603 lines
22 KiB
Markdown
603 lines
22 KiB
Markdown
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# Engine.IO: the realtime engine
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[](https://github.com/socketio/engine.io/actions)
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[](http://badge.fury.io/js/engine.io)
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`Engine.IO` is the implementation of transport-based
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cross-browser/cross-device bi-directional communication layer for
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[Socket.IO](http://github.com/socketio/socket.io).
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## How to use
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### Server
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#### (A) Listening on a port
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```js
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const engine = require('engine.io');
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const server = engine.listen(80);
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server.on('connection', socket => {
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socket.send('utf 8 string');
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socket.send(Buffer.from([0, 1, 2, 3, 4, 5])); // binary data
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});
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```
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#### (B) Intercepting requests for a http.Server
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```js
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const engine = require('engine.io');
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const http = require('http').createServer().listen(3000);
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const server = engine.attach(http);
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server.on('connection', socket => {
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socket.on('message', data => { });
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socket.on('close', () => { });
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});
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```
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#### (C) Passing in requests
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```js
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const engine = require('engine.io');
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const server = new engine.Server();
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server.on('connection', socket => {
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socket.send('hi');
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});
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// …
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httpServer.on('upgrade', (req, socket, head) => {
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server.handleUpgrade(req, socket, head);
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});
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httpServer.on('request', (req, res) => {
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server.handleRequest(req, res);
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});
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```
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### Client
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```html
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<script src="/path/to/engine.io.js"></script>
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<script>
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const socket = new eio.Socket('ws://localhost/');
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socket.on('open', () => {
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socket.on('message', data => {});
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socket.on('close', () => {});
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});
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</script>
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```
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For more information on the client refer to the
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[engine-client](http://github.com/socketio/engine.io-client) repository.
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## What features does it have?
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- **Maximum reliability**. Connections are established even in the presence of:
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- proxies and load balancers.
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- personal firewall and antivirus software.
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- for more information refer to **Goals** and **Architecture** sections
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- **Minimal client size** aided by:
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- lazy loading of flash transports.
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- lack of redundant transports.
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- **Scalable**
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- load balancer friendly
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- **Future proof**
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- **100% Node.JS core style**
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- No API sugar (left for higher level projects)
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## API
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### Server
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<hr><br>
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#### Top-level
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These are exposed by `require('engine.io')`:
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##### Events
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- `flush`
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- Called when a socket buffer is being flushed.
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- **Arguments**
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- `Socket`: socket being flushed
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- `Array`: write buffer
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- `drain`
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- Called when a socket buffer is drained
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- **Arguments**
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- `Socket`: socket being flushed
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##### Properties
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- `protocol` _(Number)_: protocol revision number
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- `Server`: Server class constructor
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- `Socket`: Socket class constructor
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- `Transport` _(Function)_: transport constructor
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- `transports` _(Object)_: map of available transports
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##### Methods
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- `()`
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- Returns a new `Server` instance. If the first argument is an `http.Server` then the
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new `Server` instance will be attached to it. Otherwise, the arguments are passed
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directly to the `Server` constructor.
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- **Parameters**
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- `http.Server`: optional, server to attach to.
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- `Object`: optional, options object (see `Server#constructor` api docs below)
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The following are identical ways to instantiate a server and then attach it.
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```js
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const httpServer; // previously created with `http.createServer();` from node.js api.
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// create a server first, and then attach
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const eioServer = require('engine.io').Server();
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eioServer.attach(httpServer);
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// or call the module as a function to get `Server`
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const eioServer = require('engine.io')();
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eioServer.attach(httpServer);
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// immediately attach
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const eioServer = require('engine.io')(httpServer);
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// with custom options
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const eioServer = require('engine.io')(httpServer, {
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maxHttpBufferSize: 1e3
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});
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```
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- `listen`
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- Creates an `http.Server` which listens on the given port and attaches WS
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to it. It returns `501 Not Implemented` for regular http requests.
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- **Parameters**
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- `Number`: port to listen on.
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- `Object`: optional, options object
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- `Function`: callback for `listen`.
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- **Options**
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- All options from `Server.attach` method, documented below.
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- **Additionally** See Server `constructor` below for options you can pass for creating the new Server
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- **Returns** `Server`
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```js
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const engine = require('engine.io');
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const server = engine.listen(3000, {
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pingTimeout: 2000,
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pingInterval: 10000
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});
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server.on('connection', /* ... */);
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```
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- `attach`
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- Captures `upgrade` requests for a `http.Server`. In other words, makes
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a regular http.Server WebSocket-compatible.
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- **Parameters**
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- `http.Server`: server to attach to.
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- `Object`: optional, options object
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- **Options**
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- All options from `Server.attach` method, documented below.
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- **Additionally** See Server `constructor` below for options you can pass for creating the new Server
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- **Returns** `Server` a new Server instance.
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```js
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const engine = require('engine.io');
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const httpServer = require('http').createServer().listen(3000);
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const server = engine.attach(httpServer, {
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wsEngine: require('eiows').Server // requires having eiows as dependency
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});
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server.on('connection', /* ... */);
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```
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#### Server
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The main server/manager. _Inherits from EventEmitter_.
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##### Events
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- `connection`
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- Fired when a new connection is established.
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- **Arguments**
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- `Socket`: a Socket object
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- `initial_headers`
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- Fired on the first request of the connection, before writing the response headers
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- **Arguments**
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- `headers` (`Object`): a hash of headers
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- `req` (`http.IncomingMessage`): the request
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- `headers`
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- Fired on the all requests of the connection, before writing the response headers
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- **Arguments**
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- `headers` (`Object`): a hash of headers
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- `req` (`http.IncomingMessage`): the request
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- `connection_error`
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- Fired when an error occurs when establishing the connection.
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- **Arguments**
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- `error`: an object with following properties:
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- `req` (`http.IncomingMessage`): the request that was dropped
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- `code` (`Number`): one of `Server.errors`
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- `message` (`string`): one of `Server.errorMessages`
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- `context` (`Object`): extra info about the error
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| Code | Message |
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| ---- | ------- |
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| 0 | "Transport unknown"
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| 1 | "Session ID unknown"
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| 2 | "Bad handshake method"
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| 3 | "Bad request"
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| 4 | "Forbidden"
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| 5 | "Unsupported protocol version"
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##### Properties
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**Important**: if you plan to use Engine.IO in a scalable way, please
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keep in mind the properties below will only reflect the clients connected
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to a single process.
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- `clients` _(Object)_: hash of connected clients by id.
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- `clientsCount` _(Number)_: number of connected clients.
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##### Methods
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- **constructor**
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- Initializes the server
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- **Parameters**
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- `Object`: optional, options object
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- **Options**
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- `pingTimeout` (`Number`): how many ms without a pong packet to
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consider the connection closed (`20000`)
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- `pingInterval` (`Number`): how many ms before sending a new ping
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packet (`25000`)
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- `upgradeTimeout` (`Number`): how many ms before an uncompleted transport upgrade is cancelled (`10000`)
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- `maxHttpBufferSize` (`Number`): how many bytes or characters a message
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can be, before closing the session (to avoid DoS). Default
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value is `1E6`.
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- `allowRequest` (`Function`): A function that receives a given handshake
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or upgrade request as its first parameter, and can decide whether to
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continue or not. The second argument is a function that needs to be
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called with the decided information: `fn(err, success)`, where
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`success` is a boolean value where false means that the request is
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rejected, and err is an error code.
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- `transports` (`<Array> String`): transports to allow connections
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to (`['polling', 'websocket']`)
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- `allowUpgrades` (`Boolean`): whether to allow transport upgrades
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(`true`)
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- `perMessageDeflate` (`Object|Boolean`): parameters of the WebSocket permessage-deflate extension
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(see [ws module](https://github.com/einaros/ws) api docs). Set to `true` to enable. (defaults to `false`)
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- `threshold` (`Number`): data is compressed only if the byte size is above this value (`1024`)
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- `httpCompression` (`Object|Boolean`): parameters of the http compression for the polling transports
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(see [zlib](http://nodejs.org/api/zlib.html#zlib_options) api docs). Set to `false` to disable. (`true`)
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- `threshold` (`Number`): data is compressed only if the byte size is above this value (`1024`)
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- `cookie` (`Object|Boolean`): configuration of the cookie that
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contains the client sid to send as part of handshake response
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headers. This cookie might be used for sticky-session. Defaults to not sending any cookie (`false`).
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See [here](https://github.com/jshttp/cookie#options-1) for all supported options.
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- `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.
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- `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.
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- `initialPacket` (`Object`): an optional packet which will be concatenated to the handshake packet emitted by Engine.IO.
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- `allowEIO3` (`Boolean`): whether to support v3 Engine.IO clients (defaults to `false`)
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- `close`
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- Closes all clients
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- **Returns** `Server` for chaining
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- `handleRequest`
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- Called internally when a `Engine` request is intercepted.
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- **Parameters**
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- `http.IncomingMessage`: a node request object
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- `http.ServerResponse`: a node response object
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- **Returns** `Server` for chaining
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- `handleUpgrade`
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- Called internally when a `Engine` ws upgrade is intercepted.
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- **Parameters** (same as `upgrade` event)
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- `http.IncomingMessage`: a node request object
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- `net.Stream`: TCP socket for the request
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- `Buffer`: legacy tail bytes
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- **Returns** `Server` for chaining
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- `attach`
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- Attach this Server instance to an `http.Server`
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- Captures `upgrade` requests for a `http.Server`. In other words, makes
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a regular http.Server WebSocket-compatible.
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- **Parameters**
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- `http.Server`: server to attach to.
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- `Object`: optional, options object
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- **Options**
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- `path` (`String`): name of the path to capture (`/engine.io`).
