# contentTracing

> Collect tracing data from Chromium to find performance bottlenecks and slow operations.

Process: [Main](../glossary.md#main-process)

This module does not include a web interface. To view recorded traces, use
[trace viewer][], available at `chrome://tracing` in Chrome.

**Note:** You should not use this module until the `ready` event of the app
module is emitted.

```javascript
const { app, contentTracing } = require('electron')

app.on('ready', () => {
  (async () => {
    await contentTracing.startRecording({
      include_categories: ['*']
    })
    console.log('Tracing started')
    await new Promise(resolve => setTimeout(resolve, 5000))
    const path = await contentTracing.stopRecording()
    console.log('Tracing data recorded to ' + path)
  })()
})
```

## Methods

The `contentTracing` module has the following methods:

### `contentTracing.getCategories()`

Returns `Promise<String[]>` - resolves with an array of category groups once all child processes have acknowledged the `getCategories` request

Get a set of category groups. The category groups can change as new code paths
are reached. See also the [list of built-in tracing
categories](https://chromium.googlesource.com/chromium/src/+/master/base/trace_event/builtin_categories.h).

### `contentTracing.startRecording(options)`

* `options` ([TraceConfig](structures/trace-config.md) | [TraceCategoriesAndOptions](structures/trace-categories-and-options.md))

Returns `Promise<void>` - resolved once all child processes have acknowledged the `startRecording` request.

Start recording on all processes.

Recording begins immediately locally and asynchronously on child processes
as soon as they receive the EnableRecording request.

If a recording is already running, the promise will be immediately resolved, as
only one trace operation can be in progress at a time.

### `contentTracing.stopRecording([resultFilePath])`

* `resultFilePath` String (optional)

Returns `Promise<String>` - resolves with a path to a file that contains the traced data once all child processes have acknowledged the `stopRecording` request

Stop recording on all processes.

Child processes typically cache trace data and only rarely flush and send
trace data back to the main process. This helps to minimize the runtime overhead
of tracing since sending trace data over IPC can be an expensive operation. So,
to end tracing, Chromium asynchronously asks all child processes to flush any
pending trace data.

Trace data will be written into `resultFilePath`. If `resultFilePath` is empty
or not provided, trace data will be written to a temporary file, and the path
will be returned in the promise.

### `contentTracing.getTraceBufferUsage()`

Returns `Promise<Object>` - Resolves with an object containing the `value` and `percentage` of trace buffer maximum usage

* `value` Number
* `percentage` Number

Get the maximum usage across processes of trace buffer as a percentage of the
full state.

[trace viewer]: https://github.com/catapult-project/catapult/blob/master/tracing
