123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504505506507508509510511512513514515516517518519520521522523524525526527528529530531532533534535536537538539540541542543544545 |
- <!DOCTYPE HTML>
- <html lang="en" class="sidebar-visible no-js light">
- <head>
- <!-- Book generated using mdBook -->
- <meta charset="UTF-8">
- <title>Faster remote joins - 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 "><a href="../../modules/spam_checker_callbacks.html">Spam checker callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/third_party_rules_callbacks.html">Third-party rules callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/presence_router_callbacks.html">Presence router callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/account_validity_callbacks.html">Account validity callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/password_auth_provider_callbacks.html">Password auth provider callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/background_update_controller_callbacks.html">Background update controller callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/account_data_callbacks.html">Account data callbacks</a></li><li class="chapter-item expanded "><a href="../../modules/porting_legacy_module.html">Porting a legacy module to the new interface</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../../workers.html">Workers</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../synctl_workers.html">Using synctl with Workers</a></li><li class="chapter-item expanded "><a href="../../systemd-with-workers/index.html">Systemd</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../../usage/administration/index.html">Administration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../usage/administration/admin_api/index.html">Admin API</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../admin_api/account_validity.html">Account Validity</a></li><li class="chapter-item expanded "><a href="../../usage/administration/admin_api/background_updates.html">Background Updates</a></li><li class="chapter-item expanded "><a href="../../admin_api/event_reports.html">Event Reports</a></li><li class="chapter-item expanded "><a href="../../admin_api/experimental_features.html">Experimental Features</a></li><li class="chapter-item expanded "><a href="../../admin_api/media_admin_api.html">Media</a></li><li class="chapter-item expanded "><a href="../../admin_api/purge_history_api.html">Purge History</a></li><li class="chapter-item expanded "><a href="../../admin_api/register_api.html">Register Users</a></li><li class="chapter-item expanded "><a href="../../usage/administration/admin_api/registration_tokens.html">Registration Tokens</a></li><li class="chapter-item expanded "><a href="../../admin_api/room_membership.html">Manipulate Room Membership</a></li><li class="chapter-item expanded "><a href="../../admin_api/rooms.html">Rooms</a></li><li class="chapter-item expanded "><a href="../../admin_api/server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="../../admin_api/statistics.html">Statistics</a></li><li class="chapter-item expanded "><a href="../../admin_api/user_admin_api.html">Users</a></li><li class="chapter-item expanded "><a href="../../admin_api/version_api.html">Server Version</a></li><li class="chapter-item expanded "><a href="../../usage/administration/admin_api/federation.html">Federation</a></li></ol></li><li class="chapter-item expanded "><a href="../../manhole.html">Manhole</a></li><li class="chapter-item expanded "><a href="../../metrics-howto.html">Monitoring</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../usage/administration/monitoring/reporting_homeserver_usage_statistics.html">Reporting Homeserver Usage Statistics</a></li></ol></li><li class="chapter-item expanded "><a href="../../usage/administration/monthly_active_users.html">Monthly Active Users</a></li><li class="chapter-item expanded "><a href="../../usage/administration/understanding_synapse_through_grafana_graphs.html">Understanding Synapse Through Grafana Graphs</a></li><li class="chapter-item expanded "><a href="../../usage/administration/useful_sql_for_admins.html">Useful SQL for Admins</a></li><li class="chapter-item expanded "><a href="../../usage/administration/database_maintenance_tools.html">Database Maintenance Tools</a></li><li class="chapter-item expanded "><a href="../../usage/administration/state_groups.html">State