mirror of
https://github.com/element-hq/synapse.git
synced 2024-12-22 12:44:30 +03:00
338 lines
34 KiB
HTML
338 lines
34 KiB
HTML
|
<!DOCTYPE HTML>
|
||
|
|
<html lang="en" class="sidebar-visible no-js light">
|
||
|
|
<head>
|
||
|
|
<!-- Book generated using mdBook -->
|
||
|
|
<meta charset="UTF-8">
|
||
|
|
<title>Streams - 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/writ
|
||
|
|
</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/synapse_architecture/streams.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>
|
||
|
|
|
||
|
|
<h2 id="streams"><a class="header" href="#streams">Streams</a></h2>
|
||
|
|
<p>Synapse has a concept of "streams", which are roughly described in <a href="https://github.com/element-hq/synapse/blob/develop/synapse/storage/util/id_generators.py"><code>id_generators.py</code></a>.
|
||
|
|
Generally speaking, streams are a series of notifications that something in Synapse's database has changed that the application might need to respond to.
|
||
|
|
For example:</p>
|
||
|
|
<ul>
|
||
|
|
<li>The events stream reports new events (PDUs) that Synapse creates, or that Synapse accepts from another homeserver.</li>
|
||
|
|
<li>The account data stream reports changes to users' <a href="https://spec.matrix.org/v1.7/client-server-api/#client-config">account data</a>.</li>
|
||
|
|
<li>The to-device stream reports when a device has a new <a href="https://spec.matrix.org/v1.7/client-server-api/#send-to-device-messaging">to-device message</a>.</li>
|
||
|
|
</ul>
|
||
|
|
<p>See <a href="https://github.com/element-hq/synapse/blob/develop/synapse/replication/tcp/streams/__init__.py"><code>synapse.replication.tcp.streams</code></a> for the full list of streams.</p>
|
||
|
|
<p>It is very helpful to understand the streams mechanism when working on any part of Synapse that needs to respond to changes—especially if those changes are made by different workers.
|
||
|
|
To that end, let's describe streams formally, paraphrasing from the docstring of <a href="https://github.com/element-hq/synapse/blob/a719b703d9bd0dade2565ddcad0e2f3a7a9d4c37/synapse/storage/util/id_generators.py#L96"><code>AbstractStreamIdGenerator</code></a>.</p>
|
||
|
|
<h3 id="definition"><a class="header" href="#definition">Definition</a></h3>
|
||
|
|
<p>A stream is an append-only log <code>T1, T2, ..., Tn, ...</code> of facts<sup class="footnote-reference"><a href="#1">1</a></sup> which grows over time.
|
||
|
|
Only "writers" can add facts to a stream, and there may be multiple writers.</p>
|
||
|
|
<p>Each fact has an ID, called its "stream ID".
|
||
|
|
Readers should only process facts in ascending stream ID order.</p>
|
||
|
|
<p>Roughly speaking, each stream is backed by a database table.
|
||
|
|
It should have a <code>stream_id</code> (or similar) bigint column holding stream IDs, plus additional columns as necessary to describe the fact.
|
||
|
|
Typically, a fact is expressed with a single row in its backing table.<sup class="footnote-reference"><a href="#2">2</a></sup>
|
||
|
|
Within a stream, no two facts may have the same stream_id.</p>
|
||
|
|
<blockquote>
|
||
|
|
<p><em>Aside</em>. Some additional notes on streams' backing tables.</p>
|
||
|
|
<ol>
|
||
|
|
<li>Rich would like to <a href="https://github.com/matrix-org/synapse/issues/13456">ditch the backing tables</a>.</li>
|
||
|
|
<li>The backing tables may have other uses.
|
||
|
|
> For example, the events table serves backs the events stream, and is read when processing new events.
|
||
|
|
> But old rows are read from the table all the time, whenever Synapse needs to lookup some facts about an event.</li>
|
||
|
|
<li>Rich suspects that sometimes the stream is backed by multiple tables, so the stream proper is the union of those tables.</li>
|
||
|
|
</ol>
|
||
|
|
</blockquote>
|
||
|
|
<p>Stream writers can "reserve" a stream ID, and then later mark it as having being completed.
|
||
|
|
Stream writers need to track the completion of each stream fact.
|
||
|
|
In the happy case, completion means a fact has been written to the stream table.
|
||
|
|
But unhappy cases (e.g. transaction rollback due to an error) also count as completion.
