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<title>4.&nbsp;Sampling</title><link rel="stylesheet" type="text/css" href="css/manual-multipage.css"><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="multi_spring-cloud-sleuth.html" title="Spring Cloud Sleuth"><link rel="up" href="multi_spring-cloud-sleuth.html" title="Spring Cloud Sleuth"><link rel="prev" href="multi__features.html" title="3.&nbsp;Features"><link rel="next" href="multi__instrumentation.html" title="5.&nbsp;Instrumentation"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">4.&nbsp;Sampling</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="multi__features.html">Prev</a>&nbsp;</td><th width="60%" align="center">&nbsp;</th><td width="20%" align="right">&nbsp;<a accesskey="n" href="multi__instrumentation.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="_sampling" href="#_sampling"></a>4.&nbsp;Sampling</h1></div></div></div><p>In distributed tracing the data volumes can be very high so sampling
can be important (you usually don&#8217;t need to export all spans to get a
good picture of what is happening). Spring Cloud Sleuth has a
<code class="literal">Sampler</code> strategy that you can implement to take control of the
sampling algorithm. Samplers do not stop span (correlation) ids from
being generated, but they do prevent the tags and events being
attached and exported. By default you get a strategy that continues to
trace if a span is already active, but new ones are always marked as
non-exportable. If all your apps run with this sampler you will see
traces in logs, but not in any remote store. For testing the default
is often enough, and it probably is all you need if you are only using
the logs (e.g. with an ELK aggregator). If you are exporting span data
to Zipkin or Spring Cloud Stream, there is also an <code class="literal">AlwaysSampler</code>
that exports everything and a <code class="literal">PercentageBasedSampler</code> that samples a
fixed fraction of spans.</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Note"><tr><td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="images/note.png"></td><th align="left">Note</th></tr><tr><td align="left" valign="top"><p>the <code class="literal">PercentageBasedSampler</code> is the default if you are using
<code class="literal">spring-cloud-sleuth-zipkin</code> or <code class="literal">spring-cloud-sleuth-stream</code>. You can
configure the exports using <code class="literal">spring.sleuth.sampler.percentage</code>. The passed
value needs to be a double from <code class="literal">0.0</code> to <code class="literal">1.0</code> so it&#8217;s not a percentage.
For backwards compatibility reasons we&#8217;re not changing the property name.</p></td></tr></table></div><p>A sampler can be installed just by creating a bean definition, e.g:</p><pre class="programlisting"><em><span class="hl-annotation" style="color: gray">@Bean</span></em>
<title>4.&nbsp;Sampling</title><link rel="stylesheet" type="text/css" href="css/manual-multipage.css"><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="multi_spring-cloud-sleuth.html" title="Spring Cloud Sleuth"><link rel="up" href="multi_spring-cloud-sleuth.html" title="Spring Cloud Sleuth"><link rel="prev" href="multi__features.html" title="3.&nbsp;Features"><link rel="next" href="multi__propagation.html" title="5.&nbsp;Propagation"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">4.&nbsp;Sampling</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="multi__features.html">Prev</a>&nbsp;</td><th width="60%" align="center">&nbsp;</th><td width="20%" align="right">&nbsp;<a accesskey="n" href="multi__propagation.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="_sampling" href="#_sampling"></a>4.&nbsp;Sampling</h1></div></div></div><p>Sampling may be employed to reduce the data collected and reported out of process.
When a span is not sampled, it adds no overhead (a noop).</p><p>Sampling is an up-front decision, meaning that the decision to report data is made at the first operation in a trace and that decision is propagated downstream.</p><p>By default, a global sampler applies a single rate to all traced operations.
<code class="literal">Tracer.Builder.sampler</code> controls this setting, and it defaults to tracing every request.</p><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_declarative_sampling" href="#_declarative_sampling"></a>4.1&nbsp;Declarative sampling</h2></div></div></div><p>Some applications need to sample based on the type or annotations of a java method.</p><p>Most users use a framework interceptor to automate this sort of policy.
The following example shows how that might work internally:</p><pre class="programlisting"><em><span class="hl-annotation" style="color: gray">@Autowired</span></em> Tracing tracing;
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-comment">// derives a sample rate from an annotation on a java method</span>
DeclarativeSampler&lt;Traced&gt; sampler = DeclarativeSampler.create(Traced::sampleRate);
<em><span class="hl-annotation" style="color: gray">@Around("@annotation(traced)")</span></em>
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">public</span> Object traceThing(ProceedingJoinPoint pjp, Traced traced) <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">throws</span> Throwable {
Span span = tracing.tracer().newTrace(sampler.sample(traced))...