Groups</a></li><li class="chapter-item expanded "><a href="../../usage/administration/request_log.html">Request log format</a></li><li class="chapter-item expanded "><a href="../../usage/administration/admin_faq.html">Admin FAQ</a></li><li class="chapter-item expanded "><div>Scripts</div></li></ol></li><li class="chapter-item expanded "><li class="part-title">Development</li><li class="chapter-item expanded "><a href="../../development/contributing_guide.html">Contributing Guide</a></li><li class="chapter-item expanded "><a href="../../code_style.html">Code Style</a></li><li class="chapter-item expanded "><a href="../../development/reviews.html">Reviewing Code</a></li><li class="chapter-item expanded "><a href="../../development/releases.html">Release Cycle</a></li><li class="chapter-item expanded "><a href="../../development/git.html">Git Usage</a></li><li class="chapter-item expanded "><div>Testing</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../development/demo.html">Demo scripts</a></li></ol></li><li class="chapter-item expanded "><a href="../../opentracing.html">OpenTracing</a></li><li class="chapter-item expanded "><a href="../../development/database_schema.html">Database Schemas</a></li><li class="chapter-item expanded "><a href="../../development/experimental_features.html">Experimental features</a></li><li class="chapter-item expanded "><a href="../../development/dependencies.html">Dependency management</a></li><li class="chapter-item expanded "><div>Synapse Architecture</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../development/synapse_architecture/cancellation.html">Cancellation</a></li><li class="chapter-item expanded "><a href="../../log_contexts.html">Log Contexts</a></li><li class="chapter-item expanded "><a href="../../replication.html">Replication</a></li><li class="chapter-item expanded "><a href="../../tcp_replication.html">TCP Replication</a></li><li class="chapter-item expanded "><a href="../../development/synapse_architecture/faster_joins.html" class="active">Faster remote joins</a></li></ol></li><li class="chapter-item expanded "><a href="../../development/internal_documentation/index.html">Internal Documentation</a></li><li><ol class="section"><li class="chapter-item expanded "><div>Single Sign-On</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../development/saml.html">SAML</a></li><li class="chapter-item expanded "><a href="../../development/cas.html">CAS</a></li></ol></li><li class="chapter-item expanded "><a href="../../development/room-dag-concepts.html">Room DAG concepts</a></li><li class="chapter-item expanded "><div>State Resolution</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../auth_chain_difference_algorithm.html">The Auth Chain Difference Algorithm</a></li></ol></li><li class="chapter-item expanded "><a href="../../media_repository.html">Media Repository</a></li><li class="chapter-item expanded "><a href="../../room_and_user_statistics.html">Room and User Statistics</a></li></ol></li><li class="chapter-item expanded "><div>Scripts</div></li><li class="chapter-item expanded affix "><li class="part-title">Other</li><li class="chapter-item expanded "><a href="../../deprecation_policy.html">Dependency Deprecation Policy</a></li><li class="chapter-item expanded "><a href="../../other/running_synapse_on_single_board_computers.html">Running Synapse on a Single-Board Computer</a></li></ol>
- </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/matrix-org/synapse" title="Git repository" aria-label="Git repository">
- <i id="git-repository-button" class="fa fa-github"></i>
- </a>
- <a href="https://github.com/matrix-org/synapse/edit/develop/docs/development/synapse_architecture/faster_joins.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="how-do-faster-joins-work"><a class="header" href="#how-do-faster-joins-work">How do faster joins work?</a></h1>
- <p>This is a work-in-progress set of notes with two goals:</p>
- <ul>
- <li>act as a reference, explaining how Synapse implements faster joins; and</li>
- <li>record the rationale behind our choices.</li>
- </ul>
- <p>See also <a href="https://github.com/matrix-org/matrix-spec-proposals/pull/3902">MSC3902</a>.</p>
- <p>The key idea is described by <a href="https://github.com/matrix-org/matrix-spec-proposals/pull/3902">MSC706</a>. This allows servers to
- request a lightweight response to the federation <code>/send_join</code> endpoint.
- This is called a <strong>faster join</strong>, also known as a <strong>partial join</strong>. In these
- notes we'll usually use the word "partial" as it matches the database schema.</p>
- <h2 id="overview-processing-events-in-a-partially-joined-room"><a class="header" href="#overview-processing-events-in-a-partially-joined-room">Overview: processing events in a partially-joined room</a></h2>
- <p>The response to a partial join consists of</p>
- <ul>
- <li>the requested join event <code>J</code>,</li>
- <li>a list of the servers in the room (according to the state before <code>J</code>),</li>
- <li>a subset of the state of the room before <code>J</code>,</li>
- <li>the full auth chain of that state subset.</li>
- </ul>
- <p>Synapse marks the room as partially joined by adding a row to the database table
- <code>partial_state_rooms</code>. It also marks the join event <code>J</code> as "partially stated",
- meaning that we have neither received nor computed the full state before/after
- <code>J</code>. This is done by adding a row to <code>partial_state_events</code>.</p>
- <details><summary>DB schema</summary>
- <pre><code>matrix=> \d partial_state_events
- Table "matrix.partial_state_events"
- Column │ Type │ Collation │ Nullable │ Default
- ══════════╪══════╪═══════════╪══════════╪═════════
- room_id │ text │ │ not null │
- event_id │ text │ │ not null │
-
- matrix=> \d partial_state_rooms
- Table "matrix.partial_state_rooms"
- Column │ Type │ Collation │ Nullable │ Default
- ════════════════════════╪════════╪═══════════╪══════════╪═════════
- room_id │ text │ │ not null │
- device_lists_stream_id │ bigint │ │ not null │ 0
- join_event_id │ text │ │ │
- joined_via │ text │ │ │
- matrix=> \d partial_state_rooms_servers
- Table "matrix.partial_state_rooms_servers"
- Column │ Type │ Collation │ Nullable │ Default
- ═════════════╪══════╪═══════════╪══════════╪═════════
- room_id │ text │ │ not null │
- server_name │ text │ │ not null │
- </code></pre>
- <p>Indices, foreign-keys and check constraints are omitted for brevity.</p>
- </details>
- <p>While partially joined to a room, Synapse receives events <code>E</code> from remote
- homeservers as normal, and can create events at the request of its local users.
- However, we run into trouble when we enforce the <a href="https://spec.matrix.org/v1.5/server-server-api/#checks-performed-on-receipt-of-a-pdu">checks on an event</a>.</p>
- <blockquote>
- <ol>
- <li>Is a valid event, otherwise it is dropped. For an event to be valid, it
- must contain a room_id, and it must comply with the event format of that
- room version.</li>
- <li>Passes signature checks, otherwise it is dropped.</li>
- <li>Passes hash checks, otherwise it is redacted before being processed further.</li>
- <li>Passes authorization rules based on the event’s auth events, otherwise it
- is rejected.</li>
- <li><strong>Passes authorization rules based on the state before the event, otherwise
- it is rejected.</strong></li>
- <li><strong>Passes authorization rules based on the current state of the room,
- otherwise it is “soft failed”.</strong></li>
- </ol>
- </blockquote>
- <p>We can enforce checks 1--4 without any problems.
- But we cannot enforce checks 5 or 6 with complete certainty, since Synapse does
- not know the full state before <code>E</code>, nor that of the room.</p>
- <h3 id="partial-state"><a class="header" href="#partial-state">Partial state</a></h3>
- <p>Instead, we make a best-effort approximation.
- While the room is considered partially joined, Synapse tracks the "partial
- state" before events.
- This works in a similar way as regular state:</p>
- <ul>
- <li>The partial state before <code>J</code> is that given to us by the partial join response.</li>
- <li>The partial state before an event <code>E</code> is the resolution of the partial states
- after each of <code>E</code>'s <code>prev_event</code>s.</li>
- <li>If <code>E</code> is rejected or a message event, the partial state after <code>E</code> is the
- partial state before <code>E</code>.</li>
- <li>Otherwise, the partial state after <code>E</code> is the partial state before <code>E</code>, plus
- <code>E</code> itself.</li>
- </ul>
- <p>More concisely, partial state propagates just like full state; the only
- difference is that we "seed" it with an incomplete initial state.
