TypeScript icon, indicating that this package has built-in type declarations

6.5.4 • Public • Published

Engine.IO: the realtime engine

Build Status NPM version

Engine.IO is the implementation of transport-based cross-browser/cross-device bi-directional communication layer for Socket.IO.

How to use


(A) Listening on a port

const engine = require('engine.io');
const server = engine.listen(80);

server.on('connection', socket => {
  socket.send('utf 8 string');
  socket.send(Buffer.from([0, 1, 2, 3, 4, 5])); // binary data

(B) Intercepting requests for a http.Server

const engine = require('engine.io');
const http = require('http').createServer().listen(3000);
const server = engine.attach(http);

server.on('connection', socket => {
  socket.on('message', data => { });
  socket.on('close', () => { });

(C) Passing in requests

const engine = require('engine.io');
const server = new engine.Server();

server.on('connection', socket => {

// …
httpServer.on('upgrade', (req, socket, head) => {
  server.handleUpgrade(req, socket, head);

httpServer.on('request', (req, res) => {
  server.handleRequest(req, res);


<script src="/path/to/engine.io.js"></script>
  const socket = new eio.Socket('ws://localhost/');
  socket.on('open', () => {
    socket.on('message', data => {});
    socket.on('close', () => {});

For more information on the client refer to the engine-client repository.

What features does it have?

  • Maximum reliability. Connections are established even in the presence of:
    • proxies and load balancers.
    • personal firewall and antivirus software.
    • for more information refer to Goals and Architecture sections
  • Minimal client size aided by:
    • lazy loading of flash transports.
    • lack of redundant transports.
  • Scalable
    • load balancer friendly
  • Future proof
  • 100% Node.JS core style
    • No API sugar (left for higher level projects)




These are exposed by require('engine.io'):

  • flush
    • Called when a socket buffer is being flushed.
    • Arguments
      • Socket: socket being flushed
      • Array: write buffer
  • drain
    • Called when a socket buffer is drained
    • Arguments
      • Socket: socket being flushed
  • protocol (Number): protocol revision number
  • Server: Server class constructor
  • Socket: Socket class constructor
  • Transport (Function): transport constructor
  • transports (Object): map of available transports
  • ()

    • Returns a new Server instance. If the first argument is an http.Server then the new Server instance will be attached to it. Otherwise, the arguments are passed directly to the Server constructor.
    • Parameters
      • http.Server: optional, server to attach to.
      • Object: optional, options object (see Server#constructor api docs below)

    The following are identical ways to instantiate a server and then attach it.

const httpServer; // previously created with `http.createServer();` from node.js api.

// create a server first, and then attach
const eioServer = require('engine.io').Server();

// or call the module as a function to get `Server`
const eioServer = require('engine.io')();

// immediately attach
const eioServer = require('engine.io')(httpServer);

// with custom options
const eioServer = require('engine.io')(httpServer, {
  maxHttpBufferSize: 1e3
  • listen
    • Creates an http.Server which listens on the given port and attaches WS to it. It returns 501 Not Implemented for regular http requests.
    • Parameters
      • Number: port to listen on.
      • Object: optional, options object
      • Function: callback for listen.
    • Options
      • All options from Server.attach method, documented below.
      • Additionally See Server constructor below for options you can pass for creating the new Server
    • Returns Server
const engine = require('engine.io');
const server = engine.listen(3000, {
  pingTimeout: 2000,
  pingInterval: 10000

server.on('connection', /* ... */);
  • attach
    • Captures upgrade requests for a http.Server. In other words, makes a regular http.Server WebSocket-compatible.
    • Parameters
      • http.Server: server to attach to.
      • Object: optional, options object
    • Options
      • All options from Server.attach method, documented below.
      • Additionally See Server constructor below for options you can pass for creating the new Server
    • Returns Server a new Server instance.
const engine = require('engine.io');
const httpServer = require('http').createServer().listen(3000);
const server = engine.attach(httpServer, {
  wsEngine: require('eiows').Server // requires having eiows as dependency

server.on('connection', /* ... */);


The main server/manager. Inherits from EventEmitter.

