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<h2>Table of Contents</h2>
<ul>
<li>
<p><span class="stability_1"><a href="#tracing_trace_events">Trace events</a></span></p>
<ul>
<li>
<p><a href="#tracing_the_trace_events_module">The <code>trace_events</code> module</a></p>
<ul>
<li>
<p><a href="#tracing_tracing_object"><code>Tracing</code> object</a></p>
<ul>
<li><a href="#tracing_tracing_categories"><code>tracing.categories</code></a></li>
<li><a href="#tracing_tracing_disable"><code>tracing.disable()</code></a></li>
<li><a href="#tracing_tracing_enable"><code>tracing.enable()</code></a></li>
<li><a href="#tracing_tracing_enabled"><code>tracing.enabled</code></a></li>
</ul>
</li>
<li><a href="#tracing_trace_events_createtracing_options"><code>trace_events.createTracing(options)</code></a></li>
<li><a href="#tracing_trace_events_getenabledcategories"><code>trace_events.getEnabledCategories()</code></a></li>
</ul>
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<div id="apicontent">
<h1>Trace events<span><a class="mark" href="#tracing_trace_events" id="tracing_trace_events">#</a></span></h1>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p>
<p>The <code>trace_events</code> module provides a mechanism to centralize tracing information
generated by V8, Node.js core, and userspace code.</p>
<p>Tracing can be enabled with the <code>--trace-event-categories</code> command-line flag
or by using the <code>trace_events</code> module. The <code>--trace-event-categories</code> flag
accepts a list of comma-separated category names.</p>
<p>The available categories are:</p>
<ul>
<li><code>node</code>: An empty placeholder.</li>
<li><code>node.async_hooks</code>: Enables capture of detailed <a href="async_hooks.html"><code>async_hooks</code></a> trace data.
The <a href="async_hooks.html"><code>async_hooks</code></a> events have a unique <code>asyncId</code> and a special <code>triggerId</code>
<code>triggerAsyncId</code> property.</li>
<li><code>node.bootstrap</code>: Enables capture of Node.js bootstrap milestones.</li>
<li><code>node.console</code>: Enables capture of <code>console.time()</code> and <code>console.count()</code>
output.</li>
<li><code>node.dns.native</code>: Enables capture of trace data for DNS queries.</li>
<li><code>node.environment</code>: Enables capture of Node.js Environment milestones.</li>
<li><code>node.fs.sync</code>: Enables capture of trace data for file system sync methods.</li>
<li>
<p><code>node.perf</code>: Enables capture of <a href="perf_hooks.html">Performance API</a> measurements.</p>
<ul>
<li><code>node.perf.usertiming</code>: Enables capture of only Performance API User Timing
measures and marks.</li>
<li><code>node.perf.timerify</code>: Enables capture of only Performance API timerify
measurements.</li>
</ul>
</li>
<li><code>node.promises.rejections</code>: Enables capture of trace data tracking the number
of unhandled Promise rejections and handled-after-rejections.</li>
<li><code>node.vm.script</code>: Enables capture of trace data for the <code>vm</code> module's
<code>runInNewContext()</code>, <code>runInContext()</code>, and <code>runInThisContext()</code> methods.</li>
<li><code>v8</code>: The <a href="v8.html">V8</a> events are GC, compiling, and execution related.</li>
</ul>
<p>By default the <code>node</code>, <code>node.async_hooks</code>, and <code>v8</code> categories are enabled.</p>
<pre><code class="language-bash">node --trace-event-categories v8,node,node.async_hooks server.js
</code></pre>
<p>Prior versions of Node.js required the use of the <code>--trace-events-enabled</code>
flag to enable trace events. This requirement has been removed. However, the
<code>--trace-events-enabled</code> flag <em>may</em> still be used and will enable the
<code>node</code>, <code>node.async_hooks</code>, and <code>v8</code> trace event categories by default.</p>
<pre><code class="language-bash">node --trace-events-enabled
# is equivalent to
node --trace-event-categories v8,node,node.async_hooks
</code></pre>
<p>Alternatively, trace events may be enabled using the <code>trace_events</code> module:</p>
<pre><code class="language-js">const trace_events = require('trace_events');
const tracing = trace_events.createTracing({ categories: ['node.perf'] });
tracing.enable(); // Enable trace event capture for the 'node.perf' category
// do work
tracing.disable(); // Disable trace event capture for the 'node.perf' category
</code></pre>
<p>Running Node.js with tracing enabled will produce log files that can be opened
in the <a href="https://www.chromium.org/developers/how-tos/trace-event-profiling-tool"><code>chrome://tracing</code></a>
tab of Chrome.</p>
<p>The logging file is by default called <code>node_trace.${rotation}.log</code>, where
<code>${rotation}</code> is an incrementing log-rotation id. The filepath pattern can
be specified with <code>--trace-event-file-pattern</code> that accepts a template
string that supports <code>${rotation}</code> and <code>${pid}</code>:</p>
<pre><code class="language-bash">node --trace-event-categories v8 --trace-event-file-pattern '${pid}-${rotation}.log' server.js
</code></pre>
<p>Starting with Node.js 10.0.0, the tracing system uses the same time source
