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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 transaction 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 (> 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 |