  • connection

    • Fired when a new connection is established.
    • Arguments
      • Socket: a Socket object
  • initial_headers

    • Fired on the first request of the connection, before writing the response headers
    • Arguments
      • headers (Object): a hash of headers
      • req (http.IncomingMessage): the request
  • headers

    • Fired on the all requests of the connection, before writing the response headers
    • Arguments
      • headers (Object): a hash of headers
      • req (http.IncomingMessage): the request
  • connection_error

    • Fired when an error occurs when establishing the connection.
    • Arguments
      • error: an object with following properties:
        • req (http.IncomingMessage): the request that was dropped
        • code (Number): one of Server.errors
        • message (string): one of Server.errorMessages
        • context (Object): extra info about the error
Code Message
0 "Transport unknown"
1 "Session ID unknown"
2 "Bad handshake method"
3 "Bad request"
4 "Forbidden"
5 "Unsupported protocol version"

Important: if you plan to use Engine.IO in a scalable way, please keep in mind the properties below will only reflect the clients connected to a single process.

  • clients (Object): hash of connected clients by id.
  • clientsCount (Number): number of connected clients.
  • constructor
    • Initializes the server
    • Parameters
      • Object: optional, options object
    • Options
      • pingTimeout (Number): how many ms without a pong packet to consider the connection closed (20000)
      • pingInterval (Number): how many ms before sending a new ping packet (25000)
      • upgradeTimeout (Number): how many ms before an uncompleted transport upgrade is cancelled (10000)
      • maxHttpBufferSize (Number): how many bytes or characters a message can be, before closing the session (to avoid DoS). Default value is 1E6.
      • allowRequest (Function): A function that receives a given handshake or upgrade request as its first parameter, and can decide whether to continue or not. The second argument is a function that needs to be called with the decided information: fn(err, success), where success is a boolean value where false means that the request is rejected, and err is an error code.
      • transports (<Array> String): transports to allow connections to (['polling', 'websocket'])
      • allowUpgrades (Boolean): whether to allow transport upgrades (true)
      • perMessageDeflate (Object|Boolean): parameters of the WebSocket permessage-deflate extension (see ws module api docs). Set to true to enable. (defaults to false)
        • threshold (Number): data is compressed only if the byte size is above this value (1024)
      • httpCompression (Object|Boolean): parameters of the http compression for the polling transports (see zlib api docs). Set to false to disable. (true)
        • threshold (Number): data is compressed only if the byte size is above this value (1024)
      • cookie (Object|Boolean): configuration of the cookie that contains the client sid to send as part of handshake response headers. This cookie might be used for sticky-session. Defaults to not sending any cookie (false). See here for all supported options.
      • wsEngine (Function): what WebSocket server implementation to use. Specified module must conform to the ws interface (see ws module api docs). Default value is ws. An alternative c++ addon is also available by installing eiows module.
      • cors (Object): the options that will be forwarded to the cors module. See there for all available options. Defaults to no CORS allowed.
      • initialPacket (Object): an optional packet which will be concatenated to the handshake packet emitted by Engine.IO.
      • allowEIO3 (Boolean): whether to support v3 Engine.IO clients (defaults to false)
  • close
    • Closes all clients
    • Returns Server for chaining
  • handleRequest
    • Called internally when a Engine request is intercepted.
    • Parameters
      • http.IncomingMessage: a node request object
      • http.ServerResponse: a node response object
    • Returns Server for chaining
  • handleUpgrade
    • Called internally when a Engine ws upgrade is intercepted.
    • Parameters (same as upgrade event)
      • http.IncomingMessage: a node request object
      • net.Stream: TCP socket for the request
      • Buffer: legacy tail bytes
    • Returns Server for chaining
  • attach
    • Attach this Server instance to an http.Server
    • Captures upgrade requests for a http.Server. In other words, makes a regular http.Server WebSocket-compatible.
    • Parameters
      • http.Server: server to attach to.
      • Object: optional, options object
    • Options
      • path (String): name of the path to capture (/engine.io).
      • destroyUpgrade (Boolean): destroy unhandled upgrade requests (true)
      • destroyUpgradeTimeout (Number): milliseconds after which unhandled requests are ended (1000)
  • generateId
    • Generate a socket id.
    • Overwrite this method to generate your custom socket id.
    • Parameters
      • http.IncomingMessage: a node request object
    • Returns A socket id for connected client.


