diff options
Diffstat (limited to 'dev-docs/develop/modules/federation_sender.html')
| -rw-r--r-- | dev-docs/develop/modules/federation_sender.html | 331 |
1 files changed, 331 insertions, 0 deletions
diff --git a/dev-docs/develop/modules/federation_sender.html b/dev-docs/develop/modules/federation_sender.html new file mode 100644 index 0000000000..2d9dc7e7c9 --- /dev/null +++ b/dev-docs/develop/modules/federation_sender.html @@ -0,0 +1,331 @@ +<!doctype html> +<html class="no-js" lang="en"> + <head><meta charset="utf-8"/> + <meta name="viewport" content="width=device-width,initial-scale=1"/> + <meta name="color-scheme" content="light dark"><meta name="generator" content="Docutils 0.19: https://docutils.sourceforge.io/" /> +<link rel="index" title="Index" href="../genindex.html" /><link rel="search" title="Search" href="../search.html" /><link rel="prev" title="Welcome to the Synapse Developer Documentation!" href="../index.html" /> + + <!-- Generated with Sphinx 6.1.3 and Furo 2023.03.23 --> + <title>Federation Sender - Synapse development documentation</title> + <link rel="stylesheet" type="text/css" href="../_static/pygments.css" /> + <link rel="stylesheet" type="text/css" href="../_static/styles/furo.css?digest=fad236701ea90a88636c2a8c73b44ae642ed2a53" /> + <link rel="stylesheet" type="text/css" href="../_static/styles/furo-extensions.css?digest=30d1aed668e5c3a91c3e3bf6a60b675221979f0e" /> + + + + +<style> + body { + --color-code-background: #f8f8f8; + --color-code-foreground: black; + + } + @media not print { + body[data-theme="dark"] { + --color-code-background: #202020; + --color-code-foreground: #d0d0d0; + + } + @media (prefers-color-scheme: dark) { + body:not([data-theme="light"]) { + --color-code-background: #202020; + --color-code-foreground: #d0d0d0; + + } + } + } +</style></head> + <body> + + <script> + document.body.dataset.theme = localStorage.getItem("theme") || "auto"; + </script> + + +<svg xmlns="http://www.w3.org/2000/svg" style="display: none;"> + <symbol id="svg-toc" viewBox="0 0 24 24"> + <title>Contents</title> + <svg stroke="currentColor" fill="currentColor" stroke-width="0" viewBox="0 0 1024 1024"> + <path d="M408 442h480c4.4 0 8-3.6 8-8v-56c0-4.4-3.6-8-8-8H408c-4.4 0-8 3.6-8 8v56c0 4.4 3.6 8 8 8zm-8 204c0 4.4 3.6 8 8 8h480c4.4 0 8-3.6 8-8v-56c0-4.4-3.6-8-8-8H408c-4.4 0-8 3.6-8 8v56zm504-486H120c-4.4 0-8 3.6-8 8v56c0 4.4 3.6 8 8 8h784c4.4 0 8-3.6 8-8v-56c0-4.4-3.6-8-8-8zm0 632H120c-4.4 0-8 3.6-8 8v56c0 4.4 3.6 8 8 8h784c4.4 0 8-3.6 8-8v-56c0-4.4-3.6-8-8-8zM115.4 518.9L271.7 642c5.8 4.6 14.4.5 14.4-6.9V388.9c0-7.4-8.5-11.5-14.4-6.9L115.4 505.1a8.74 8.74 0 0 0 0 13.8z"/> + </svg> + </symbol> + <symbol id="svg-menu" viewBox="0 0 24 24"> + <title>Menu</title> + <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" + stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather-menu"> + <line x1="3" y1="12" x2="21" y2="12"></line> + <line x1="3" y1="6" x2="21" y2="6"></line> + <line x1="3" y1="18" x2="21" y2="18"></line> + </svg> + </symbol> + <symbol id="svg-arrow-right" viewBox="0 0 24 24"> + <title>Expand</title> + <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" + stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather-chevron-right"> + <polyline points="9 18 15 12 9 6"></polyline> + </svg> + </symbol> + <symbol id="svg-sun" viewBox="0 0 24 24"> + <title>Light mode</title> + <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" + stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" class="feather-sun"> + <circle cx="12" cy="12" r="5"></circle> + <line x1="12" y1="1" x2="12" y2="3"></line> + <line x1="12" y1="21" x2="12" y2="23"></line> + <line x1="4.22" y1="4.22" x2="5.64" y2="5.64"></line> + <line x1="18.36" y1="18.36" x2="19.78" y2="19.78"></line> + <line x1="1" y1="12" x2="3" y2="12"></line> + <line x1="21" y1="12" x2="23" y2="12"></line> + <line x1="4.22" y1="19.78" x2="5.64" y2="18.36"></line> + <line x1="18.36" y1="5.64" x2="19.78" y2="4.22"></line> + </svg> + </symbol> + <symbol id="svg-moon" viewBox="0 0 24 24"> + <title>Dark mode</title> + <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" + stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" class="icon-tabler-moon"> + <path stroke="none" d="M0 0h24v24H0z" fill="none" /> + <path d="M12 3c.132 0 .263 0 .393 0a7.5 7.5 0 0 0 7.92 12.446a9 9 0 1 1 -8.313 -12.454z" /> + </svg> + </symbol> + <symbol id="svg-sun-half" viewBox="0 0 24 24"> + <title>Auto light/dark mode</title> + <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" + stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" class="icon-tabler-shadow"> + <path