cnysa (unpronouncible) is a module that allows you to see information about what the async_hooks module is doing under the covers. AsyncResources and their lifecycle events are displayed neatly in rows (see bottom for examples).

Pre-Require Hook

Pre-require cynsa/register in your application:

node --require cynsa/register app.js

If cnysa.json exists in the current working directory, it will be used as the configuration passed to the Cnysa constructor as described below.

API

const { Cnysa } = require('cnysa');

new Cnysa()

Constructs a new Cnysa instance.

Cnysa#enable()

Starts recording async events.

Cnysa#disable()

Stops recording async events.

Cnysa#mark(tag?: string|number)

Generate a special AsyncResource with the given tag. If tag is not specified, a monotonically increasing number is assigned to it.

The special AsyncResource will be displayed as a single event in cyan. This is good for determining the current running async resource.

Cnysa#createAsyncSnapshot(options: CnysaSnapshotOptions)

• options.width: Maximum number of characters to print on a single line before wrapping. Defaults to the current terminal width.
• options.ignoreTypes: A string or RegExp to filter out AsyncResource types.
• options.roots: A list of AsyncResource IDs that must be an ancestor of a given AsyncResource for it to be displayed. The default value, an empty list, is equivalent to specifying no constraint on ancestry.
• options.padding: Number that represents the amount of space between each depicted event.
• options.format: A string that represents how the output should be formatted. Currently, the available options are 'default' and 'svg' (which uses ansi-to-svg).

Returns a formatted async ancestry tree. Great for printing in the console.

All options are optional.

Cnysa#createAsyncStackTrace(options: CnysaStackTraceOptions)

• options.ignoreTypes: A string or RegExp to filter out AsyncResource types.

Experimental. Returns a formatted async stack trace.

All options are optional.

Cnysa.get()

Gets the most recently constructed Cnysa instance. If none were constructed, one is constructed automatically and returned. Therefore, this method is guaranteed to return a Cnysa instance.

This is useful when it is unknown whether cnysa has been used earlier in the application, especially as a command-line require.

Understanding output

For each AsyncResource, a timeline will be printed, with a number of colored symbols:

• Green * represents the async resource creation.
• Red * represents its destruction.
• Blue {...} represent running in an async scope.
• Gray - indicates the lifetime of the resource creation, and is bookended by * symbols.
• Cyan * represents a location where Cnysa#mark was called.

Examples

node --require cnysa/register -e "                 \
  fs.readFile('package.json', (err, contents) => { \
    require('cnysa').Cnysa.get().mark('hi');       \
  })"