A representation of a client. Inherits from EventEmitter.

  • close
    • Fired when the client is disconnected.
    • Arguments
      • String: reason for closing
      • Object: description object (optional)
  • message
    • Fired when the client sends a message.
    • Arguments
      • String or Buffer: Unicode string or Buffer with binary contents
  • error
    • Fired when an error occurs.
    • Arguments
      • Error: error object
  • upgrading
    • Fired when the client starts the upgrade to a better transport like WebSocket.
    • Arguments
      • Object: the transport
  • upgrade
    • Fired when the client completes the upgrade to a better transport like WebSocket.
    • Arguments
      • Object: the transport
  • flush
    • Called when the write buffer is being flushed.
    • Arguments
      • Array: write buffer
  • drain
    • Called when the write buffer is drained
  • packet
    • Called when a socket received a packet (message, ping)
    • Arguments
      • type: packet type
      • data: packet data (if type is message)
  • packetCreate
    • Called before a socket sends a packet (message, ping)
    • Arguments
      • type: packet type
      • data: packet data (if type is message)
  • heartbeat
    • Called when ping or pong packed is received (depends of client version)
  • id (String): unique identifier
  • server (Server): engine parent reference
  • request (http.IncomingMessage): request that originated the Socket
  • upgraded (Boolean): whether the transport has been upgraded
  • readyState (String): opening|open|closing|closed
  • transport (Transport): transport reference
  • send:
    • Sends a message, performing message = toString(arguments[0]) unless sending binary data, which is sent as is.
    • Parameters
      • StringBuffer | ArrayBuffer | ArrayBufferView: a string or any object implementing toString(), with outgoing data, or a Buffer or ArrayBuffer with binary data. Also any ArrayBufferView can be sent as is.
      • Object: optional, options object
      • Function: optional, a callback executed when the message gets flushed out by the transport
    • Options
      • compress (Boolean): whether to compress sending data. This option might be ignored and forced to be true when using polling. (true)
    • Returns Socket for chaining
  • close
    • Disconnects the client
    • Returns Socket for chaining


Exposed in the eio global namespace (in the browser), or by require('engine.io-client') (in Node.JS).

For the client API refer to the engine-client repository.

Debug / logging

Engine.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 Engine.IO's debugging scopes you can use:

DEBUG=engine* node myapp


  • polling: XHR / JSONP polling transport.
  • websocket: WebSocket transport.



The support channels for engine.io are the same as socket.io:


To contribute patches, run tests or benchmarks, make sure to clone the repository:

git clone git://github.com/LearnBoost/engine.io.git


cd engine.io
npm install


Tests run with npm test. It runs the server tests that are aided by the usage of engine.io-client.

Make sure npm install is run first.


The main goal of Engine is ensuring the most reliable realtime communication. Unlike the previous Socket.IO core, it always establishes a long-polling connection first, then tries to upgrade to better transports that are "tested" on the side.

During the lifetime of the Socket.IO projects, we've found countless drawbacks to relying on HTML5 WebSocket or Flash Socket as the first connection mechanisms.

Both are clearly the right way of establishing a bidirectional communication, with HTML5 WebSocket being the way of the future. However, to answer most business needs, alternative traditional HTTP 1.1 mechanisms are just as good as delivering the same solution.