stroke="none" d="M0 0h24v24H0z" fill="none"/> + <circle cx="12" cy="12" r="9" /> + <path d="M13 12h5" /> + <path d="M13 15h4" /> + <path d="M13 18h1" /> + <path d="M13 9h4" /> + <path d="M13 6h1" /> + </svg> + </symbol> +</svg> + +<input type="checkbox" class="sidebar-toggle" name="__navigation" id="__navigation"> +<input type="checkbox" class="sidebar-toggle" name="__toc" id="__toc"> +<label class="overlay sidebar-overlay" for="__navigation"> + <div class="visually-hidden">Hide navigation sidebar</div> +</label> +<label class="overlay toc-overlay" for="__toc"> + <div class="visually-hidden">Hide table of contents sidebar</div> +</label> + + + +<div class="page"> + <header class="mobile-header"> + <div class="header-left"> + <label class="nav-overlay-icon" for="__navigation"> + <div class="visually-hidden">Toggle site navigation sidebar</div> + <i class="icon"><svg><use href="#svg-menu"></use></svg></i> + </label> + </div> + <div class="header-center"> + <a href="../index.html"><div class="brand">Synapse development documentation</div></a> + </div> + <div class="header-right"> + <div class="theme-toggle-container theme-toggle-header"> + <button class="theme-toggle"> + <div class="visually-hidden">Toggle Light / Dark / Auto color theme</div> + <svg class="theme-icon-when-auto"><use href="#svg-sun-half"></use></svg> + <svg class="theme-icon-when-dark"><use href="#svg-moon"></use></svg> + <svg class="theme-icon-when-light"><use href="#svg-sun"></use></svg> + </button> + </div> + <label class="toc-overlay-icon toc-header-icon no-toc" for="__toc"> + <div class="visually-hidden">Toggle table of contents sidebar</div> + <i class="icon"><svg><use href="#svg-toc"></use></svg></i> + </label> + </div> + </header> + <aside class="sidebar-drawer"> + <div class="sidebar-container"> + + <div class="sidebar-sticky"><a class="sidebar-brand" href="../index.html"> + + + <span class="sidebar-brand-text">Synapse development documentation</span> + +</a><form class="sidebar-search-container" method="get" action="../search.html" role="search"> + <input class="sidebar-search" placeholder="Search" name="q" aria-label="Search"> + <input type="hidden" name="check_keywords" value="yes"> + <input type="hidden" name="area" value="default"> +</form> +<div id="searchbox"></div><div class="sidebar-scroll"><div class="sidebar-tree"> + <p class="caption" role="heading"><span class="caption-text">Contents:</span></p> +<ul class="current"> +<li class="toctree-l1 current current-page"><a class="current reference internal" href="#">Federation Sender</a></li> +</ul> + +</div> +</div> + + </div> + + </div> + </aside> + <div class="main"> + <div class="content"> + <div class="article-container"> + <a href="#" class="back-to-top muted-link"> + <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"> + <path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12z"></path> + </svg> + <span>Back to top</span> + </a> + <div class="content-icon-container"> + +<div class="theme-toggle-container theme-toggle-content"> + <button class="theme-toggle"> + <div class="visually-hidden">Toggle Light / Dark / Auto color theme</div> + <svg class="theme-icon-when-auto"><use href="#svg-sun-half"></use></svg> + <svg class="theme-icon-when-dark"><use href="#svg-moon"></use></svg> + <svg class="theme-icon-when-light"><use href="#svg-sun"></use></svg> + </button> + </div> + <label class="toc-overlay-icon toc-content-icon no-toc" for="__toc"> + <div class="visually-hidden">Toggle table of contents sidebar</div> + <i class="icon"><svg><use href="#svg-toc"></use></svg></i> + </label> + </div> + <article role="main"> + <section id="federation-sender"> +<h1>Federation Sender<a class="headerlink" href="#federation-sender" title="Permalink to this heading">#</a></h1> +<p>The Federation Sender is responsible for sending Persistent Data Units (PDUs) +and Ephemeral Data Units (EDUs) to other homeservers using +the <code class="docutils literal notranslate"><span class="pre">/send</span></code> Federation API.</p> +<h2 class="rubric" id="how-do-pdus-get-sent">How do PDUs get sent?</h2> +<p>The Federation Sender is made aware of new PDUs due to <code class="docutils literal notranslate"><span class="pre">FederationSender.notify_new_events</span></code>. +When the sender is notified about a newly-persisted PDU that originates from this homeserver +and is not an out-of-band event, we pass the PDU to the <code class="docutils literal notranslate"><span class="pre">_PerDestinationQueue</span></code> for each +remote homeserver that is in the room at that point in the DAG.