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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/tcp_replication.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> + + <h1 id="tcp-replication"><a class="header" href="#tcp-replication">TCP Replication</a></h1> +<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2> +<p>Previously the workers used an HTTP long poll mechanism to get updates +from the master, which had the problem of causing a lot of duplicate +work on the server. This TCP protocol replaces those APIs with the aim +of increased efficiency.</p> +<h2 id="overview"><a class="header" href="#overview">Overview</a></h2> +<p>The protocol is based on fire and forget, line based commands. An +example flow would be (where '>' indicates master to worker and +'<' worker to master flows):</p> +<pre><code>> SERVER example.com +< REPLICATE +> POSITION events master 53 53 +> RDATA events master 54 ["$foo1:bar.com", ...] +> RDATA events master 55 ["$foo4:bar.com", ...] +</code></pre> +<p>The example shows the server accepting a new connection and sending its identity +with the <code>SERVER</code> command, followed by the client server to respond with the +position of all streams. The server then periodically sends <code>RDATA</code> commands +which have the format <code>RDATA <stream_name> <instance_name> <token> <row></code>, where +the format of <code><row></code> is defined by the individual streams. The +<code><instance_name></code> is the name of the Synapse process that generated the data +(usually "master").</p> +<p>Error reporting happens by either the client or server sending an ERROR +command, and usually the connection will be closed.</p> +<p>Since the protocol is a simple line based, its possible to manually +connect to the server using a tool like netcat. A few things should be +noted when manually using the protocol:</p> +<ul> +<li>The federation stream is only available if federation sending has +been disabled on the main process.</li> +<li>The server will only time connections out that have sent a <code>PING</code> +command. If a ping is sent then the connection will be closed if no +further commands are receieved within 15s. Both the client and +server protocol implementations will send an initial PING on +connection and ensure at least one command every 5s is sent (not +necessarily <code>PING</code>).</li> +<li><code>RDATA</code> commands <em>usually</em> include a numeric token, however if the +stream has multiple rows to replicate per token the server will send +multiple <code>RDATA</code> commands, with all but the last having a token of +<code>batch</code>. See the documentation on <code>commands.RdataCommand</code> for +further details.</li> +</ul> +<h2 id="architecture"><a class="header" href="#architecture">Architecture</a></h2> +<p>The basic structure of the protocol is line based, where the initial +word of each line specifies the command. The rest of the line is parsed +based on the command. For example, the RDATA command is defined as:</p> +<pre><code>RDATA <stream_name> <instance_name> <token> <row_json> +</code></pre> +<p>(Note that <row_json> may contains spaces, but cannot contain +newlines.)</p> +<p>Blank lines are ignored.</p> +<h3 id="keep-alives"><a class="header" href="#keep-alives">Keep alives</a></h3> +<p>Both sides are expected to send at least one command every 5s or so, and +should send a <code>PING</code> command if necessary. If either side do not receive +a command within e.g. 15s then the connection should be closed.</p> +<p>Because the server may be connected to manually using e.g. netcat, the +timeouts aren't enabled until an initial <code>PING</code> command is seen. Both +the client and server implementations below send a <code>PING</code> command +immediately on connection to ensure the timeouts are enabled.</p> +<p>This ensures that both sides can quickly realize if the tcp connection +has gone and handle the situation appropriately.</p> +<h3 id="start-up"><a class="header" href="#start-up">Start up</a></h3> +<p>When a new connection is made, the server:</p> +<ul> +<li>Sends a <code>SERVER</code> command, which includes the identity of the server, +allowing the client to detect if its connected to the expected +server</li> +<li>Sends a <code>PING</code> command as above, to enable the client to time out +connections promptly.</li> +</ul> +<p>The client:</p> +<ul> +<li>Sends a <code>NAME</code> command, allowing the server to associate a human +friendly name with the connection. This is optional.</li> +<li>Sends a <code>PING</code> as above</li> +<li>Sends a <code>REPLICATE</code> to get the current position of all streams.</li> +<li>On receipt of a <code>SERVER</code> command, checks that the server name +matches the expected server name.</li> +</ul> +<h3 id="error-handling"><a class="header" href="#error-handling">Error handling</a></h3> +<p>If either side detects an error it can send an <code>ERROR</code> command and close +the connection.</p> +<p>If the client side loses the connection to the server it should +reconnect, following the steps above.</p> +<h3 id="congestion"><a class="header" href="#congestion">Congestion</a></h3> +<p>If the server sends messages faster than the client can consume them the +server will first buffer a (fairly large) number of commands and then +disconnect the client. This ensures that we don't queue up an unbounded +number of commands in memory and gives us a potential oppurtunity to +squawk loudly. When/if the client recovers it can reconnect to the +server and ask for missed messages.</p> +<h3 id="reliability"><a class="header" href="#reliability">Reliability</a></h3> +<p>In general the replication stream should be considered an unreliable +transport since e.g. commands are not resent if the connection +disappears.</p> +<p>The exception to that are the replication streams, i.e. RDATA commands, +since these include tokens which can be used to restart the stream on +connection errors.</p> +<p>The client should keep track of the token in the last RDATA command +received for each stream so that on reconneciton it can start streaming +from the correct place. Note: not all RDATA have valid tokens due to +batching. See <code>RdataCommand</code> for more details.