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    Realtime application framework for Node.JS, with HTML5 WebSockets and cross-browser fallbacks support.
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    socket.io

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    Features

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

    Some implementations in other languages are also available:

    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

    An 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:

    Simple and convenient API

    Sample code:

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

    Cross-browser

    Browser support is tested in Saucelabs:

    Sauce Test Status

    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 --save
    

    How to use

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

    var server = require('http').createServer();
    var io = require('socket.io')(server);
    io.on('connection', function(client){
      client.on('event', function(data){});
      client.on('disconnect', function(){});
    });
    server.listen(3000);
    

    Standalone

    var io = require('socket.io')();
    io.on('connection', function(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.

    var app = require('express')();
    var server = require('http').createServer(app);
    var io = require('socket.io')(server);
    io.on('connection', function(){ /* … */ });
    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.

    var app = require('koa')();
    var server = require('http').createServer(app.callback());
    var io = require('socket.io')(server);
    io.on('connection', function(){ /* … */ });
    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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    License

    MIT

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