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<ol class="chapter"><li class="chapter-item expanded affix "><li class="part-title">Introduction</li><li class="chapter-item expanded "><a href="welcome_and_overview.html">Welcome and Overview</a></li><li class="chapter-item expanded affix "><li class="part-title">Setup</li><li class="chapter-item expanded "><a href="setup/installation.html">Installation</a></li><li class="chapter-item expanded "><a href="postgres.html">Using Postgres</a></li><li class="chapter-item expanded "><a href="reverse_proxy.html">Configuring a Reverse Proxy</a></li><li class="chapter-item expanded "><a href="setup/forward_proxy.html">Configuring a Forward/Outbound Proxy</a></li><li class="chapter-item expanded "><a href="turn-howto.html">Configuring a Turn Server</a></li><li class="chapter-item expanded "><a href="delegate.html">Delegation</a></li><li class="chapter-item expanded affix "><li class="part-title">Upgrading</li><li class="chapter-item expanded "><a href="upgrade.html">Upgrading between Synapse 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<h1 id="opentracing"><a class="header" href="#opentracing">OpenTracing</a></h1>
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<h2 id="background"><a class="header" href="#background">Background</a></h2>
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<p>OpenTracing is a semi-standard being adopted by a number of distributed
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tracing platforms. It is a common api for facilitating vendor-agnostic
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tracing instrumentation. That is, we can use the OpenTracing api and
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select one of a number of tracer implementations to do the heavy lifting
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in the background. Our current selected implementation is Jaeger.</p>
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<p>OpenTracing is a tool which gives an insight into the causal
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relationship of work done in and between servers. The servers each track
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events and report them to a centralised server - in Synapse's case:
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Jaeger. The basic unit used to represent events is the span. The span
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roughly represents a single piece of work that was done and the time at
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which it occurred. A span can have child spans, meaning that the work of
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the child had to be completed for the parent span to complete, or it can
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have follow-on spans which represent work that is undertaken as a result
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of the parent but is not depended on by the parent to in order to
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finish.</p>
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<p>Since this is undertaken in a distributed environment a request to
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another server, such as an RPC or a simple GET, can be considered a span
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(a unit or work) for the local server. This causal link is what
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OpenTracing aims to capture and visualise. In order to do this metadata
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about the local server's span, i.e the 'span context', needs to be
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included with the request to the remote.</p>
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<p>It is up to the remote server to decide what it does with the spans it
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creates. This is called the sampling policy and it can be configured
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through Jaeger's settings.</p>
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<p>For OpenTracing concepts see
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<a href="https://opentracing.io/docs/overview/what-is-tracing/">https://opentracing.io/docs/overview/what-is-tracing/</a>.</p>
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<p>For more information about Jaeger's implementation see
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<a href="https://www.jaegertracing.io/docs/">https://www.jaegertracing.io/docs/</a></p>
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<h2 id="setting-up-opentracing"><a class="header" href="#setting-up-opentracing">Setting up OpenTracing</a></h2>
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<p>To receive OpenTracing spans, start up a Jaeger server. This can be done
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using docker like so:</p>
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<pre><code class="language-sh">docker run -d --name jaeger \
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-p 6831:6831/udp \
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-p 6832:6832/udp \
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-p 5778:5778 \
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-p 16686:16686 \
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-p 14268:14268 \
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jaegertracing/all-in-one:1
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</code></pre>
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<p>Latest documentation is probably at
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https://www.jaegertracing.io/docs/latest/getting-started.</p>
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<h2 id="enable-opentracing-in-synapse"><a class="header" href="#enable-opentracing-in-synapse">Enable OpenTracing in Synapse</a></h2>
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<p>OpenTracing is not enabled by default. It must be enabled in the
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homeserver config by uncommenting the config options under <code>opentracing</code>
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as shown in the <a href="./sample_config.yaml">sample config</a>. For example:</p>
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<pre><code class="language-yaml">opentracing:
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enabled: true
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homeserver_whitelist:
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- "mytrustedhomeserver.org"
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- "*.myotherhomeservers.com"
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</code></pre>
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<h2 id="homeserver-whitelisting"><a class="header" href="#homeserver-whitelisting">Homeserver whitelisting</a></h2>
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<p>The homeserver whitelist is configured using regular expressions. A list
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of regular expressions can be given and their union will be compared
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when propagating any spans contexts to another homeserver.</p>
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<p>Though it's mostly safe to send and receive span contexts to and from
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untrusted users since span contexts are usually opaque ids it can lead
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to two problems, namely:</p>
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<ul>
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<li>If the span context is marked as sampled by the sending homeserver
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the receiver will sample it. Therefore two homeservers with wildly
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different sampling policies could incur higher sampling counts than
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intended.</li>
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<li>Sending servers can attach arbitrary data to spans, known as
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'baggage'. For safety this has been disabled in Synapse but that
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doesn't prevent another server sending you baggage which will be
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logged to OpenTracing's logs.</li>
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</ul>
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<h2 id="configuring-jaeger"><a class="header" href="#configuring-jaeger">Configuring Jaeger</a></h2>
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<p>Sampling strategies can be set as in this document:
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<a href="https://www.jaegertracing.io/docs/latest/sampling/">https://www.jaegertracing.io/docs/latest/sampling/</a>.</p>
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