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1# socket.io
2[![Run on Repl.it](https://repl.it/badge/github/socketio/socket.io)](https://replit.com/@socketio/socketio-minimal-example)
3[![Backers on Open Collective](https://opencollective.com/socketio/backers/badge.svg)](#backers) [![Sponsors on Open Collective](https://opencollective.com/socketio/sponsors/badge.svg)](#sponsors)
4[![Build Status](https://github.com/socketio/socket.io/workflows/CI/badge.svg)](https://github.com/socketio/socket.io/actions)
5[![NPM version](https://badge.fury.io/js/socket.io.svg)](https://www.npmjs.com/package/socket.io)
6![Downloads](https://img.shields.io/npm/dm/socket.io.svg?style=flat)
7[![](https://slackin-socketio.now.sh/badge.svg)](https://slackin-socketio.now.sh)
8
9## Features
10
11Socket.IO enables real-time bidirectional event-based communication. It consists of:
12
13- a Node.js server (this repository)
14- a [Javascript client library](https://github.com/socketio/socket.io-client) for the browser (or a Node.js client)
15
16Some implementations in other languages are also available:
17
18- [Java](https://github.com/socketio/socket.io-client-java)
19- [C++](https://github.com/socketio/socket.io-client-cpp)
20- [Swift](https://github.com/socketio/socket.io-client-swift)
21- [Dart](https://github.com/rikulo/socket.io-client-dart)
22- [Python](https://github.com/miguelgrinberg/python-socketio)
23- [.NET](https://github.com/doghappy/socket.io-client-csharp)
24- [Rust](https://github.com/1c3t3a/rust-socketio)
25- [PHP](https://github.com/ElephantIO/elephant.io)
26
27Its main features are:
28
29#### Reliability
30
31Connections are established even in the presence of:
32 - proxies and load balancers.
33 - personal firewall and antivirus software.
34
35For this purpose, it relies on [Engine.IO](https://github.com/socketio/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](https://github.com/socketio/engine.io#goals) section for more information.
36
37#### Auto-reconnection support
38
39Unless instructed otherwise a disconnected client will try to reconnect forever, until the server is available again. Please see the available reconnection options [here](https://socket.io/docs/v3/client-api/#new-Manager-url-options).
40
41#### Disconnection detection
42
43A 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.
44
45That 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.
46
47#### Binary support
48
49Any serializable data structures can be emitted, including:
50
51- [ArrayBuffer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer) and [Blob](https://developer.mozilla.org/en-US/docs/Web/API/Blob) in the browser
52- [ArrayBuffer](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer) and [Buffer](https://nodejs.org/api/buffer.html) in Node.js
53
54#### Simple and convenient API
55
56Sample code:
57
58```js
59io.on('connection', socket => {
60 socket.emit('request', /* … */); // emit an event to the socket
61 io.emit('broadcast', /* … */); // emit an event to all connected sockets
62 socket.on('reply', () => { /* … */ }); // listen to the event
63});
64```
65
66#### Cross-browser
67
68Browser support is tested in Sauce Labs:
69
70[![Sauce Test Status](https://saucelabs.com/browser-matrix/socket.svg)](https://saucelabs.com/u/socket)
71
72#### Multiplexing support
73
74In 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.
75
76#### Room support
77
78Within 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.
79
80This is a useful feature to send notifications to a group of users, or to a given user connected on several devices for example.
81
82
83**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](https://github.com/socketio/socket.io-protocol).
84
85## Installation
86
87```bash
88// with npm
89npm install socket.io
90
91// with yarn
92yarn add socket.io
93```
94
95## How to use
96
97The following example attaches socket.io to a plain Node.JS
98HTTP server listening on port `3000`.
99
100```js
101const server = require('http').createServer();
102const io = require('socket.io')(server);
103io.on('connection', client => {
104 client.on('event', data => { /* … */ });
105 client.on('disconnect', () => { /* … */ });
106});
107server.listen(3000);
108```
109
110### Standalone
111
112```js
113const io = require('socket.io')();
114io.on('connection', client => { ... });
115io.listen(3000);
116```
117
118### Module syntax
119
120```js
121import { Server } from "socket.io";
122const io = new Server(server);
123io.listen(3000);
124```
125
126### In conjunction with Express
127
128Starting with **3.0**, express applications have become request handler
129functions that you pass to `http` or `http` `Server` instances. You need
130to pass the `Server` to `socket.io`, not the express application
131function. Also make sure to call `.listen` on the `server`, not the `app`.
132
133```js
134const app = require('express')();
135const server = require('http').createServer(app);
136const io = require('socket.io')(server);
137io.on('connection', () => { /* … */ });
138server.listen(3000);
139```
140
141### In conjunction with Koa
142
143Like Express.JS, Koa works by exposing an application as a request
144handler function, but only by calling the `callback` method.
145
146```js
147const app = require('koa')();
148const server = require('http').createServer(app.callback());
149const io = require('socket.io')(server);
150io.on('connection', () => { /* … */ });
151server.listen(3000);
152```
153
154### In conjunction with Fastify
155
156To integrate Socket.io in your Fastify application you just need to
157register `fastify-socket.io` plugin. It will create a `decorator`
158called `io`.
159
160```js
161const app = require('fastify')();
162app.register(require('fastify-socket.io'));
163app.io.on('connection', () => { /* … */ });
164app.listen(3000);
165```
166
167## Documentation
168
169Please see the documentation [here](https://socket.io/docs/).
170
171The source code of the website can be found [here](https://github.com/socketio/socket.io-website). Contributions are welcome!
172
173## Debug / logging
174
175Socket.IO is powered by [debug](https://github.com/visionmedia/debug).
176In order to see all the debug output, run your app with the environment variable
177`DEBUG` including the desired scope.
178
179To see the output from all of Socket.IO's debugging scopes you can use:
180
181```
182DEBUG=socket.io* node myapp
183```
184
185## Testing
186
187```
188npm test
189```
190This runs the `gulp` task `test`. By default the test will be run with the source code in `lib` directory.
191
192Set the environmental variable `TEST_VERSION` to `compat` to test the transpiled es5-compat version of the code.
193
194The `gulp` task `test` will always transpile the source code into es5 and export to `dist` first before running the test.
195
196
197## Backers
198
199Support us with a monthly donation and help us continue our activities. [[Become a backer](https://opencollective.com/socketio#backer)]
200
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231
232
233## Sponsors
234
235Become a sponsor and get your logo on our README on Github with a link to your site. [[Become a sponsor](https://opencollective.com/socketio#sponsor)]
236
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267
268
269## License
270
271[MIT](LICENSE)