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

Socket.IO is a Node.JS project that makes WebSockets and realtime possible in all browsers. It also enhances WebSockets by providing built-in multiplexing, horizontal scalability, automatic JSON encoding/decoding, and more.

How to Install

npm install socket.io

How to use

First, require socket.io:

var io = require('socket.io');

Next, attach it to a HTTP/HTTPS server. If you're using the fantastic express web framework:

Express 3.x

var app = express()
  , server = require('http').createServer(app)
  , io = io.listen(server);

server.listen(80);

io.sockets.on('connection', function (socket) {
  socket.emit('news', { hello: 'world' });
  socket.on('my other event', function (data) {
    console.log(data);
  });
});

Express 2.x

var app = express.createServer()
  , io = io.listen(app);

app.listen(80);

io.sockets.on('connection', function (socket) {
  socket.emit('news', { hello: 'world' });
  socket.on('my other event', function (data) {
    console.log(data);
  });
});

Finally, load it from the client side code:

<script src="/socket.io/socket.io.js"></script>
<script>
  var socket = io.connect('http://localhost');
  socket.on('news', function (data) {
    console.log(data);
    socket.emit('my other event', { my: 'data' });
  });
</script>

For more thorough examples, look at the examples/ directory.

Short recipes

Sending and receiving events.

Socket.IO allows you to emit and receive custom events. Besides connect, message and disconnect, you can emit custom events:

// note, io.listen(<port>) will create a http server for you
var io = require('socket.io').listen(80);

io.sockets.on('connection', function (socket) {
  io.sockets.emit('this', { will: 'be received by everyone' });

  socket.on('private message', function (from, msg) {
    console.log('I received a private message by ', from, ' saying ', msg);
  });

  socket.on('disconnect', function () {
    io.sockets.emit('user disconnected');
  });
});

Storing data associated to a client

Sometimes it's necessary to store data associated with a client that's necessary for the duration of the session.

Server side

var io = require('socket.io').listen(80);

io.sockets.on('connection', function (socket) {
  socket.on('set nickname', function (name) {
    socket.set('nickname', name, function () { socket.emit('ready'); });
  });

  socket.on('msg', function () {
    socket.get('nickname', function (err, name) {
      console.log('Chat message by ', name);
    });
  });
});

Client side

<script>
  var socket = io.connect('http://localhost');

  socket.on('connect', function () {
    socket.emit('set nickname', prompt('What is your nickname?'));
    socket.on('ready', function () {
      console.log('Connected !');
      socket.emit('msg', prompt('What is your message?'));
    });
  });
</script>

Restricting yourself to a namespace

If you have control over all the messages and events emitted for a particular application, using the default / namespace works.

If you want to leverage 3rd-party code, or produce code to share with others, socket.io provides a way of namespacing a socket.

This has the benefit of multiplexing a single connection. Instead of socket.io using two WebSocket connections, it'll use one.

The following example defines a socket that listens on '/chat' and one for '/news':

Server side

var io = require('socket.io').listen(80);

var chat = io
  .of('/chat')
  .on('connection', function (socket) {
    socket.emit('a message', { that: 'only', '/chat': 'will get' });
    chat.emit('a message', { everyone: 'in', '/chat': 'will get' });
  });

var news = io
  .of('/news');
  .on('connection', function (socket) {
    socket.emit('item', { news: 'item' });
  });

Client side:

<script>
  var chat = io.connect('http://localhost/chat')
    , news = io.connect('http://localhost/news');

  chat.on('connect', function () {
    chat.emit('hi!');
  });

  news.on('news', function () {
    news.emit('woot');
  });
</script>

Sending volatile messages.

Sometimes certain messages can be dropped. Let's say you have an app that shows realtime tweets for the keyword bieber.

If a certain client is not ready to receive messages (because of network slowness or other issues, or because he's connected through long polling and is in the middle of a request-response cycle), if he doesn't receive ALL the tweets related to bieber your application won't suffer.

In that case, you might want to send those messages as volatile messages.

Server side

var io = require('socket.io').listen(80);

io.sockets.on('connection', function (socket) {
  var tweets = setInterval(function () {
    getBieberTweet(function (tweet) {
      socket.volatile.emit('bieber tweet', tweet);
    });
  }, 100);

  socket.on('disconnect', function () {
    clearInterval(tweets);
  });
});

Client side

In the client side, messages are received the same way whether they're volatile or not.

Getting acknowledgements

Sometimes, you might want to get a callback when the client confirmed the message reception.

To do this, simply pass a function as the last parameter of .send or .emit. What's more, when you use .emit, the acknowledgement is done by you, which means you can also pass data along:

Server side

var io = require('socket.io').listen(80);

io.sockets.on('connection', function (socket) {
  socket.on('ferret', function (name, fn) {
    fn('woot');
  });
});

Client side

<script>
  var socket = io.connect(); // TIP: .connect with no args does auto-discovery
  socket.on('connect', function () { // TIP: you can avoid listening on `connect` and listen on events directly too!
    socket.emit('ferret', 'tobi', function (data) {
      console.log(data); // data will be 'woot'
    });
  });
</script>

Broadcasting messages

To broadcast, simply add a broadcast flag to emit and send method calls. Broadcasting means sending a message to everyone else except for the socket that starts it.

Server side

var io = require('socket.io').listen(80);

io.sockets.on('connection', function (socket) {
  socket.broadcast.emit('user connected');
  socket.broadcast.json.send({ a: 'message' });
});

Rooms

Sometimes you want to put certain sockets in the same room, so that it's easy to broadcast to all of them together.

Think of this as built-in channels for sockets. Sockets join and leave rooms in each socket.

Server side

var io = require('socket.io').listen(80);

io.sockets.on('connection', function (socket) {
  socket.join('justin bieber fans');
  socket.broadcast.to('justin bieber fans').emit('new fan');
  io.sockets.in('rammstein fans').emit('new non-fan');
});

Using it just as a cross-browser WebSocket

If you just want the WebSocket semantics, you can do that too. Simply leverage send and listen on the message event:

Server side

var io = require('socket.io').listen(80);

io.sockets.on('connection', function (socket) {
  socket.on('message', function () { });
  socket.on('disconnect', function () { });
});

Client side

<script>
  var socket = io.connect('http://localhost/');
  socket.on('connect', function () {
    socket.send('hi');

    socket.on('message', function (msg) {
      // my msg
    });
  });
</script>

Changing configuration

Configuration in socket.io is TJ-style:

Server side

var io = require('socket.io').listen(80);

io.configure(function () {
  io.set('transports', ['websocket', 'flashsocket', 'xhr-polling']);
});

io.configure('development', function () {
  io.set('transports', ['websocket', 'xhr-polling']);
  io.enable('log');
});

License

(The MIT License)

Copyright (c) 2011 Guillermo Rauch <[e-mail]>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.