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# Engine.IO: the realtime engine

[![Build Status](https://github.com/socketio/engine.io/workflows/CI/badge.svg?branch=master)](https://github.com/socketio/engine.io/actions)
[![NPM version](https://badge.fury.io/js/engine.io.svg)](http://badge.fury.io/js/engine.io)

`Engine.IO` is the implementation of transport-based
cross-browser/cross-device bi-directional communication layer for
[Socket.IO](http://github.com/socketio/socket.io).

## How to use

### Server

#### (A) Listening on a port

```js
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

```js
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

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

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

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

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

### Client

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

For more information on the client refer to the
[engine-client](http://github.com/socketio/engine.io-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.

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

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

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

// 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`

```js
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.

```js
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', /* ... */);
```

#### Server

The main server/manager. _Inherits from EventEmitter_.

##### Events

- `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"

##### 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`.
      - `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](https://github.com/einaros/ws) 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](http://nodejs.org/api/zlib.html#zlib_options) 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](https://github.com/jshttp/cookie#options-1) 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](https://github.com/websockets/ws/blob/master/doc/ws.md)). 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](https://github.com/expressjs/cors#configuration-options) 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.

#### 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`, `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)

##### 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](http://github.com/learnboost/engine.io-client) repository.

## Debug / logging

Engine.IO is powered by [debug](http://github.com/visionmedia/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](https://github.com/EugenDueck/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](http://groups.google.com/group/socket_io)
  - [Website](http://socket.io)

## 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:

1. **Better server performance**
  - _A: Load balancers_<br>
      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_<br>
      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_<br>
      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.

2. **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**<br>
    Many corporate proxies block WebSocket traffic.

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

3. **Cloud application platforms**<br>
    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

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

has you covered.

### Can I implement `Engine` in other languages?

Absolutely. The [engine.io-protocol](https://github.com/socketio/engine.io-protocol)
repository contains the most up-to-date description of the specification
at all times.

## 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.

cmd: