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  136. <h1 id="opentracing"><a class="header" href="#opentracing">OpenTracing</a></h1>
  137. <h2 id="background"><a class="header" href="#background">Background</a></h2>
  138. <p>OpenTracing is a semi-standard being adopted by a number of distributed
  139. tracing platforms. It is a common api for facilitating vendor-agnostic
  140. tracing instrumentation. That is, we can use the OpenTracing api and
  141. select one of a number of tracer implementations to do the heavy lifting
  142. in the background. Our current selected implementation is Jaeger.</p>
  143. <p>OpenTracing is a tool which gives an insight into the causal
  144. relationship of work done in and between servers. The servers each track
  145. events and report them to a centralised server - in Synapse's case:
  146. Jaeger. The basic unit used to represent events is the span. The span
  147. roughly represents a single piece of work that was done and the time at
  148. which it occurred. A span can have child spans, meaning that the work of
  149. the child had to be completed for the parent span to complete, or it can
  150. have follow-on spans which represent work that is undertaken as a result
  151. of the parent but is not depended on by the parent to in order to
  152. finish.</p>
  153. <p>Since this is undertaken in a distributed environment a request to
  154. another server, such as an RPC or a simple GET, can be considered a span
  155. (a unit or work) for the local server. This causal link is what
  156. OpenTracing aims to capture and visualise. In order to do this metadata
  157. about the local server's span, i.e the 'span context', needs to be
  158. included with the request to the remote.</p>
  159. <p>It is up to the remote server to decide what it does with the spans it
  160. creates. This is called the sampling policy and it can be configured
  161. through Jaeger's settings.</p>
  162. <p>For OpenTracing concepts see
  163. <a href="https://opentracing.io/docs/overview/what-is-tracing/">https://opentracing.io/docs/overview/what-is-tracing/</a>.</p>
  164. <p>For more information about Jaeger's implementation see
  165. <a href="https://www.jaegertracing.io/docs/">https://www.jaegertracing.io/docs/</a></p>
  166. <h2 id="setting-up-opentracing"><a class="header" href="#setting-up-opentracing">Setting up OpenTracing</a></h2>
  167. <p>To receive OpenTracing spans, start up a Jaeger server. This can be done
  168. using docker like so:</p>
  169. <pre><code class="language-sh">docker run -d --name jaeger \
  170. -p 6831:6831/udp \
  171. -p 6832:6832/udp \
  172. -p 5778:5778 \
  173. -p 16686:16686 \
  174. -p 14268:14268 \
  175. jaegertracing/all-in-one:1
  176. </code></pre>
  177. <p>Latest documentation is probably at
  178. https://www.jaegertracing.io/docs/latest/getting-started.</p>
  179. <h2 id="enable-opentracing-in-synapse"><a class="header" href="#enable-opentracing-in-synapse">Enable OpenTracing in Synapse</a></h2>
  180. <p>OpenTracing is not enabled by default. It must be enabled in the
  181. homeserver config by uncommenting the config options under <code>opentracing</code>
  182. as shown in the <a href="./sample_config.yaml">sample config</a>. For example:</p>
  183. <pre><code class="language-yaml">opentracing:
  184. enabled: true
  185. homeserver_whitelist:
  186. - &quot;mytrustedhomeserver.org&quot;
  187. - &quot;*.myotherhomeservers.com&quot;
  188. </code></pre>
  189. <h2 id="homeserver-whitelisting"><a class="header" href="#homeserver-whitelisting">Homeserver whitelisting</a></h2>
  190. <p>The homeserver whitelist is configured using regular expressions. A list
  191. of regular expressions can be given and their union will be compared
  192. when propagating any spans contexts to another homeserver.</p>
  193. <p>Though it's mostly safe to send and receive span contexts to and from
  194. untrusted users since span contexts are usually opaque ids it can lead
  195. to two problems, namely:</p>
  196. <ul>
  197. <li>If the span context is marked as sampled by the sending homeserver
  198. the receiver will sample it. Therefore two homeservers with wildly
  199. different sampling policies could incur higher sampling counts than
  200. intended.</li>
  201. <li>Sending servers can attach arbitrary data to spans, known as
  202. 'baggage'. For safety this has been disabled in Synapse but that
  203. doesn't prevent another server sending you baggage which will be
  204. logged to OpenTracing's logs.</li>
  205. </ul>
  206. <h2 id="configuring-jaeger"><a class="header" href="#configuring-jaeger">Configuring Jaeger</a></h2>
  207. <p>Sampling strategies can be set as in this document:
  208. <a href="https://www.jaegertracing.io/docs/latest/sampling/">https://www.jaegertracing.io/docs/latest/sampling/</a>.</p>
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