- Synapse records that we have only calculated partial state for this event with
- a row in <code>partial_state_events</code>.</p>
- <p>While the room remains partially stated, check 5 on incoming events to that
- room becomes:</p>
- <blockquote>
- <ol start="5">
- <li>Passes authorization rules based on <strong>the resolution between the partial
- state before <code>E</code> and <code>E</code>'s auth events.</strong> If the event fails to pass
- authorization rules, it is rejected.</li>
- </ol>
- </blockquote>
- <p>Additionally, check 6 is deleted: no soft-failures are enforced.</p>
- <p>While partially joined, the current partial state of the room is defined as the
- resolution across the partial states after all forward extremities in the room.</p>
- <p><em>Remark.</em> Events with partial state are <em>not</em> considered
- <a href="../room-dag-concepts.html#outliers">outliers</a>.</p>
- <h3 id="approximation-error"><a class="header" href="#approximation-error">Approximation error</a></h3>
- <p>Using partial state means the auth checks can fail in a few different ways<sup class="footnote-reference"><a href="#2">1</a></sup>.</p>
- <div class="footnote-definition" id="2"><sup class="footnote-definition-label">1</sup>
- <p>Is this exhaustive?</p>
- </div>
- <ul>
- <li>We may erroneously accept an incoming event in check 5 based on partial state
- when it would have been rejected based on full state, or vice versa.</li>
- <li>This means that an event could erroneously be added to the current partial
- state of the room when it would not be present in the full state of the room,
- or vice versa.</li>
- <li>Additionally, we may have skipped soft-failing an event that would have been
- soft-failed based on full state.</li>
- </ul>
- <p>(Note that the discrepancies described in the last two bullets are user-visible.)</p>
- <p>This means that we have to be very careful when we want to lookup pieces of room
- state in a partially-joined room. Our approximation of the state may be
- incorrect or missing. But we can make some educated guesses. If</p>
- <ul>
- <li>our partial state is likely to be correct, or</li>
- <li>the consequences of our partial state being incorrect are minor,</li>
- </ul>
- <p>then we proceed as normal, and let the resync process fix up any mistakes (see
- below).</p>
- <p>When is our partial state likely to be correct?</p>
- <ul>
- <li>It's more accurate the closer we are to the partial join event. (So we should
- ideally complete the resync as soon as possible.)</li>
- <li>Non-member events: we will have received them as part of the partial join
- response, if they were part of the room state at that point. We may
- incorrectly accept or reject updates to that state (at first because we lack
- remote membership information; later because of compounding errors), so these
- can become incorrect over time.</li>
- <li>Local members' memberships: we are the only ones who can create join and
- knock events for our users. We can't be completely confident in the
- correctness of bans, invites and kicks from other homeservers, but the resync
- process should correct any mistakes.</li>
- <li>Remote members' memberships: we did not receive these in the /send_join
- response, so we have essentially no idea if these are correct or not.</li>
- </ul>
- <p>In short, we deem it acceptable to trust the partial state for non-membership
- and local membership events. For remote membership events, we wait for the
- resync to complete, at which point we have the full state of the room and can
- proceed as normal.</p>
- <h3 id="fixing-the-approximation-with-a-resync"><a class="header" href="#fixing-the-approximation-with-a-resync">Fixing the approximation with a resync</a></h3>
- <p>The partial-state approximation is only a temporary affair. In the background,
- synapse beings a "resync" process. This is a continuous loop, starting at the
- partial join event and proceeding downwards through the event graph. For each
- <code>E</code> seen in the room since partial join, Synapse will fetch </p>
- <ul>
- <li>the event ids in the state of the room before <code>E</code>, via
- <a href="https://spec.matrix.org/v1.5/server-server-api/#get_matrixfederationv1state_idsroomid"><code>/state_ids</code></a>;</li>
- <li>the event ids in the full auth chain of <code>E</code>, included in the <code>/state_ids</code>
- response; and</li>
- <li>any events from the previous two bullets that Synapse hasn't persisted, via
- <a href="https://spec.matrix.org/v1.5/server-server-api/#get_matrixfederationv1stateroomid">`/state</a>.</li>
- </ul>
- <p>This means Synapse has (or can compute) the full state before <code>E</code>, which allows
