socket.io

What is socket.io?

The socket.io npm package enables real-time, bidirectional and event-based communication between web clients and servers. It is primarily used to build real-time web applications and has features like broadcasting to multiple sockets, storing data associated with each client, and asynchronous I/O.

What are socket.io's main functionalities?

Real-time bidirectional event-based communication

This feature allows the server to establish a WebSocket connection with the client for real-time communication. The server listens for events like 'connection', 'chat message', and 'disconnect' to react accordingly.

const io = require('socket.io')(3000);

io.on('connection', (socket) => {
  console.log('a user connected');
  socket.on('chat message', (msg) => {
    io.emit('chat message', msg);
  });
  socket.on('disconnect', () => {
    console.log('user disconnected');
  });
});

Broadcasting

Broadcasting allows a server to send a message to all connected clients except for the one that triggered the message. This is useful for notifying all users about the actions of one.

io.on('connection', (socket) => {
  socket.broadcast.emit('user connected', 'A new user has joined the chat');
});

Namespaces and Rooms

Socket.IO allows for the creation of Namespaces and Rooms which can be used to divide the clients into different groups for targeted broadcasting and communication.

const chat = io.of('/chat').on('connection', (socket) => {
  socket.join('some room');
  chat.to('some room').emit('some event');
});

Other packages similar to socket.io

ws

The 'ws' package is a simple WebSocket library for Node.js. Unlike socket.io, it does not provide high-level features like broadcasting to multiple sockets or automatic reconnection.

engine.io

Engine.io is the low-level engine that powers socket.io. It provides the bare WebSocket-like API and is responsible for handling the transport logistics. It is less feature-rich compared to socket.io.

sockjs

SockJS is a JavaScript library that provides a WebSocket-like object. It is similar to socket.io in that it offers a fallback mechanism for environments where WebSockets are not supported.

faye-websocket

Faye is a set of tools for simple publish-subscribe messaging between web clients. It's more focused on the pub/sub paradigm and lacks some of the real-time communication features that socket.io offers.

README

socket.io

Features

Socket.IO enables real-time bidirectional event-based communication. It consists of:

• a Node.js server (this repository)
• a Javascript client library for the browser (or a Node.js client)

Some implementations in other languages are also available:

• Java
• C++
• Swift
• Dart

Its main features are:

Reliability

Connections are established even in the presence of:

• proxies and load balancers.
• personal firewall and antivirus software.

For this purpose, it relies on Engine.IO, which first establishes a long-polling connection, then tries to upgrade to better transports that are "tested" on the side, like WebSocket. Please see the Goals section for more information.

Auto-reconnection support

Unless instructed otherwise a disconnected client will try to reconnect forever, until the server is available again. Please see the available reconnection options here.

Disconnection detection

A heartbeat mechanism is implemented at the Engine.IO level, allowing both the server and the client to know when the other one is not responding anymore.

That functionality is achieved with timers set on both the server and the client, with timeout values (the pingInterval and pingTimeout parameters) shared during the connection handshake. Those timers require any subsequent client calls to be directed to the same server, hence the sticky-session requirement when using multiples nodes.

Binary support

Any serializable data structures can be emitted, including:

• ArrayBuffer and Blob in the browser
• ArrayBuffer and Buffer in Node.js

Simple and convenient API

Sample code:

io.on('connection', socket => {
  socket.emit('request', /* … */); // emit an event to the socket
  io.emit('broadcast', /* … */); // emit an event to all connected sockets
  socket.on('reply', () => { /* … */ }); // listen to the event
});

Cross-browser

Browser support is tested in Saucelabs:

Multiplexing support

In order to create separation of concerns within your application (for example per module, or based on permissions), Socket.IO allows you to create several Namespaces, which will act as separate communication channels but will share the same underlying connection.

Room support

Within each Namespace, you can define arbitrary channels, called Rooms, that sockets can join and leave. You can then broadcast to any given room, reaching every socket that has joined it.

This is a useful feature to send notifications to a group of users, or to a given user connected on several devices for example.

Note: Socket.IO is not a WebSocket implementation. Although Socket.IO indeed uses WebSocket as a transport when possible, it adds some metadata to each packet: the packet type, the namespace and the ack id when a message acknowledgement is needed. That is why a WebSocket client will not be able to successfully connect to a Socket.IO server, and a Socket.IO client will not be able to connect to a WebSocket server (like ws://echo.websocket.org) either. Please see the protocol specification here.

Installation

npm install socket.io

How to use

The following example attaches socket.io to a plain Node.JS HTTP server listening on port 3000.

const server = require('http').createServer();
const io = require('socket.io')(server);
io.on('connection', client => {
  client.on('event', data => { /* … */ });
  client.on('disconnect', () => { /* … */ });
});
server.listen(3000);

Standalone

const io = require('socket.io')();
io.on('connection', client => { ... });
io.listen(3000);

In conjunction with Express

Starting with 3.0, express applications have become request handler functions that you pass to http or http Server instances. You need to pass the Server to socket.io, and not the express application function. Also make sure to call .listen on the server, not the app.

const app = require('express')();
const server = require('http').createServer(app);
const io = require('socket.io')(server);
io.on('connection', () => { /* … */ });
server.listen(3000);

In conjunction with Koa

Like Express.JS, Koa works by exposing an application as a request handler function, but only by calling the callback method.

const app = require('koa')();
const server = require('http').createServer(app.callback());
const io = require('socket.io')(server);
io.on('connection', () => { /* … */ });
server.listen(3000);

Documentation

Please see the documentation here. Contributions are welcome!

Debug / logging

Socket.IO is powered by debug. In order to see all the debug output, run your app with the environment variable DEBUG including the desired scope.

To see the output from all of Socket.IO's debugging scopes you can use:

DEBUG=socket.io* node myapp

Testing

npm test

This runs the gulp task test. By default the test will be run with the source code in lib directory.

Set the environmental variable TEST_VERSION to compat to test the transpiled es5-compat version of the code.

The gulp task test will always transpile the source code into es5 and export to dist first before running the test.

Backers

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Sponsors

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License

MIT