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<div><h1 class="title"><a id="id-1"></a>Transaction sampling</h1><a class="edit_me" rel="nofollow" title="Edit this page on GitHub" href="https://github.com/elastic/apm-server/edit/7.10/docs/guide/trace-sampling.asciidoc">edit</a></div>
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<div class="titlepage"><div><div>
<div class="position-relative"><h2 class="title"><a id="trace-sampling"></a>Transaction sampling</h2><a class="edit_me" rel="nofollow" title="Edit this page on GitHub" href="https://github.com/elastic/apm-server/edit/7.10/docs/guide/trace-sampling.asciidoc">edit</a></div>
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<p>Elastic APM supports head-based, probability sampling.
<em>Head-based</em> means the sampling decision for each trace is made when that trace is initiated.
<em>Probability sampling</em> means that each trace has a defined and equal probability of being sampled.</p>
<p>For example, a sampling value of <code class="literal">.2</code> indicates a transaction sample rate of <code class="literal">20%</code>.
This means that only <code class="literal">20%</code> of traces will send and retain all of their associated information.
The remaining traces will drop contextual information to reduce the transfer and storage size of the trace.</p>
<div class="position-relative"><h4><a id="_why_sample"></a>Why sample?</h4><a class="edit_me" rel="nofollow" title="Edit this page on GitHub" href="https://github.com/elastic/apm-server/edit/7.10/docs/guide/trace-sampling.asciidoc">edit</a></div>
<p>Distributed tracing can generate a substantial amount of data,
and storage can be a concern for users running <code class="literal">100%</code> sampling — especially as they scale.</p>
<p>The goal of probability sampling is to provide you with a representative set of data that allows
you to make statistical inferences about the entire group of data.
In other words, in most cases, you can still find anomalous patterns in your applications, detect outages, track errors,
and lower MTTR, even when sampling at less than <code class="literal">100%</code>.</p>
<div class="position-relative"><h4><a id="_what_data_is_sampled"></a>What data is sampled?</h4><a class="edit_me" rel="nofollow" title="Edit this page on GitHub" href="https://github.com/elastic/apm-server/edit/7.10/docs/guide/trace-sampling.asciidoc">edit</a></div>
<p>A sampled trace retains all data associated with it.</p>
<p>Non-sampled traces drop <a class="xref" href="transaction-spans.html" title="Spans"><code class="literal">span</code></a> data.
Spans contain more granular information about what is happening within a transaction,
like external requests or database calls.
Spans also contain contextual information and labels.</p>
<p>Regardless of the sampling decision, all traces retain transaction and error data.
This means the following data will always accurately reflect <span class="strong strong"><strong>all</strong></span> of your application’s requests, regardless of the configured sampling rate:</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
Transaction duration and transactions per minute
</li>
<li class="listitem">
Transaction breakdown metrics
</li>
<li class="listitem">
Errors, error occurrence, and error rate
</li>
</ul>
</div>
<div class="position-relative"><h4><a id="_sample_rates"></a>Sample rates</h4><a class="edit_me" rel="nofollow" title="Edit this page on GitHub" href="https://github.com/elastic/apm-server/edit/7.10/docs/guide/trace-sampling.asciidoc">edit</a></div>
<p>What’s the best sampling rate? Unfortunately, there isn’t one.
Sampling is dependent on your data, the throughput of your application, data retainment policies, and other factors.
A sampling rate from <code class="literal">.1%</code> to <code class="literal">100%</code> would all be considered normal.
You may even decide to have a unique sample rate per service — for example, if a certain service
experiences considerably more or less traffic than another.</p>
<div class="position-relative"><h4><a id="_sampling_with_distributed_tracing"></a>Sampling with distributed tracing</h4><a class="edit_me" rel="nofollow" title="Edit this page on GitHub" href="https://github.com/elastic/apm-server/edit/7.10/docs/guide/trace-sampling.asciidoc">edit</a></div>
<p>The initiating service makes the sampling decision in a distributed trace,
and all downstream services respect that decision.</p>
<p>In each example below, <code class="literal">Service A</code> initiates four transactions.
