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<section id="federation-sender">
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<h1>Federation Sender<a class="headerlink" href="#federation-sender" title="Permalink to this heading">#</a></h1>
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<p>The Federation Sender is responsible for sending Persistent Data Units (PDUs)
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and Ephemeral Data Units (EDUs) to other homeservers using
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the <code class="docutils literal notranslate"><span class="pre">/send</span></code> Federation API.</p>
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<h2 class="rubric" id="how-do-pdus-get-sent">How do PDUs get sent?</h2>
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<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>.
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When the sender is notified about a newly-persisted PDU that originates from this homeserver
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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
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remote homeserver that is in the room at that point in the DAG.</p>
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<h3 class="rubric" id="per-destination-queues">Per-Destination Queues</h3>
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<p>There is one <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> per ‘destination’ homeserver.
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The <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> maintains the following information about the destination:</p>
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<ul class="simple">
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<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>
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<li><p>a queue of PDUs to be sent to the destination; and</p></li>
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<li><p>a queue of EDUs to be sent to the destination (not considered in this section).</p></li>
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</ul>
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<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
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transaction if there is not already one in progress.</p>
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<h3 class="rubric" id="transactions-and-the-transaction-transmission-loop">Transactions and the Transaction Transmission Loop</h3>
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<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’.
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The body of the HTTP request contains a list of PDUs and EDUs to send to the destination.</p>
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<p>The <em>Transaction Transmission Loop</em> (<code class="docutils literal notranslate"><span class="pre">_transaction_transmission_loop</span></code>) is responsible
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for emptying the queued PDUs (and EDUs) from a <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> by sending
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them to the destination.</p>
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<p>There can only be one transaction in flight for a given destination at any time.
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(Other than preventing us from overloading the destination, this also makes it easier to
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reason about because we process events sequentially for each destination.
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This is useful for <em>Catch-Up Mode</em>, described later.)</p>
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<p>The loop continues so long as there is anything to send. At each iteration of the loop, we:</p>
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<ul class="simple">
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<li><p>dequeue up to 50 PDUs (and up to 100 EDUs).</p></li>
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<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>
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<li><p>if successful, make note of the fact that we succeeded in transmitting PDUs up to
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the given <code class="docutils literal notranslate"><span class="pre">stream_ordering</span></code> of the latest PDU by</p></li>
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<li><p>if unsuccessful, back off from the remote homeserver for some time.
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If we have been unsuccessful for too long (when the backoff interval grows to exceed 1 hour),
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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>
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</ul>
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<h3 class="rubric" id="catch-up-mode">Catch-Up Mode</h3>
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<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>
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(<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>.
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(Only once the catch-up mode has been exited can the regular tranaction transmission behaviour
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be resumed.)</p>
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<p><em>Catch-Up Mode</em>, entered upon Synapse startup or once a homeserver has fallen behind due to
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connection problems, is responsible for sending PDUs that have been missed by the destination
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homeserver. (PDUs can be missed because the <code class="docutils literal notranslate"><span class="pre">PerDestinationQueue</span></code> is volatile — i.e. resets
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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>
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<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
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<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
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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,
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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
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destination).</p>
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<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
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<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>.
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In other words, from the set of latest PDUs in each room to be sent to the destination,
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the 50 oldest such PDUs are pulled out.</p>
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<p>These PDUs could, in principle, now be directly sent to the destination. However, as an
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optimisation intended to prevent overloading destination homeservers, we instead attempt
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to send the latest forward extremities so long as the destination homeserver is still
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eligible to receive those.
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This reduces load on the destination <strong>in aggregate</strong> because all Synapse homeservers
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will behave according to this principle and therefore avoid sending lots of different PDUs
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at different points in the DAG to a recovering homeserver.
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<em>This optimisation is not currently valid in rooms which are partial-state on this homeserver,
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since we are unable to determine whether the destination homeserver is eligible to receive
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the latest forward extremities unless this homeserver sent those PDUs — in this case, we
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just send the latest PDUs originating from this server and skip this optimisation.</em></p>
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<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>
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is advanced as normal.
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Once there are no longer any rooms containing outstanding PDUs to be sent to the destination
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<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
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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>
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<h4 class="rubric" id="a-note-on-failures-and-back-offs">A note on failures and back-offs</h4>
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<p>If a remote server is unreachable over federation, we back off from that server,
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with an exponentially-increasing retry interval.
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We automatically retry after the retry interval expires (roughly, the logic to do so
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being triggered every minute).</p>
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<p>If the backoff grows too large (> 1 hour), the in-memory queue is emptied (to prevent
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unbounded growth) and Catch-Up Mode is entered.</p>
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<p>It is worth noting that the back-off for a remote server is cleared once an inbound
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request from that remote server is received (see <code class="docutils literal notranslate"><span class="pre">notify_remote_server_up</span></code>).
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At this point, the transaction transmission loop is also started up, to proactively
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send missed PDUs and EDUs to the destination (i.e. you don’t need to wait for a new PDU
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or EDU, destined for that destination, to be created in order to send out missed PDUs and
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EDUs).</p>
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