# Trace Events

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> 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.

```txt
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.

```txt
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:

```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
```

Running Node.js with tracing enabled will produce log files that can be opened
in the [`chrome://tracing`](https://www.chromium.org/developers/how-tos/trace-event-profiling-tool)
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}`:

```txt
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
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### `Tracing` object
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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`
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* {string}

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

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

```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());
```

#### `tracing.enable()`
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Enables this `Tracing` object for the set of categories covered by the
`Tracing` object.

#### `tracing.enabled`
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* {boolean} `true` only if the `Tracing` object has been enabled.

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

```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();
```

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

```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());
```

[Performance API]: perf_hooks.html
[V8]: v8.html
[`async_hooks`]: async_hooks.html
