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<h1 id="tcp-replication"><a class="header" href="#tcp-replication">TCP Replication</a></h1>
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<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
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<p>Previously the workers used an HTTP long poll mechanism to get updates
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from the master, which had the problem of causing a lot of duplicate
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work on the server. This TCP protocol replaces those APIs with the aim
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of increased efficiency.</p>
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<h2 id="overview"><a class="header" href="#overview">Overview</a></h2>
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<p>The protocol is based on fire and forget, line based commands. An
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example flow would be (where '>' indicates master to worker and
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'<' worker to master flows):</p>
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<pre><code>> SERVER example.com
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< REPLICATE
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> POSITION events master 53 53
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> RDATA events master 54 ["$foo1:bar.com", ...]
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> RDATA events master 55 ["$foo4:bar.com", ...]
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</code></pre>
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<p>The example shows the server accepting a new connection and sending its identity
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with the <code>SERVER</code> command, followed by the client server to respond with the
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position of all streams. The server then periodically sends <code>RDATA</code> commands
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which have the format <code>RDATA <stream_name> <instance_name> <token> <row></code>, where
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the format of <code><row></code> is defined by the individual streams. The
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<code><instance_name></code> is the name of the Synapse process that generated the data
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(usually "master").</p>
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<p>Error reporting happens by either the client or server sending an ERROR
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command, and usually the connection will be closed.</p>
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<p>Since the protocol is a simple line based, its possible to manually
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connect to the server using a tool like netcat. A few things should be
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noted when manually using the protocol:</p>
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<ul>
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<li>The federation stream is only available if federation sending has
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been disabled on the main process.</li>
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<li>The server will only time connections out that have sent a <code>PING</code>
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command. If a ping is sent then the connection will be closed if no
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further commands are receieved within 15s. Both the client and
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server protocol implementations will send an initial PING on
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connection and ensure at least one command every 5s is sent (not
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necessarily <code>PING</code>).</li>
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<li><code>RDATA</code> commands <em>usually</em> include a numeric token, however if the
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stream has multiple rows to replicate per token the server will send
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multiple <code>RDATA</code> commands, with all but the last having a token of
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<code>batch</code>. See the documentation on <code>commands.RdataCommand</code> for
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further details.</li>
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</ul>
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<h2 id="architecture"><a class="header" href="#architecture">Architecture</a></h2>
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<p>The basic structure of the protocol is line based, where the initial
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word of each line specifies the command. The rest of the line is parsed
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based on the command. For example, the RDATA command is defined as:</p>
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<pre><code>RDATA <stream_name> <instance_name> <token> <row_json>
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</code></pre>
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<p>(Note that <row_json> may contains spaces, but cannot contain
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newlines.)</p>
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<p>Blank lines are ignored.</p>
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<h3 id="keep-alives"><a class="header" href="#keep-alives">Keep alives</a></h3>
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<p>Both sides are expected to send at least one command every 5s or so, and
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should send a <code>PING</code> command if necessary. If either side do not receive
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a command within e.g. 15s then the connection should be closed.</p>
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<p>Because the server may be connected to manually using e.g. netcat, the
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timeouts aren't enabled until an initial <code>PING</code> command is seen. Both
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the client and server implementations below send a <code>PING</code> command
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immediately on connection to ensure the timeouts are enabled.</p>
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<p>This ensures that both sides can quickly realize if the tcp connection
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has gone and handle the situation appropriately.</p>
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<h3 id="start-up"><a class="header" href="#start-up">Start up</a></h3>
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<p>When a new connection is made, the server:</p>
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<ul>
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<li>Sends a <code>SERVER</code> command, which includes the identity of the server,
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allowing the client to detect if its connected to the expected
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server</li>
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<li>Sends a <code>PING</code> command as above, to enable the client to time out
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connections promptly.</li>
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</ul>
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<p>The client:</p>
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<ul>
|
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<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>
|
|
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|
|
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