WebSocket based connections have two fundamental benefits:

  1. Better server performance
  • A: Load balancers
    Load balancing a long polling connection poses a serious architectural nightmare since requests can come from any number of open sockets by the user agent, but they all need to be routed to the process and computer that owns the Engine connection. This negatively impacts RAM and CPU usage.
  • B: Network traffic
    WebSocket is designed around the premise that each message frame has to be surrounded by the least amount of data. In HTTP 1.1 transports, each message frame is surrounded by HTTP headers and chunked encoding frames. If you try to send the message "Hello world" with xhr-polling, the message ultimately becomes larger than if you were to send it with WebSocket.
  • C: Lightweight parser
    As an effect of B, the server has to do a lot more work to parse the network data and figure out the message when traditional HTTP requests are used (as in long polling). This means that another advantage of WebSocket is less server CPU usage.
  1. Better user experience

    Due to the reasons stated in point 1, the most important effect of being able to establish a WebSocket connection is raw data transfer speed, which translates in some cases in better user experience.

    Applications with heavy realtime interaction (such as games) will benefit greatly, whereas applications like realtime chat (Gmail/Facebook), newsfeeds (Facebook) or timelines (Twitter) will have negligible user experience improvements.

Having said this, attempting to establish a WebSocket connection directly so far has proven problematic:

  1. Proxies
    Many corporate proxies block WebSocket traffic.

  2. Personal firewall and antivirus software
    As a result of our research, we've found that at least 3 personal security applications block WebSocket traffic.

  3. Cloud application platforms
    Platforms like Heroku or No.de have had trouble keeping up with the fast-paced nature of the evolution of the WebSocket protocol. Applications therefore end up inevitably using long polling, but the seamless installation experience of Socket.IO we strive for ("require() it and it just works") disappears.

Some of these problems have solutions. In the case of proxies and personal programs, however, the solutions many times involve upgrading software. Experience has shown that relying on client software upgrades to deliver a business solution is fruitless: the very existence of this project has to do with a fragmented panorama of user agent distribution, with clients connecting with latest versions of the most modern user agents (Chrome, Firefox and Safari), but others with versions as low as IE 5.5.

From the user perspective, an unsuccessful WebSocket connection can translate in up to at least 10 seconds of waiting for the realtime application to begin exchanging data. This perceptively hurts user experience.

To summarize, Engine focuses on reliability and user experience first, marginal potential UX improvements and increased server performance second. Engine is the result of all the lessons learned with WebSocket in the wild.


The main premise of Engine, and the core of its existence, is the ability to swap transports on the fly. A connection starts as xhr-polling, but it can switch to WebSocket.

The central problem this poses is: how do we switch transports without losing messages?

Engine only switches from polling to another transport in between polling cycles. Since the server closes the connection after a certain timeout when there's no activity, and the polling transport implementation buffers messages in between connections, this ensures no message loss and optimal performance.

Another benefit of this design is that we workaround almost all the limitations of Flash Socket, such as slow connection times, increased file size (we can safely lazy load it without hurting user experience), etc.


Can I use engine without Socket.IO ?

Absolutely. Although the recommended framework for building realtime applications is Socket.IO, since it provides fundamental features for real-world applications such as multiplexing, reconnection support, etc.

Engine is to Socket.IO what Connect is to Express. An essential piece for building realtime frameworks, but something you probably won't be using for building actual applications.

Does the server serve the client?

No. The main reason is that Engine is meant to be bundled with frameworks. Socket.IO includes Engine, therefore serving two clients is not necessary. If you use Socket.IO, including

<script src="/socket.io/socket.io.js">

has you covered.

Can I implement Engine in other languages?

Absolutely. The engine.io-protocol repository contains the most up-to-date description of the specification at all times.


(The MIT License)

Copyright (c) 2014 Guillermo Rauch <guillermo@learnboost.com>

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.





Package Sidebar


npm i engine.io

Weekly Downloads






Unpacked Size

162 kB

Total Files


Last publish


  • rauchg
  • darrachequesne