as the one used by <code>process.hrtime()</code>
however the trace-event timestamps are expressed in microseconds,
unlike <code>process.hrtime()</code> which returns nanoseconds.</p>
<p>The features from this module are not available in <a href="worker_threads.html#worker_threads_class_worker"><code>Worker</code></a> threads.</p>
<h2>The <code>trace_events</code> module<span><a class="mark" href="#tracing_the_trace_events_module" id="tracing_the_trace_events_module">#</a></span></h2>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div>
<h3><code>Tracing</code> object<span><a class="mark" href="#tracing_tracing_object" id="tracing_tracing_object">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div>
<p>The <code>Tracing</code> object is used to enable or disable tracing for sets of
categories. Instances are created using the <code>trace_events.createTracing()</code>
method.</p>
<p>When created, the <code>Tracing</code> object is disabled. Calling the
<code>tracing.enable()</code> method adds the categories to the set of enabled trace event
categories. Calling <code>tracing.disable()</code> will remove the categories from the
set of enabled trace event categories.</p>
<h4><code>tracing.categories</code><span><a class="mark" href="#tracing_tracing_categories" id="tracing_tracing_categories">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
</ul>
<p>A comma-separated list of the trace event categories covered by this
<code>Tracing</code> object.</p>
<h4><code>tracing.disable()</code><span><a class="mark" href="#tracing_tracing_disable" id="tracing_tracing_disable">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div>
<p>Disables this <code>Tracing</code> object.</p>
<p>Only trace event categories <em>not</em> covered by other enabled <code>Tracing</code> objects
and <em>not</em> specified by the <code>--trace-event-categories</code> flag will be disabled.</p>
<pre><code class="language-js">const trace_events = require('trace_events');
const t1 = trace_events.createTracing({ categories: ['node', 'v8'] });
const t2 = trace_events.createTracing({ categories: ['node.perf', 'node'] });
t1.enable();
t2.enable();
// Prints 'node,node.perf,v8'
console.log(trace_events.getEnabledCategories());
t2.disable(); // Will only disable emission of the 'node.perf' category
// Prints 'node,v8'
console.log(trace_events.getEnabledCategories());
</code></pre>
<h4><code>tracing.enable()</code><span><a class="mark" href="#tracing_tracing_enable" id="tracing_tracing_enable">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div>
<p>Enables this <code>Tracing</code> object for the set of categories covered by the
<code>Tracing</code> object.</p>
<h4><code>tracing.enabled</code><span><a class="mark" href="#tracing_tracing_enabled" id="tracing_tracing_enabled">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> <code>true</code> only if the <code>Tracing</code> object has been enabled.</li>
</ul>
<h3><code>trace_events.createTracing(options)</code><span><a class="mark" href="#tracing_trace_events_createtracing_options" id="tracing_trace_events_createtracing_options">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div>
<ul>
<li>
<p><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></p>
<ul>
<li><code>categories</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a> An array of trace category names. Values included
in the array are coerced to a string when possible. An error will be
thrown if the value cannot be coerced.</li>
</ul>
</li>
<li>Returns: <a href="tracing.html#tracing_tracing_object" class="type"><Tracing></a>.</li>
</ul>
<p>Creates and returns a <code>Tracing</code> object for the given set of <code>categories</code>.</p>
<pre><code class="language-js">const trace_events = require('trace_events');
const categories = ['node.perf', 'node.async_hooks'];
const tracing = trace_events.createTracing({ categories });
tracing.enable();
// do stuff
tracing.disable();
</code></pre>
<h3><code>trace_events.getEnabledCategories()</code><span><a class="mark" href="#tracing_trace_events_getenabledcategories" id="tracing_trace_events_getenabledcategories">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v10.0.0</span>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a></li>
</ul>
<p>Returns a comma-separated list of all currently-enabled trace event
categories. The current set of enabled trace event categories is determined
by the <em>union</em> of all currently-enabled <code>Tracing</code> objects and any categories
enabled using the <code>--trace-event-categories</code> flag.</p>
<p>Given the file <code>test.js</code> below, the command
<code>node --trace-event-categories node.perf test.js</code> will print
<code>'node.async_hooks,node.perf'</code> to the console.</p>
<pre><code class="language-js">const trace_events = require('trace_events');
const t1 = trace_events.createTracing({ categories: ['node.async_hooks'] });
const t2 = trace_events.createTracing({ categories: ['node.perf'] });
const t3 = trace_events.createTracing({ categories: ['v8'] });
t1.enable();
t2.enable();
console.log(trace_events.getEnabledCategories());
</code></pre>
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