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">try</span> {
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">return</span> pjp.proceed();
} <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">finally</span> {
span.finish();
}
}</pre></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_custom_sampling" href="#_custom_sampling"></a>4.2&nbsp;Custom sampling</h2></div></div></div><p>Depending on what the operation is, you may want to apply different policies.
For example, you might not want to trace requests to static resources such as images, or you might want to trace all requests to a new api.</p><p>Most users use a framework interceptor to automate this sort of policy.
The following example shows how that might work internally:</p><pre class="programlisting"><em><span class="hl-annotation" style="color: gray">@Autowired</span></em> Tracer tracer;
Span newTrace(Request input) {
SamplingFlags flags = SamplingFlags.NONE;
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">if</span> (input.url().startsWith(<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"/experimental"</span>)) {
flags = SamplingFlags.SAMPLED;
} <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">else</span> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">if</span> (input.url().startsWith(<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"/static"</span>)) {
flags = SamplingFlags.NOT_SAMPLED;
}
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">return</span> tracer.newTrace(flags);
}</pre></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_sampling_in_spring_cloud_sleuth" href="#_sampling_in_spring_cloud_sleuth"></a>4.3&nbsp;Sampling in Spring Cloud Sleuth</h2></div></div></div><p>By default Spring Cloud Sleuth sets all spans to non-exportable.
That means that traces appear in logs but not in any remote store.
For testing the default is often enough, and it probably is all you need if you use only the logs (for example, with an ELK aggregator).
If you export span data to Zipkin, there is also an <code class="literal">Sampler.ALWAYS_SAMPLE</code> setting that exports everything and a <code class="literal">ProbabilityBasedSampler</code> setting that samples a fixed fraction of spans.</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Note"><tr><td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="images/note.png"></td><th align="left">Note</th></tr><tr><td align="left" valign="top"><p>The <code class="literal">ProbabilityBasedSampler</code> is the default if you use <code class="literal">spring-cloud-sleuth-zipkin</code>.
You can configure the exports by setting <code class="literal">spring.sleuth.sampler.probability</code>.
The passed value needs to be a double from <code class="literal">0.0</code> to <code class="literal">1.0</code>.</p></td></tr></table></div><p>A sampler can be installed by creating a bean definition, as shown in the following example:</p><pre class="programlisting"><em><span class="hl-annotation" style="color: gray">@Bean</span></em>
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">public</span> Sampler defaultSampler() {
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">return</span> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">new</span> AlwaysSampler();
}</pre><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Tip"><tr><td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="images/tip.png"></td><th align="left">Tip</th></tr><tr><td align="left" valign="top"><p>You can set the HTTP header <code class="literal">X-B3-Flags</code> to <code class="literal">1</code> or when doing messaging you can
set <code class="literal">spanFlags</code> header to <code class="literal">1</code>. Then the current span will be forced to be exportable
regardless of the sampling decision.</p></td></tr></table></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="multi__features.html">Prev</a>&nbsp;</td><td width="20%" align="center">&nbsp;</td><td width="40%" align="right">&nbsp;<a accesskey="n" href="multi__instrumentation.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">3.&nbsp;Features&nbsp;</td><td width="20%" align="center"><a accesskey="h" href="multi_spring-cloud-sleuth.html">Home</a></td><td width="40%" align="right" valign="top">&nbsp;5.&nbsp;Instrumentation</td></tr></table></div></body></html>
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">return</span> Sampler.ALWAYS_SAMPLE;
}</pre><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><table border="0" summary="Tip"><tr><td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="images/tip.png"></td><th align="left">Tip</th></tr><tr><td align="left" valign="top"><p>You can set the HTTP header <code class="literal">X-B3-Flags</code> to <code class="literal">1</code>, or, when doing messaging, you can set the <code class="literal">spanFlags</code> header to <code class="literal">1</code>.
Doing so forces the current span to be exportable regardless of the sampling decision.</p></td></tr></table></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="multi__features.html">Prev</a>&nbsp;</td><td width="20%" align="center">&nbsp;</td><td width="40%" align="right">&nbsp;<a accesskey="n" href="multi__propagation.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">3.&nbsp;Features&nbsp;</td><td width="20%" align="center"><a accesskey="h" href="multi_spring-cloud-sleuth.html">Home</a></td><td width="40%" align="right" valign="top">&nbsp;5.&nbsp;Propagation</td></tr></table></div></body></html>