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(Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar"> + <i class="fa fa-search"></i> + </button> + </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/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/matrix-org/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/matrix-org/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/matrix-org/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>The 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>The "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><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; 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