</p> +<h3 class="rubric" id="per-destination-queues">Per-Destination Queues</h3> +<p>There is one <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> per ‘destination’ homeserver. +The <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> maintains the following information about the destination:</p> +<ul class="simple"> +<li><p>whether the destination is currently in <a class="reference internal" href="#catch-up-mode"><span class="xref myst">catch-up mode (see below)</span></a>;</p></li> +<li><p>a queue of PDUs to be sent to the destination; and</p></li> +<li><p>a queue of EDUs to be sent to the destination (not considered in this section).</p></li> +</ul> +<p>Upon a new PDU being enqueued, <code class="docutils literal notranslate"><span class="pre">attempt_new_transaction</span></code> is called to start a new +transaction if there is not already one in progress.</p> +<h3 class="rubric" id="transactions-and-the-transaction-transmission-loop">Transactions and the Transaction Transmission Loop</h3> +<p>Each federation HTTP request to the <code class="docutils literal notranslate"><span class="pre">/send</span></code> endpoint is referred to as a ‘transaction’. +The body of the HTTP request contains a list of PDUs and EDUs to send to the destination.</p> +<p>The <em>Transaction Transmission Loop</em> (<code class="docutils literal notranslate"><span class="pre">_transaction_transmission_loop</span></code>) is responsible +for emptying the queued PDUs (and EDUs) from a <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> by sending +them to the destination.</p> +<p>There can only be one transaction in flight for a given destination at any time. +(Other than preventing us from overloading the destination, this also makes it easier to +reason about because we process events sequentially for each destination. +This is useful for <em>Catch-Up Mode</em>, described later.)</p> +<p>The loop continues so long as there is anything to send. At each iteration of the loop, we:</p> +<ul class="simple"> +<li><p>dequeue up to 50 PDUs (and up to 100 EDUs).</p></li> +<li><p>make the <code class="docutils literal notranslate"><span class="pre">/send</span></code> request to the destination homeserver with the dequeued PDUs and EDUs.</p></li> +<li><p>if successful, make note of the fact that we succeeded in transmitting PDUs up to +the given <code class="docutils literal notranslate"><span class="pre">stream_ordering</span></code> of the latest PDU by</p></li> +<li><p>if unsuccessful, back off from the remote homeserver for some time. +If we have been unsuccessful for too long (when the backoff interval grows to exceed 1 hour), +the in-memory queues are emptied and we enter <a class="reference internal" href="#catch-up-mode"><span class="xref myst"><em>Catch-Up Mode</em>, described below</span></a>.</p></li> +</ul> +<h3 class="rubric" id="catch-up-mode">Catch-Up Mode</h3> +<p>When the <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> has the catch-up flag set, the <em>Catch-Up Transmission Loop</em> +(<code class="docutils literal notranslate"><span class="pre">_catch_up_transmission_loop</span></code>) is used in lieu of the regular <code class="docutils literal notranslate"><span class="pre">_transaction_transmission_loop</span></code>. +(Only once the catch-up mode has been exited can the regular tranaction transmission behaviour +be resumed.)</p> +<p><em>Catch-Up Mode</em>, entered upon Synapse startup or once a homeserver has fallen behind due to +connection problems, is responsible for sending PDUs that have been missed by the destination +homeserver. (PDUs can be missed because the <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> is volatile — i.e. resets +on startup — and it does not hold PDUs forever if <code class="docutils literal notranslate"><span class="pre">/send</span></code> requests to the destination fail.)</p> +<p>The catch-up mechanism makes use of the <code class="docutils literal notranslate"><span class="pre">last_successful_stream_ordering</span></code> column in the +<code class="docutils literal notranslate"><span class="pre">destinations</span></code> table (which gives the <code class="docutils literal notranslate"><span class="pre">stream_ordering</span></code> of the most recent successfully +sent PDU) and the <code class="docutils literal notranslate"><span class="pre">stream_ordering</span></code> column in the <code class="docutils literal notranslate"><span class="pre">destination_rooms</span></code> table (which gives, +for each room, the <code class="docutils literal notranslate"><span class="pre">stream_ordering</span></code> of the most recent PDU that needs to be sent to this +destination).