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<!doctype html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width"> <title>Trace Events | Node.js v10.24.1 Documentation</title> <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Lato:400,700,400italic"> <link rel="stylesheet" href="assets/style.css"> <link rel="stylesheet" href="assets/sh.css"> <link rel="canonical" href="https://nodejs.org/api/tracing.html"> </head> <body class="alt apidoc" id="api-section-tracing"> <div id="content" class="clearfix"> <div id="column2" class="interior"> <div id="intro" class="interior"> <a href="/" title="Go back to the home page"> Node.js </a> </div> <ul> <li><a href="documentation.html" class="nav-documentation">About these Docs</a></li> <li><a href="synopsis.html" class="nav-synopsis">Usage & Example</a></li> </ul> <div class="line"></div> <ul> <li><a href="assert.html" class="nav-assert">Assertion Testing</a></li> <li><a href="async_hooks.html" class="nav-async_hooks">Async 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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> </li> </ul> </li> </ul> </div> <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>Trace Event 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.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-txt">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-txt">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-txt">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> <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> </div> </div> </div> <script src="assets/sh_main.js"></script> <script src="assets/sh_javascript.min.js"></script> <script>highlight(undefined, undefined, 'pre');</script> </body> </html>