In the first example, <code class="literal">Service A</code> samples at <code class="literal">.5</code> (<code class="literal">50%</code>). In the second, <code class="literal">Service A</code> samples at <code class="literal">1</code> (<code class="literal">100%</code>).
Each subsequent service respects the initial sampling decision, regardless of their configured sample rate.
The result is a sampling percentage that matches the initiating service:</p>
<div class="imageblock">
<div class="content">
<img src="images/dt-sampling-example.png" alt="How sampling impacts distributed tracing">
</div>
</div>
<div class="position-relative"><h4><a id="_apm_app_implications"></a>APM app implications</h4><a class="edit_me" rel="nofollow" title="Edit this page on GitHub" href="https://github.com/elastic/apm-server/edit/7.10/docs/guide/trace-sampling.asciidoc">edit</a></div>
<p>Because the transaction sample rate is respected by downstream services,
the APM app always knows which transactions have and haven’t been sampled.
This prevents the app from showing broken traces.
In addition, because transaction and error data is never sampled,
you can always expect metrics and errors to be accurately reflected in the APM app.</p>
<p><span class="strong strong"><strong>Service maps</strong></span></p>
<p>Service maps rely on distributed traces to draw connections between services.
A minimum required version of APM agents is required for Service maps to work.
See <a href="/guide/en/kibana/7.10/service-maps.html" class="ulink" target="_top">Service maps</a> for more information.</p>
<div class="position-relative"><h4><a id="_adjust_the_sample_rate"></a>Adjust the sample rate</h4><a class="edit_me" rel="nofollow" title="Edit this page on GitHub" href="https://github.com/elastic/apm-server/edit/7.10/docs/guide/trace-sampling.asciidoc">edit</a></div>
<p>There are three ways to adjust the transaction sample rate of your APM agents:</p>
<div class="variablelist">
<dl class="variablelist">
<dt>
<span class="term">
Dynamic
</span>
</dt>
<dd>
The transaction sample rate can be changed dynamically (no redeployment necessary) on a per-service and per-environment
basis with <a href="/guide/en/kibana/7.10/agent-configuration.html" class="ulink" target="_top">APM Agent Configuration</a> in Kibana.
</dd>
<dt>
<span class="term">
Kibana API
</span>
</dt>
<dd>
APM Agent configuration exposes an API that can be used to programmatically change
your agents' sampling rate.
An example is provided in the <a href="/guide/en/kibana/7.10/agent-config-api.html" class="ulink" target="_top">Agent configuration API reference</a>.
</dd>
<dt>
<span class="term">
Configuration
</span>
</dt>
<dd>
<p>
Each agent provides a configuration value used to set the transaction sample rate.
See the relevant agent’s documentation for more details:
</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
Go: <a href="/guide/en/apm/agent/go/1.x/configuration.html#config-transaction-sample-rate" class="ulink" target="_top"><code class="literal">ELASTIC_APM_TRANSACTION_SAMPLE_RATE</code></a>
</li>
<li class="listitem">
Java: <a href="/guide/en/apm/agent/java/1.x/config-core.html#config-transaction-sample-rate" class="ulink" target="_top"><code class="literal">transaction_sample_rate</code></a>
</li>
<li class="listitem">
.NET: <a href="/guide/en/apm/agent/dotnet/1.x/config-core.html#config-transaction-sample-rate" class="ulink" target="_top"><code class="literal">TransactionSampleRate</code></a>
</li>
<li class="listitem">
Node.js: <a href="/guide/en/apm/agent/nodejs/4.x/configuration.html#transaction-sample-rate" class="ulink" target="_top"><code class="literal">transactionSampleRate</code></a>
</li>
<li class="listitem">
Python: <a href="/guide/en/apm/agent/python/5.x/configuration.html#config-transaction-sample-rate" class="ulink" target="_top"><code class="literal">transaction_sample_rate</code></a>
</li>
<li class="listitem">
Ruby: <a href="/guide/en/apm/agent/ruby/3.x/configuration.html#config-transaction-sample-rate" class="ulink" target="_top"><code class="literal">transaction_sample_rate</code></a>
</li>
</ul>
</div>
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</dl>
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