</p> +<p>Each iteration of the loop pulls out 50 <code class="docutils literal notranslate"><span class="pre">destination_rooms</span></code> entries with the oldest +<code class="docutils literal notranslate"><span class="pre">stream_ordering</span></code>s that are greater than the <code class="docutils literal notranslate"><span class="pre">last_successful_stream_ordering</span></code>. +In other words, from the set of latest PDUs in each room to be sent to the destination, +the 50 oldest such PDUs are pulled out.</p> +<p>These PDUs could, in principle, now be directly sent to the destination. However, as an +optimisation intended to prevent overloading destination homeservers, we instead attempt +to send the latest forward extremities so long as the destination homeserver is still +eligible to receive those. +This reduces load on the destination <strong>in aggregate</strong> because all Synapse homeservers +will behave according to this principle and therefore avoid sending lots of different PDUs +at different points in the DAG to a recovering homeserver. +<em>This optimisation is not currently valid in rooms which are partial-state on this homeserver, +since we are unable to determine whether the destination homeserver is eligible to receive +the latest forward extremities unless this homeserver sent those PDUs — in this case, we +just send the latest PDUs originating from this server and skip this optimisation.</em></p> +<p>Whilst PDUs are sent through this mechanism, the position of <code class="docutils literal notranslate"><span class="pre">last_successful_stream_ordering</span></code> +is advanced as normal. +Once there are no longer any rooms containing outstanding PDUs to be sent to the destination +<em>that are not already in the <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> because they arrived since Catch-Up Mode +was enabled</em>, Catch-Up Mode is exited and we return to <code class="docutils literal notranslate"><span class="pre">_transaction_transmission_loop</span></code>.</p> +<h4 class="rubric" id="a-note-on-failures-and-back-offs">A note on failures and back-offs</h4> +<p>If a remote server is unreachable over federation, we back off from that server, +with an exponentially-increasing retry interval. +Whilst we don’t automatically retry after the interval, we prevent making new attempts +until such time as the back-off has cleared. +Once the back-off is cleared and a new PDU or EDU arrives for transmission, the transmission +loop resumes and empties the queue by making federation requests.</p> +<p>If the backoff grows too large (> 1 hour), the in-memory queue is emptied (to prevent +unbounded growth) and Catch-Up Mode is entered.</p> +<p>It is worth noting that the back-off for a remote server is cleared once an inbound +request from that remote server is received (see <code class="docutils literal notranslate"><span class="pre">notify_remote_server_up</span></code>). +At this point, the transaction transmission loop is also started up, to proactively +send missed PDUs and EDUs to the destination (i.e. you don’t need to wait for a new PDU +or EDU, destined for that destination, to be created in order to send out missed PDUs and +EDUs).</p> +</section> + + </article> + </div> + <footer> + + <div class="related-pages"> + + <a class="prev-page" href="../index.html"> + <svg class="furo-related-icon"><use href="#svg-arrow-right"></use></svg> + <div class="page-info"> + <div class="context"> + <span>Previous</span> + </div> + + <div class="title">Home</div> + + </div> + </a> + </div> + <div class="bottom-of-page"> + <div class="left-details"> + <div class="copyright"> + Copyright © 2023, The Matrix.org Foundation C.I.C. + </div> + Made with <a href="https://www.sphinx-doc.org/">Sphinx</a> and <a class="muted-link" href="https://pradyunsg.me">@pradyunsg</a>'s + + <a href="https://github.com/pradyunsg/furo">Furo</a> + + </div> + <div class="right-details"> + + </div> + </div> + + </footer> + </div> + <aside class="toc-drawer no-toc"> + + + + </aside> + </div> +</div><script data-url_root="../" id="documentation_options" src="../_static/documentation_options.js"></script> + <script src="../_static/doctools.js"></script> + <script src="../_static/sphinx_highlight.js"></script> + <script src="../_static/scripts/furo.js"></script> + </body> +</html> \ No newline at end of file |