deploy: 8b50a9d01da2c84bb9838287519fa3e0a4e955ce

author: DMRobertson <DMRobertson@users.noreply.github.com> 2023-10-03 10:55:03 +0000
committer: DMRobertson <DMRobertson@users.noreply.github.com> 2023-10-03 10:55:03 +0000
commit: aefc379e30b30cc5db568d70a74cd06d8c7a17a8 (patch)
tree: 209b1740f0996c896f22236d072a036c46b1c3c1 /dev-docs/v1.94/modules
parent: deploy: 8b50a9d01da2c84bb9838287519fa3e0a4e955ce (diff)
download: synapse-aefc379e30b30cc5db568d70a74cd06d8c7a17a8.tar.xz
1 files changed, 329 insertions, 0 deletions
diff --git a/dev-docs/v1.94/modules/federation_sender.html b/dev-docs/v1.94/modules/federation_sender.html
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+<!doctype html>
+<html class="no-js" lang="en" data-content_root="">
+  <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 6.2.1 and Furo 2023.09.10 -->
+        <title>Federation Sender - Synapse development documentation</title>
+      <link rel="stylesheet" type="text/css" href="../_static/pygments.css" />
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+  body {
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+  <p class="caption" role="heading"><span class="caption-text">Contents:</span></p>
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+<li class="toctree-l1 current current-page"><a class="current reference internal" href="#">Federation Sender</a></li>
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+        <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.
+We automatically retry after the retry interval expires (roughly, the logic to do so
+being triggered every minute).</p>
+<p>If the backoff grows too large (&gt; 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>
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author	DMRobertson <DMRobertson@users.noreply.github.com>	2023-10-03 10:55:03 +0000
committer	DMRobertson <DMRobertson@users.noreply.github.com>	2023-10-03 10:55:03 +0000
commit	aefc379e30b30cc5db568d70a74cd06d8c7a17a8 (patch)
tree	209b1740f0996c896f22236d072a036c46b1c3c1 /dev-docs/v1.94/modules
parent	deploy: 8b50a9d01da2c84bb9838287519fa3e0a4e955ce (diff)
download	synapse-aefc379e30b30cc5db568d70a74cd06d8c7a17a8.tar.xz