|
<!-- Versions will be added dynamically in version-picker.js -->
|
||
|
|
</ul>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h1 class="menu-title">Synapse</h1>
|
||
|
|
|
||
|
|
<div class="right-buttons">
|
||
|
|
<a href="../print.html" title="Print this book" aria-label="Print this book">
|
||
|
|
<i id="print-button" class="fa fa-print"></i>
|
||
|
|
</a>
|
||
|
|
<a href="https://github.com/element-hq/synapse" title="Git repository" aria-label="Git repository">
|
||
|
|
<i id="git-repository-button" class="fa fa-github"></i>
|
||
|
|
</a>
|
||
|
|
<a href="https://github.com/element-hq/synapse/edit/develop/docs/development/room-dag-concepts.md" title="Suggest an edit" aria-label="Suggest an edit">
|
||
|
|
<i id="git-edit-button" class="fa fa-edit"></i>
|
||
|
|
</a>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<div id="search-wrapper" class="hidden">
|
||
|
|
<form id="searchbar-outer" class="searchbar-outer">
|
||
|
|
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
|
||
|
|
</form>
|
||
|
|
<div id="searchresults-outer" class="searchresults-outer hidden">
|
||
|
|
<div id="searchresults-header" class="searchresults-header"></div>
|
||
|
|
<ul id="searchresults">
|
||
|
|
</ul>
|
||
|
|
</div>
|
||
|
|
</div>
|
||
|
|
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
|
||
|
|
<script type="text/javascript">
|
||
|
|
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
|
||
|
|
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
|
||
|
|
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
|
||
|
|
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
|
||
|
|
});
|
||
|
|
</script>
|
||
|
|
|
||
|
|
<div id="content" class="content">
|
||
|
|
<main>
|
||
|
|
<!-- Page table of contents -->
|
||
|
|
<div class="sidetoc">
|
||
|
|
<nav class="pagetoc"></nav>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<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>Incremental <code>/sync?since=xxx</code> returns things in the order they arrive at the server
|
||
|
|
(<code>stream_ordering</code>).</li>
|
||
|
|
<li>Initial <code>/sync</code>, <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>
|
||
|
|
<script type="text/javascript" src="../docs/website_files/version-picker.js"></script>
|
||
|
|
<script type="text/javascript" src="../docs/website_files/version.js"></script>
|
||
|
|
</body>
|
||
|
|
</html>
|