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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__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;
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 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-comment">// When there is no trace in progress, this decides using an annotation</span>
Sampler decideUsingAnnotation = declarativeSampler.toSampler(traced);
Tracer tracer = tracer.withSampler(decideUsingAnnotation);
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-comment">// This code looks the same as if there was no declarative override</span>
ScopedSpan span = tracer.startScopedSpan(spanName(pjp));
<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">catch</span> (RuntimeException | Error e) {
span.error(e);
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">throw</span> e;
} <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;
<em><span class="hl-annotation" style="color: gray">@Autowired</span></em> Sampler fallback;
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);
Span nextSpan(<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">final</span> Request input) {
Sampler requestBased = Sampler() {
<em><span class="hl-annotation" style="color: gray">@Override</span></em> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">public</span> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">boolean</span> isSampled(<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">long</span> traceId) {
<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>)) {
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">return</span> true;
} <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>)) {
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">return</span> false;
}
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">return</span> fallback.isSampled(traceId);
}
};
<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">return</span> tracer.withSampler(requestBased).nextSpan();
}</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).