|
||
|
|
Once completed, the rows written with that stream ID are fixed, and no new rows
|
||
|
|
will be inserted with that ID.</p>
|
||
|
|
<h3 id="current-stream-id"><a class="header" href="#current-stream-id">Current stream ID</a></h3>
|
||
|
|
<p>For any given stream reader (including writers themselves), we may define a per-writer current stream ID:</p>
|
||
|
|
<blockquote>
|
||
|
|
<p>A current stream ID <em>for a writer W</em> is the largest stream ID such that
|
||
|
|
all transactions added by W with equal or smaller ID have completed.</p>
|
||
|
|
</blockquote>
|
||
|
|
<p>Similarly, there is a "linear" notion of current stream ID:</p>
|
||
|
|
<blockquote>
|
||
|
|
<p>A "linear" current stream ID is the largest stream ID such that
|
||
|
|
all facts (added by any writer) with equal or smaller ID have completed.</p>
|
||
|
|
</blockquote>
|
||
|
|
<p>Because different stream readers A and B learn about new facts at different times, A and B may disagree about current stream IDs.
|
||
|
|
Put differently: we should think of stream readers as being independent of each other, proceeding through a stream of facts at different rates.</p>
|
||
|
|
<p>The above definition does not give a unique current stream ID, in fact there can
|
||
|
|
be a range of current stream IDs. Synapse uses both the minimum and maximum IDs
|
||
|
|
for different purposes. Most often the maximum is used, as its generally
|
||
|
|
beneficial for workers to advance their IDs as soon as possible. However, the
|
||
|
|
minimum is used in situations where e.g. another worker is going to wait until
|
||
|
|
the stream advances past a position.</p>
|
||
|
|
<p><strong>NB.</strong> For both senses of "current", that if a writer opens a transaction that never completes, the current stream ID will never advance beyond that writer's last written stream ID.</p>
|
||
|
|
<p>For single-writer streams, the per-writer current ID and the linear current ID are the same.
|
||
|
|
Both senses of current ID are monotonic, but they may "skip" or jump over IDs because facts complete out of order.</p>
|
||
|
|
<p><em>Example</em>.
|
||
|
|
Consider a single-writer stream which is initially at ID 1.</p>
|
||
|
|
<table><thead><tr><th>Action</th><th>Current stream ID</th><th>Notes</th></tr></thead><tbody>
|
||
|
|
<tr><td></td><td>1</td><td></td></tr>
|
||
|
|
<tr><td>Reserve 2</td><td>1</td><td></td></tr>
|
||
|
|
<tr><td>Reserve 3</td><td>1</td><td></td></tr>
|
||
|
|
<tr><td>Complete 3</td><td>1</td><td>current ID unchanged, waiting for 2 to complete</td></tr>
|
||
|
|
<tr><td>Complete 2</td><td>3</td><td>current ID jumps from 1 -> 3</td></tr>
|
||
|
|
<tr><td>Reserve 4</td><td>3</td><td></td></tr>
|
||
|
|
<tr><td>Reserve 5</td><td>3</td><td></td></tr>
|
||
|
|
<tr><td>Reserve 6</td><td>3</td><td></td></tr>
|
||
|
|
<tr><td>Complete 5</td><td>3</td><td></td></tr>
|
||
|
|
<tr><td>Complete 4</td><td>5</td><td>current ID jumps 3->5, even though 6 is pending</td></tr>
|
||
|
|
<tr><td>Complete 6</td><td>6</td><td></td></tr>
|
||
|
|
</tbody></table>
|
||
|
|
<h3 id="multi-writer-streams"><a class="header" href="#multi-writer-streams">Multi-writer streams</a></h3>
|
||
|
|
<p>There are two ways to view a multi-writer stream.</p>
|
||
|
|
<ol>
|
||
|
|
<li>Treat it as a collection of distinct single-writer streams, one
|
||
|
|
for each writer.</li>
|
||
|
|
<li>Treat it as a single stream.</li>
|
||
|
|
</ol>
|
||
|
|
<p>The single stream (option 2) is conceptually simpler, and easier to represent (a single stream id).
|
||
|
|
However, it requires each reader to know about the entire set of writers, to ensures that readers don't erroneously advance their current stream position too early and miss a fact from an unknown writer.
|
||
|
|
In contrast, multiple parallel streams (option 1) are more complex, requiring more state to represent (map from writer to stream id).
|
||
|
|
The payoff for doing so is that readers can "peek" ahead to facts that completed on one writer no matter the state of the others, reducing latency.</p>
|
||
|
|
<p>Note that a multi-writer stream can be viewed in both ways.
