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# Trace Events#

Stability: 1 - Experimental

Trace Event 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 `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.fs.sync` - Enables capture of trace data for file system sync 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 `vm` module's `runInNewContext()`, `runInContext()`, and `runInThisContext()` methods.
- `v8` - The V8 events are GC, compiling, and execution related.

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 `trace_events` module:

    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

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

Starting with Node.js 10.0.0, 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 `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('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('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('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());