- Synapse to properly authorise or reject <code>E</code>. At this point ,the event
- is considered to have "full state" rather than "partial state". We record this
- by removing <code>E</code> from the <code>partial_state_events</code> table.</p>
- <p>[<strong>TODO:</strong> Does Synapse persist a new state group for the full state
- before <code>E</code>, or do we alter the (partial-)state group in-place? Are state groups
- ever marked as partially-stated? ]</p>
- <p>This scheme means it is possible for us to have accepted and sent an event to
- clients, only to reject it during the resync. From a client's perspective, the
- effect is similar to a retroactive
- state change due to state resolution---i.e. a "state reset".<sup class="footnote-reference"><a href="#3">2</a></sup></p>
- <div class="footnote-definition" id="3"><sup class="footnote-definition-label">2</sup>
- <p>Clients should refresh caches to detect such a change. Rumour has it that
- sliding sync will fix this.</p>
- </div>
- <p>When all events since the join <code>J</code> have been fully-stated, the room resync
- process is complete. We record this by removing the room from
- <code>partial_state_rooms</code>.</p>
- <h2 id="faster-joins-on-workers"><a class="header" href="#faster-joins-on-workers">Faster joins on workers</a></h2>
- <p>For the time being, the resync process happens on the master worker.
- A new replication stream <code>un_partial_stated_room</code> is added. Whenever a resync
- completes and a partial-state room becomes fully stated, a new message is sent
- into that stream containing the room ID.</p>
- <h2 id="notes-on-specific-cases"><a class="header" href="#notes-on-specific-cases">Notes on specific cases</a></h2>
- <blockquote>
- <p><strong>NB.</strong> The notes below are rough. Some of them are hidden under <code><details></code>
- disclosures because they have yet to be implemented in mainline Synapse.</p>
- </blockquote>
- <h3 id="creating-events-during-a-partial-join"><a class="header" href="#creating-events-during-a-partial-join">Creating events during a partial join</a></h3>
- <p>When sending out messages during a partial join, we assume our partial state is
- accurate and proceed as normal. For this to have any hope of succeeding at all,
- our partial state must contain an entry for each of the (type, state key) pairs
- <a href="https://spec.matrix.org/v1.3/rooms/v10/#authorization-rules">specified by the auth rules</a>:</p>
- <ul>
- <li><code>m.room.create</code></li>
- <li><code>m.room.join_rules</code></li>
- <li><code>m.room.power_levels</code></li>
- <li><code>m.room.third_party_invite</code></li>
- <li><code>m.room.member</code></li>
- </ul>
- <p>The first four of these should be present in the state before <code>J</code> that is given
- to us in the partial join response; only membership events are omitted. In order
- for us to consider the user joined, we must have their membership event. That
- means the only possible omission is the target's membership in an invite, kick
- or ban.</p>
- <p>The worst possibility is that we locally invite someone who is banned according to
- the full state, because we lack their ban in our current partial state. The rest
- of the federation---at least, those who are fully joined---should correctly
- enforce the <a href="https://spec.matrix.org/v1.3/client-server-api/#room-membership">membership transition constraints</a>. So any the erroneous invite should be ignored by fully-joined
- homeservers and resolved by the resync for partially-joined homeservers.</p>
- <p>In more generality, there are two problems we're worrying about here:</p>
- <ul>
- <li>We might create an event that is valid under our partial state, only to later
- find out that is actually invalid according to the full state.</li>
- <li>Or: we might refuse to create an event that is invalid under our partial
- state, even though it would be perfectly valid under the full state.</li>
- </ul>
- <p>However we expect such problems to be unlikely in practise, because</p>
- <ul>
- <li>We trust that the room has sensible power levels, e.g. that bad actors with
- high power levels are demoted before their ban.</li>
- <li>We trust that the resident server provides us up-to-date power levels, join
- rules, etc.</li>
- <li>State changes in rooms are relatively infrequent, and the resync period is
- relatively quick.</li>
- </ul>
- <h4 id="sending-out-the-event-over-federation"><a class="header" href="#sending-out-the-event-over-federation">Sending out the event over federation</a></h4>
- <p><strong>TODO:</strong> needs prose fleshing out.</p>
- <p>Normally: send out in a fed txn to all HSes in the room.