</p> +<h3 id="example"><a class="header" href="#example">Example</a></h3> +<p>An example iteraction is shown below. Each line is prefixed with '>' +or '<' to indicate which side is sending, these are <em>not</em> included on +the wire:</p> +<pre><code>* connection established * +> SERVER localhost:8823 +> PING 1490197665618 +< NAME synapse.app.appservice +< PING 1490197665618 +< REPLICATE +> POSITION events master 1 1 +> POSITION backfill master 1 1 +> POSITION caches master 1 1 +> RDATA caches master 2 ["get_user_by_id",["@01register-user:localhost:8823"],1490197670513] +> RDATA events master 14 ["$149019767112vOHxz:localhost:8823", + "!AFDCvgApUmpdfVjIXm:localhost:8823","m.room.guest_access","",null] +< PING 1490197675618 +> ERROR server stopping +* connection closed by server * +</code></pre> +<p>The <code>POSITION</code> command sent by the server is used to set the clients +position without needing to send data with the <code>RDATA</code> command.</p> +<p>An example of a batched set of <code>RDATA</code> is:</p> +<pre><code>> RDATA caches master batch ["get_user_by_id",["@test:localhost:8823"],1490197670513] +> RDATA caches master batch ["get_user_by_id",["@test2:localhost:8823"],1490197670513] +> RDATA caches master batch ["get_user_by_id",["@test3:localhost:8823"],1490197670513] +> RDATA caches master 54 ["get_user_by_id",["@test4:localhost:8823"],1490197670513] +</code></pre> +<p>In this case the client shouldn't advance their caches token until it +sees the the last <code>RDATA</code>.</p> +<h3 id="list-of-commands"><a class="header" href="#list-of-commands">List of commands</a></h3> +<p>The list of valid commands, with which side can send it: server (S) or +client (C):</p> +<h4 id="server-s"><a class="header" href="#server-s">SERVER (S)</a></h4> +<p>Sent at the start to identify which server the client is talking to</p> +<h4 id="rdata-s"><a class="header" href="#rdata-s">RDATA (S)</a></h4> +<p>A single update in a stream</p> +<h4 id="position-s"><a class="header" href="#position-s">POSITION (S)</a></h4> +<p>On receipt of a POSITION command clients should check if they have missed any +updates, and if so then fetch them out of band. Sent in response to a +REPLICATE command (but can happen at any time).</p> +<p>The POSITION command includes the source of the stream. Currently all streams +are written by a single process (usually "master"). If fetching missing +updates via HTTP API, rather than via the DB, then processes should make the +request to the appropriate process.</p> +<p>Two positions are included, the "new" position and the last position sent respectively. +This allows servers to tell instances that the positions have advanced but no +data has been written, without clients needlessly checking to see if they +have missed any updates.</p> +<h4 id="error-s-c"><a class="header" href="#error-s-c">ERROR (S, C)</a></h4> +<p>There was an error</p> +<h4 id="ping-s-c"><a class="header" href="#ping-s-c">PING (S, C)</a></h4> +<p>Sent periodically to ensure the connection is still alive</p> +<h4 id="name-c"><a class="header" href="#name-c">NAME (C)</a></h4> +<p>Sent at the start by client to inform the server who they are</p> +<h4 id="replicate-c"><a class="header" href="#replicate-c">REPLICATE (C)</a></h4> +<p>Asks the server for the current position of all streams.</p> +<h4 id="user_sync-c"><a class="header" href="#user_sync-c">USER_SYNC (C)</a></h4> +<p>A user has started or stopped syncing on this process.</p> +<h4 id="clear_user_sync-c"><a class="header" href="#clear_user_sync-c">CLEAR_USER_SYNC (C)</a></h4> +<p>The server should clear all associated user sync data from the worker.</p> +<p>This is used when a worker is shutting down.</p> +<h4 id="federation_ack-c"><a class="header" href="#federation_ack-c">FEDERATION_ACK (C)</a></h4> +<p>Acknowledge receipt of some federation data</p> +<h3 id="remote_server_up-s-c"><a class="header" href="#remote_server_up-s-c">REMOTE_SERVER_UP (S, C)</a></h3> +<p>Inform other processes that a remote server may have come back online.</p> +<p>See <code>synapse/replication/tcp/commands.py</code> for a detailed description and +the format of each command.</p> +<h3 id="cache-invalidation-stream"><a class="header" href="#cache-invalidation-stream">Cache Invalidation Stream</a></h3> +<p>The cache invalidation stream is used to inform workers when they need +to invalidate any of their caches in the data store. This is done by +streaming all cache invalidations done on master down to the workers, +assuming that any caches on the workers also exist on the master.</p> +<p>Each individual cache invalidation results in a row being sent down +replication, which includes the cache name (the name of the function) +and they key to invalidate. For example:</p> +<pre><code>> RDATA caches master 550953771 ["get_user_by_id", ["@bob:example.com"], 1550574873251] +</code></pre> +<p>Alternatively, an entire cache can be invalidated by sending down a <code>null</code> +instead of the key. For example:</p> +<pre><code>> RDATA caches master 550953772 ["get_user_by_id", null, 1550574873252] +</code></pre> +<p>However, there are times when a number of caches need to be invalidated +at the same time with the same key. To reduce traffic we batch those +invalidations into a single poke by defining a special cache name that +workers understand to mean to expand to invalidate the correct caches.</p> +<p>Currently the special cache names are declared in +<code>synapse/storage/_base.py</code> and are:</p> +<ol> +<li><code>cs_cache_fake</code> ─ invalidates caches that depend on the current +state</li> +</ol> + + </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="development/internal_documentation/index.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="development/internal_documentation/index.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; + </script> + <script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script> + <script src="mark.min.js" type="text/javascript" charset="utf-8"></script> + <script src="searcher.js" type="text/javascript" charset="utf-8"></script> + <script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script> + <script src="highlight.js" type="text/javascript" charset="utf-8"></script> + <script src="book.js" type="text/javascript" charset="utf-8"></script> + + <!-- Custom JS scripts --> + <script type="text/javascript" src="docs/website_files/table-of-contents.js"></script> + </body> +</html> \ No newline at end of file |