|
||
|
|
For example, the events stream is treated as multiple single-writer streams (option 1) by the sync handler, so that events are sent to clients as soon as possible.
|
||
|
|
But the background process that works through events treats them as a single linear stream.</p>
|
||
|
|
<p>Another useful example is the cache invalidation stream.
|
||
|
|
The facts this stream holds are instructions to "you should now invalidate these cache entries".
|
||
|
|
We only ever treat this as a multiple single-writer streams as there is no important ordering between cache invalidations.
|
||
|
|
(Invalidations are self-contained facts; and the invalidations commute/are idempotent).</p>
|
||
|
|
<h3 id="writing-to-streams"><a class="header" href="#writing-to-streams">Writing to streams</a></h3>
|
||
|
|
<p>Writers need to track:</p>
|
||
|
|
<ul>
|
||
|
|
<li>track their current position (i.e. its own per-writer stream ID).</li>
|
||
|
|
<li>their facts currently awaiting completion.</li>
|
||
|
|
</ul>
|
||
|
|
<p>At startup,</p>
|
||
|
|
<ul>
|
||
|
|
<li>the current position of that writer can be found by querying the database (which suggests that facts need to be written to the database atomically, in a transaction); and</li>
|
||
|
|
<li>there are no facts awaiting completion.</li>
|
||
|
|
</ul>
|
||
|
|
<p>To reserve a stream ID, call <a href="https://www.postgresql.org/docs/current/functions-sequence.html"><code>nextval</code></a> on the appropriate postgres sequence.</p>
|
||
|
|
<p>To write a fact to the stream: insert the appropriate rows to the appropriate backing table.</p>
|
||
|
|
<p>To complete a fact, first remove it from your map of facts currently awaiting completion.
|
||
|
|
Then, if no earlier fact is awaiting completion, the writer can advance its current position in that stream.
|
||
|
|
Upon doing so it should emit an <code>RDATA</code> message<sup class="footnote-reference"><a href="#3">3</a></sup>, once for every fact between the old and the new stream ID.</p>
|
||
|
|
<h3 id="subscribing-to-streams"><a class="header" href="#subscribing-to-streams">Subscribing to streams</a></h3>
|
||
|
|
<p>Readers need to track the current position of every writer.</p>
|
||
|
|
<p>At startup, they can find this by contacting each writer with a <code>REPLICATE</code> message,
|
||
|
|
requesting that all writers reply describing their current position in their streams.
|
||
|
|
Writers reply with a <code>POSITION</code> message.</p>
|
||
|
|
<p>To learn about new facts, readers should listen for <code>RDATA</code> messages and process them to respond to the new fact.
|
||
|
|
The <code>RDATA</code> itself is not a self-contained representation of the fact;
|
||
|
|
readers will have to query the stream tables for the full details.
|
||
|
|
Readers must also advance their record of the writer's current position for that stream.</p>
|
||
|
|
<h1 id="summary"><a class="header" href="#summary">Summary</a></h1>
|
||
|
|
<p>In a nutshell: we have an append-only log with a "buffer/scratchpad" at the end where we have to wait for the sequence to be linear and contiguous.</p>
|
||
|
|
<hr />
|
||
|
|
<div class="footnote-definition" id="1"><sup class="footnote-definition-label">1</sup>
|
||
|
|
<p>we use the word <em>fact</em> here for two reasons.
|
||
|
|
Firstly, the word "event" is already heavily overloaded (PDUs, EDUs, account data, ...) and we don't need to make that worse.
|
||
|
|
Secondly, "fact" emphasises that the things we append to a stream cannot change after the fact.</p>
|
||
|
|
</div>
|
||
|
|
<div class="footnote-definition" id="2"><sup class="footnote-definition-label">2</sup>
|
||
|
|
<p>A fact might be expressed with 0 rows, e.g. if we opened a transaction to persist an event, but failed and rolled the transaction back before marking the fact as completed.
|
||
|
|
In principle a fact might be expressed with 2 or more rows; if so, each of those rows should share the fact's stream ID.</p>
|
||
|
|
</div>
|
||
|
|
<div class="footnote-definition" id="3"><sup class="footnote-definition-label">3</sup>
|
||
|
|
<p>This communication used to happen directly with the writers <a href="../../tcp_replication.html">over TCP</a>;
|
||
|
|
nowadays it's done via Redis's Pubsub.</p>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
</main>
|
||
|
|
|
||
|
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
||
|
|
<!-- Mobile navigation buttons -->
|
||
|
|
<a rel="prev" href="../../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="../../tcp_replication.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="../../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="../../tcp_replication.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>
|