- We only know that some HSes were in the room at some point. Wat do.
- Send it out to the list of servers from the first join.
- <strong>TODO</strong> what do we do here if we have full state?
- If the prev event was created by us, we can risk sending it to the wrong HS. (Motivation: privacy concern of the content. Not such a big deal for a public room or an encrypted room. But non-encrypted invite-only...)
- But don't want to send out sensitive data in other HS's events in this way.</p>
- <p>Suppose we discover after resync that we shouldn't have sent out one our events (not a prev_event) to a target HS. Not much we can do.
- What about if we didn't send them an event but shouldn't've?
- E.g. what if someone joined from a new HS shortly after you did? We wouldn't talk to them.
- Could imagine sending out the "Missed" events after the resync but... painful to work out what they shuld have seen if they joined/left.
- Instead, just send them the latest event (if they're still in the room after resync) and let them backfill.(?)</p>
- <ul>
- <li>Don't do this currently.</li>
- <li>If anyone who has received our messages sends a message to a HS we missed, they can backfill our messages</li>
- <li>Gap: rooms which are infrequently used and take a long time to resync.</li>
- </ul>
- <h3 id="joining-after-a-partial-join"><a class="header" href="#joining-after-a-partial-join">Joining after a partial join</a></h3>
- <p><strong>NB.</strong> Not yet implemented.</p>
- <details>
- <p><strong>TODO:</strong> needs prose fleshing out. Liase with Matthieu. Explain why /send_join
- (Rich was surprised we didn't just create it locally. Answer: to try and avoid
- a join which then gets rejected after resync.)</p>
- <p>We don't know for sure that any join we create would be accepted.
- E.g. the joined user might have been banned; the join rules might have changed in a way that we didn't realise... some way in which the partial state was mistaken.
- Instead, do another partial make-join/send-join handshake to confirm that the join works.</p>
- <ul>
- <li>Probably going to get a bunch of duplicate state events and auth events.... but the point of partial joins is that these should be small. Many are already persisted = good.</li>
- <li>What if the second send_join response includes a different list of reisdent HSes? Could ignore it.
- <ul>
- <li>Could even have a special flag that says "just make me a join", i.e. don't bother giving me state or servers in room. Deffo want the auth chain tho.</li>
- </ul>
- </li>
- <li>SQ: wrt device lists it's a lot safer to ignore it!!!!!</li>
- <li>What if the state at the second join is inconsistent with what we have? Ignore it?</li>
- </ul>
- </details>
- <h3 id="leaving-and-kicks-and-bans-after-a-partial-join"><a class="header" href="#leaving-and-kicks-and-bans-after-a-partial-join">Leaving (and kicks and bans) after a partial join</a></h3>
- <p><strong>NB.</strong> Not yet implemented.</p>
- <details>
- <p>When you're fully joined to a room, to have <code>U</code> leave a room their homeserver
- needs to</p>
- <ul>
- <li>create a new leave event for <code>U</code> which will be accepted by other homeservers,
- and</li>
- <li>send that event <code>U</code> out to the homeservers in the federation.</li>
- </ul>
- <p>When is a leave event accepted? See
- <a href="https://spec.matrix.org/v1.5/rooms/v10/#authorization-rules">v10 auth rules</a>:</p>
- <blockquote>
- <ol start="4">
- <li>If type is m.room.member: [...]
