Public API

Overview

Following is a simple overview of the public API.

MJS modules

import diagnostics_channel from 'node:diagnostics_channel';

// Get a reusable channel object
const channel = diagnostics_channel.channel('my-channel');

function onMessage(message, name) {
  // Received data
}

// Subscribe to the channel
diagnostics_channel.subscribe('my-channel', onMessage);

// Check if the channel has an active subscriber
if (channel.hasSubscribers) {
  // Publish data to the channel
  channel.publish({
    some: 'data',
  });
}

// Unsubscribe from the channel
diagnostics_channel.unsubscribe('my-channel', onMessage);

CJS modules

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

// Get a reusable channel object
const channel = diagnostics_channel.channel('my-channel');

function onMessage(message, name) {
  // Received data
}

// Subscribe to the channel
diagnostics_channel.subscribe('my-channel', onMessage);

// Check if the channel has an active subscriber
if (channel.hasSubscribers) {
  // Publish data to the channel
  channel.publish({
    some: 'data',
  });
}

// Unsubscribe from the channel
diagnostics_channel.unsubscribe('my-channel', onMessage);

diagnostics_channel.hasSubscribers(name)

• name <string> | <symbol> The channel name
• Returns: <boolean> If there are active subscribers

Check if there are active subscribers to the named channel. This is helpful if the message you want to send might be expensive to prepare.

This API is optional but helpful when trying to publish messages from very performance-sensitive code.

MJS modules

import diagnostics_channel from 'node:diagnostics_channel';

if (diagnostics_channel.hasSubscribers('my-channel')) {
  // There are subscribers, prepare and publish message
}

CJS modules

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

if (diagnostics_channel.hasSubscribers('my-channel')) {
  // There are subscribers, prepare and publish message
}

diagnostics_channel.channel(name)

• name <string> | <symbol> The channel name
• Returns: <Channel> The named channel object

This is the primary entry-point for anyone wanting to publish to a named channel. It produces a channel object which is optimized to reduce overhead at publish time as much as possible.

MJS modules

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

CJS modules

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

const channel = diagnostics_channel.channel('my-channel');

diagnostics_channel.subscribe(name, onMessage)

• name <string> | <symbol> The channel name
• onMessage <Function> The handler to receive channel messages
  • message <any> The message data
  • name <string> | <symbol> The name of the channel

Register a message handler to subscribe to this channel. This message handler will be run synchronously whenever a message is published to the channel. Any errors thrown in the message handler will trigger an 'uncaughtException'.

MJS modules

import diagnostics_channel from 'node:diagnostics_channel';

diagnostics_channel.subscribe('my-channel', (message, name) => {
  // Received data
});

CJS modules

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

diagnostics_channel.subscribe('my-channel', (message, name) => {
  // Received data
});

diagnostics_channel.unsubscribe(name, onMessage)

• name <string> | <symbol> The channel name
• onMessage <Function> The previous subscribed handler to remove
• Returns: <boolean> true if the handler was found, false otherwise.

Remove a message handler previously registered to this channel with diagnostics_channel.subscribe(name, onMessage).

MJS modules

import diagnostics_channel from 'node:diagnostics_channel';

function onMessage(message, name) {
  // Received data
}

diagnostics_channel.subscribe('my-channel', onMessage);

diagnostics_channel.unsubscribe('my-channel', onMessage);

CJS modules

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

function onMessage(message, name) {
  // Received data
}

diagnostics_channel.subscribe('my-channel', onMessage);

diagnostics_channel.unsubscribe('my-channel', onMessage);

Class: Channel

The class Channel represents an individual named channel within the data pipeline. It is used to track subscribers and to publish messages when there are subscribers present. It exists as a separate object to avoid channel lookups at publish time, enabling very fast publish speeds and allowing for heavy use while incurring very minimal cost. Channels are created with diagnostics_channel.channel(name), constructing a channel directly with new Channel(name) is not supported.

channel.hasSubscribers

• Returns: <boolean> If there are active subscribers

Check if there are active subscribers to this channel. This is helpful if the message you want to send might be expensive to prepare.

This API is optional but helpful when trying to publish messages from very performance-sensitive code.

MJS modules

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

if (channel.hasSubscribers) {
  // There are subscribers, prepare and publish message
}

CJS modules

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

const channel = diagnostics_channel.channel('my-channel');

if (channel.hasSubscribers) {
  // There are subscribers, prepare and publish message
}

channel.publish(message)

• message <any> The message to send to the channel subscribers

Publish a message to any subscribers to the channel. This will trigger message handlers synchronously so they will execute within the same context.

MJS modules

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

channel.publish({
  some: 'message',
});

CJS modules

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

const channel = diagnostics_channel.channel('my-channel');

channel.publish({
  some: 'message',
});

channel.subscribe(onMessage)

• onMessage <Function> The handler to receive channel messages
  • message <any> The message data
  • name <string> | <symbol> The name of the channel

Register a message handler to subscribe to this channel. This message handler will be run synchronously whenever a message is published to the channel. Any errors thrown in the message handler will trigger an 'uncaughtException'.

MJS modules

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

channel.subscribe((message, name) => {
  // Received data
});

CJS modules

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

const channel = diagnostics_channel.channel('my-channel');

channel.subscribe((message, name) => {
  // Received data
});

channel.unsubscribe(onMessage)

• onMessage <Function> The previous subscribed handler to remove
• Returns: <boolean> true if the handler was found, false otherwise.

Remove a message handler previously registered to this channel with channel.subscribe(onMessage).

MJS modules

import diagnostics_channel from 'node:diagnostics_channel';

const channel = diagnostics_channel.channel('my-channel');

function onMessage(message, name) {
  // Received data
}

channel.subscribe(onMessage);

channel.unsubscribe(onMessage);

CJS modules

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

const channel = diagnostics_channel.channel('my-channel');

function onMessage(message, name) {
  // Received data
}

channel.subscribe(onMessage);

channel.unsubscribe(onMessage);

Built-in Channels

While the diagnostics_channel API is now considered stable, the built-in channels currently available are not. Each channel must be declared stable independently.

HTTP

http.client.request.start

• request <http.ClientRequest>

Emitted when client starts a request.

http.client.response.finish

• request <http.ClientRequest>
• response <http.IncomingMessage>

Emitted when client receives a response.

http.server.request.start

• request <http.IncomingMessage>
• response <http.ServerResponse>
• socket <net.Socket>
• server <http.Server>

Emitted when server receives a request.

http.server.response.finish

• request <http.IncomingMessage>
• response <http.ServerResponse>
• socket <net.Socket>
• server <http.Server>

Emitted when server sends a response.

net.client.socket

• socket <net.Socket>

Emitted when a new TCP or pipe client socket is created.

net.server.socket

• socket <net.Socket>

Emitted when a new TCP or pipe connection is received.

udp.socket

• socket <dgram.Socket>

Emitted when a new UDP socket is created.