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Table of contents

- Trace events
  - The `node:trace_events` module
    - `Tracing` object
      - `tracing.categories`
      - `tracing.disable()`
      - `tracing.enable()`
      - `tracing.enabled`
    - `trace_events.createTracing(options)`
    - `trace_events.getEnabledCategories()`
  - Examples
    - Collect trace events data by inspector

## Trace events#

> Stability: 1 - Experimental

The `node:trace_events` module provides a mechanism to centralize tracing information generated by V8, Node.js core, and userspace code.

Tracing can be enabled with the `--trace-event-categories` command-line flag or by using the `node:trace_events` module. The `--trace-event-categories` flag accepts a list of comma-separated category names.

The available categories are:

- `node`: An empty placeholder.
- `node.async_hooks`: Enables capture of detailed `async_hooks` trace data. The `async_hooks` events have a unique `asyncId` and a special `triggerId` `triggerAsyncId` property.
- `node.bootstrap`: Enables capture of Node.js bootstrap milestones.
- `node.console`: Enables capture of `console.time()` and `console.count()` output.
- `node.threadpoolwork.sync`: Enables capture of trace data for threadpool synchronous operations, such as `blob`, `zlib`, `crypto` and `node_api`.
- `node.threadpoolwork.async`: Enables capture of trace data for threadpool asynchronous operations, such as `blob`, `zlib`, `crypto` and `node_api`.
- `node.dns.native`: Enables capture of trace data for DNS queries.
- `node.net.native`: Enables capture of trace data for network.
- `node.environment`: Enables capture of Node.js Environment milestones.
- `node.fs.sync`: Enables capture of trace data for file system sync methods.
- `node.fs_dir.sync`: Enables capture of trace data for file system sync directory methods.
- `node.fs.async`: Enables capture of trace data for file system async methods.
- `node.fs_dir.async`: Enables capture of trace data for file system async directory methods.
- `node.perf`: Enables capture of Performance API measurements.
  - `node.perf.usertiming`: Enables capture of only Performance API User Timing measures and marks.
  - `node.perf.timerify`: Enables capture of only Performance API timerify measurements.
- `node.promises.rejections`: Enables capture of trace data tracking the number of unhandled Promise rejections and handled-after-rejections.
- `node.vm.script`: Enables capture of trace data for the `node:vm` module's `runInNewContext()`, `runInContext()`, and `runInThisContext()` methods.
- `v8`: The V8 events are GC, compiling, and execution related.
- `node.http`: Enables capture of trace data for http request / response.
- `node.module_timer`: Enables capture of trace data for CJS Module loading.

By default the `node`, `node.async_hooks`, and `v8` categories are enabled.

    node --trace-event-categories v8,node,node.async_hooks server.js

Prior versions of Node.js required the use of the `--trace-events-enabled` flag to enable trace events. This requirement has been removed. However, the `--trace-events-enabled` flag may still be used and will enable the `node`, `node.async_hooks`, and `v8` trace event categories by default.

    node --trace-events-enabled

    # is equivalent to

    node --trace-event-categories v8,node,node.async_hooks

Alternatively, trace events may be enabled using the `node:trace_events` module:

    const trace_events = require('node: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

Running Node.js with tracing enabled will produce log files that can be opened in the `chrome://tracing` tab of Chrome.

The logging file is by default called `node_trace.${rotation}.log`, where `${rotation}` is an incrementing log-rotation id. The filepath pattern can be specified with `--trace-event-file-pattern` that accepts a template string that supports `${rotation}` and `${pid}`:

    node --trace-event-categories v8 --trace-event-file-pattern '${pid}-${rotation}.log' server.js

To guarantee that the log file is properly generated after signal events like `SIGINT`, `SIGTERM`, or `SIGBREAK`, make sure to have the appropriate handlers in your code, such as:

    process.on('SIGINT', function onSigint() {
      console.info('Received SIGINT.');
      process.exit(130);  // Or applicable exit code depending on OS and signal
    });

The tracing system uses the same time source as the one used by `process.hrtime()`. However the trace-event timestamps are expressed in microseconds, unlike `process.hrtime()` which returns nanoseconds.

The features from this module are not available in `Worker` threads.

### The `node:trace_events` module#

Added in: v10.0.0

#### `Tracing` object#

Added in: v10.0.0

The `Tracing` object is used to enable or disable tracing for sets of categories. Instances are created using the `trace_events.createTracing()` method.

When created, the `Tracing` object is disabled. Calling the `tracing.enable()` method adds the categories to the set of enabled trace event categories. Calling `tracing.disable()` will remove the categories from the set of enabled trace event categories.

##### `tracing.categories`#

Added in: v10.0.0

- <string>

A comma-separated list of the trace event categories covered by this `Tracing` object.

##### `tracing.disable()`#

Added in: v10.0.0

Disables this `Tracing` object.

Only trace event categories not covered by other enabled `Tracing` objects and not specified by the `--trace-event-categories` flag will be disabled.

    const trace_events = require('node: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());

##### `tracing.enable()`#

Added in: v10.0.0

Enables this `Tracing` object for the set of categories covered by the `Tracing` object.

##### `tracing.enabled`#

Added in: v10.0.0

- <boolean> `true` only if the `Tracing` object has been enabled.

#### `trace_events.createTracing(options)`#

Added in: v10.0.0

- `options` <Object>
  - `categories` <string[]> 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.
- Returns: <Tracing>.

Creates and returns a `Tracing` object for the given set of `categories`.

    const trace_events = require('node:trace_events');
    const categories = ['node.perf', 'node.async_hooks'];
    const tracing = trace_events.createTracing({ categories });
    tracing.enable();
    // do stuff
    tracing.disable();

#### `trace_events.getEnabledCategories()`#

Added in: v10.0.0

- Returns: <string>

Returns a comma-separated list of all currently-enabled trace event categories. The current set of enabled trace event categories is determined by the union of all currently-enabled `Tracing` objects and any categories enabled using the `--trace-event-categories` flag.

Given the file `test.js` below, the command `node --trace-event-categories node.perf test.js` will print `'node.async_hooks,node.perf'` to the console.

    const trace_events = require('node: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());

### Examples#

#### Collect trace events data by inspector#

    'use strict';

    const { Session } = require('inspector');
    const session = new Session();
    session.connect();

    function post(message, data) {
      return new Promise((resolve, reject) => {
        session.post(message, data, (err, result) => {
          if (err)
            reject(new Error(JSON.stringify(err)));
          else
            resolve(result);
        });
      });
    }

    async function collect() {
      const data = [];
      session.on('NodeTracing.dataCollected', (chunk) => data.push(chunk));
      session.on('NodeTracing.tracingComplete', () => {
        // done
      });
      const traceConfig = { includedCategories: ['v8'] };
      await post('NodeTracing.start', { traceConfig });
      // do something
      setTimeout(() => {
        post('NodeTracing.stop').then(() => {
          session.disconnect();
          console.log(data);
        });
      }, 1000);
    }

    collect();