- >
- > 5. If membership is leave:
- >
- > 1. If the sender matches state_key, allow if and only if that user’s current membership state is invite, join, or knock.
- 2. [...]</li>
- </ol>
- </blockquote>
- <p>I think this means that (well-formed!) self-leaves are governed entirely by
- 4.5.1. This means that if we correctly calculate state which says that <code>U</code> is
- invited, joined or knocked and include it in the leave's auth events, our event
- is accepted by checks 4 and 5 on incoming events.</p>
- <blockquote>
- <ol start="4">
- <li>Passes authorization rules based on the event’s auth events, otherwise
- > it is rejected.</li>
- <li>Passes authorization rules based on the state before the event, otherwise
- > it is rejected.</li>
- </ol>
- </blockquote>
- <p>The only way to fail check 6 is if the receiving server's current state of the
- room says that <code>U</code> is banned, has left, or has no membership event. But this is
- fine: the receiving server already thinks that <code>U</code> isn't in the room.</p>
- <blockquote>
- <ol start="6">
- <li>Passes authorization rules based on the current state of the room,
- > otherwise it is “soft failed”.</li>
- </ol>
- </blockquote>
- <p>For the second point (publishing the leave event), the best thing we can do is
- to is publish to all HSes we know to be currently in the room. If they miss that
- event, they might send us traffic in the room that we don't care about. This is
- a problem with leaving after a "full" join; we don't seek to fix this with
- partial joins.</p>
- <p>(With that said: there's nothing machine-readable in the /send response. I don't
- think we can deduce "destination has left the room" from a failure to /send an
- event into that room?)</p>
- <h4 id="can-we-still-do-this-during-a-partial-join"><a class="header" href="#can-we-still-do-this-during-a-partial-join">Can we still do this during a partial join?</a></h4>
- <p>We can create leave events and can choose what gets included in our auth events,
- so we can be sure that we pass check 4 on incoming events. For check 5, we might
- have an incorrect view of the state before an event.
- The only way we might erroneously think a leave is valid is if</p>
- <ul>
- <li>the partial state before the leave has <code>U</code> joined, invited or knocked, but</li>
- <li>the full state before the leave has <code>U</code> banned, left or not present,</li>
- </ul>
- <p>in which case the leave doesn't make anything worse: other HSes already consider
- us as not in the room, and will continue to do so after seeing the leave.</p>
- <p>The remaining obstacle is then: can we safely broadcast the leave event? We may
- miss servers or incorrectly think that a server is in the room. Or the
- destination server may be offline and miss the transaction containing our leave
- event.This should self-heal when they see an event whose <code>prev_events</code> descends
- from our leave.</p>
- <p>Another option we considered was to use federation <code>/send_leave</code> to ask a
- fully-joined server to send out the event on our behalf. But that introduces
- complexity without much benefit. Besides, as Rich put it,</p>
- <blockquote>
- <p>sending out leaves is pretty best-effort currently</p>
- </blockquote>
- <p>so this is probably good enough as-is.</p>
- <h4 id="cleanup-after-the-last-leave"><a class="header" href="#cleanup-after-the-last-leave">Cleanup after the last leave</a></h4>
- <p><strong>TODO</strong>: what cleanup is necessary? Is it all just nice-to-have to save unused
- work?</p>
- </details>
- </main>
- <nav class="nav-wrapper" aria-label="Page navigation">
- <!-- Mobile navigation buttons -->
- <a rel="prev" href="../../tcp_replication.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="../../development/internal_documentation/index.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="../../tcp_replication.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="../../development/internal_documentation/index.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>
|