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Engine.IO: the realtime engine

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

How to use

Server

(A) Listening on a port

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

server.on('connection', function(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

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

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

(C) Passing in requests

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

server.on('connection', function(socket){
  socket.send('hi');
});

// …
httpServer.on('upgrade', function(req, socket, head){
  server.handleUpgrade(req, socket, head);
});
httpServer.on('request', function(req, res){
  server.handleRequest(req, res);
});

Client

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

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)
Written in readable vanilla JavaScript

API

Server

Top-level

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

Events

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

Properties

protocol (Number): protocol revision number
Server: Server class constructor
Socket: Socket class constructor
Transport (Function): transport constructor
transports (Object): map of available transports

Methods

()
- 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.

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

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

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

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

// with custom options
var 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

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

var engine = require('engine.io');
var httpServer = require('http').createServer().listen(3000);
var server = engine.attach(httpServer, {
  wsEngine: 'uws' // requires having uws as dependency
});

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

Server

The main server/manager. Inherits from EventEmitter.

Events

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

Properties

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.

Methods

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 (1MB).
- 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. (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 (String|Boolean): name of the HTTP cookie that
  contains the client sid to send as part of handshake response
  headers. Set to false to not send one. (io)
- cookiePath (String|Boolean): path of the above cookie
  option. If false, no path will be sent, which means browsers will only send the cookie on the engine.io attached path (/engine.io).
  Set false to not save io cookie on all requests. (/)
- cookieHttpOnly (Boolean): If true HttpOnly io cookie cannot be accessed by client-side APIs, such as JavaScript. (true) This option has no effect if cookie or cookiePath is set to false.
- wsEngine (String): 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 uws module.
- initialPacket (Object): an optional packet which will be concatenated to the handshake packet emitted by Engine.IO.
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)
- handlePreflightRequest (Boolean|Function): whether to let engine.io handle the OPTIONS requests. You can also pass a custom function to handle the requests (true)
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.

Socket

A representation of a client. Inherits from EventEmitter.

Events

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
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, pong)
- Arguments
- type: packet type
- data: packet data (if type is message)

Properties

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

Methods

send:
- Sends a message, performing message = toString(arguments[0]) unless
  sending binary data, which is sent as is.
- Parameters
- String | Buffer | 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

Client

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

Transports

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

Plugins

engine.io-conflation: Makes conflation and aggregation of messages straightforward.

Support

The support channels for engine.io are the same as socket.io:
- irc.freenode.net #socket.io
- Google Groups
- Website

Development

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

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

Then:

cd engine.io
npm install

Tests

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.

Goals

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:

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.

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:

Proxies

Many corporate proxies block WebSocket traffic.
Personal firewall and antivirus software

As a result of our research, we've found that at least 3 personal security
applications block WebSocket traffic.
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.

Architecture

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.

FAQ

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, and the parser implementation in JavaScript.

License

(The MIT License)

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.

Engine.IO: the realtime engine

How to use

Server

(A) Listening on a port

(B) Intercepting requests for a http.Server

(C) Passing in requests

Client

What features does it have?

API

Server

Top-level

Events

Properties

Methods

Server

Events

Properties

Methods

Socket

Events

Properties

Methods

Client

Debug / logging

Transports

Plugins

Support

Development

Tests

Goals

Architecture

FAQ

Can I use engine without Socket.IO ?

Does the server serve the client?

Can I implement Engine in other languages?

License

Can I implement `Engine` in other languages?