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- `destroyUpgrade` (`Boolean`): destroy unhandled upgrade requests (`true`)
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- `destroyUpgradeTimeout` (`Number`): milliseconds after which unhandled requests are ended (`1000`)
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- `generateId`
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- Generate a socket id.
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- Overwrite this method to generate your custom socket id.
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- **Parameters**
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- `http.IncomingMessage`: a node request object
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- **Returns** A socket id for connected client.
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<hr><br>
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#### Socket
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A representation of a client. _Inherits from EventEmitter_.
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##### Events
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- `close`
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- Fired when the client is disconnected.
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- **Arguments**
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- `String`: reason for closing
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- `Object`: description object (optional)
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- `message`
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- Fired when the client sends a message.
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- **Arguments**
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- `String` or `Buffer`: Unicode string or Buffer with binary contents
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- `error`
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- Fired when an error occurs.
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- **Arguments**
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- `Error`: error object
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- `upgrading`
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- Fired when the client starts the upgrade to a better transport like WebSocket.
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- **Arguments**
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- `Object`: the transport
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- `upgrade`
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- Fired when the client completes the upgrade to a better transport like WebSocket.
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- **Arguments**
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- `Object`: the transport
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- `flush`
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- Called when the write buffer is being flushed.
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- **Arguments**
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- `Array`: write buffer
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- `drain`
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- Called when the write buffer is drained
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- `packet`
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- Called when a socket received a packet (`message`, `ping`)
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- **Arguments**
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- `type`: packet type
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- `data`: packet data (if type is message)
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- `packetCreate`
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- Called before a socket sends a packet (`message`, `ping`)
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- **Arguments**
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- `type`: packet type
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- `data`: packet data (if type is message)
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- `heartbeat`
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- Called when `ping` or `pong` packed is received (depends of client version)
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##### Properties
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- `id` _(String)_: unique identifier
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- `server` _(Server)_: engine parent reference
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- `request` _(http.IncomingMessage)_: request that originated the Socket
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- `upgraded` _(Boolean)_: whether the transport has been upgraded
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- `readyState` _(String)_: opening|open|closing|closed
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- `transport` _(Transport)_: transport reference
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##### Methods
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- `send`:
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- Sends a message, performing `message = toString(arguments[0])` unless
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sending binary data, which is sent as is.
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- **Parameters**
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- `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.
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- `Object`: optional, options object
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- `Function`: optional, a callback executed when the message gets flushed out by the transport
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- **Options**
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- `compress` (`Boolean`): whether to compress sending data. This option might be ignored and forced to be `true` when using polling. (`true`)
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- **Returns** `Socket` for chaining
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- `close`
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- Disconnects the client
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- **Returns** `Socket` for chaining
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### Client
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<hr><br>
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Exposed in the `eio` global namespace (in the browser), or by
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`require('engine.io-client')` (in Node.JS).
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For the client API refer to the
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[engine-client](http://github.com/learnboost/engine.io-client) repository.
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## Debug / logging
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Engine.IO is powered by [debug](http://github.com/visionmedia/debug).
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In order to see all the debug output, run your app with the environment variable
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`DEBUG` including the desired scope.
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To see the output from all of Engine.IO's debugging scopes you can use:
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```
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DEBUG=engine* node myapp
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```
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## Transports
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- `polling`: XHR / JSONP polling transport.
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- `websocket`: WebSocket transport.
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## Plugins
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- [engine.io-conflation](https://github.com/EugenDueck/engine.io-conflation): Makes **conflation and aggregation** of messages straightforward.
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## Support
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The support channels for `engine.io` are the same as `socket.io`:
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- irc.freenode.net **#socket.io**
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- [Google Groups](http://groups.google.com/group/socket_io)
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- [Website](http://socket.io)
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## Development
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To contribute patches, run tests or benchmarks, make sure to clone the
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repository:
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```
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git clone git://github.com/LearnBoost/engine.io.git
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```
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Then:
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```
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cd engine.io
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npm install
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```
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## Tests
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Tests run with `npm test`. It runs the server tests that are aided by
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the usage of `engine.io-client`.
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Make sure `npm install` is run first.
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## Goals
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The main goal of `Engine` is ensuring the most reliable realtime communication.
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Unlike the previous Socket.IO core, it always establishes a long-polling
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connection first, then tries to upgrade to better transports that are "tested" on
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the side.
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During the lifetime of the Socket.IO projects, we've found countless drawbacks
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to relying on `HTML5 WebSocket` or `Flash Socket` as the first connection
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mechanisms.
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Both are clearly the _right way_ of establishing a bidirectional communication,
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with HTML5 WebSocket being the way of the future. However, to answer most business
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needs, alternative traditional HTTP 1.1 mechanisms are just as good as delivering
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the same solution.
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WebSocket based connections have two fundamental benefits:
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1. **Better server performance**
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- _A: Load balancers_<br>
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Load balancing a long polling connection poses a serious architectural nightmare
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since requests can come from any number of open sockets by the user agent, but
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they all need to be routed to the process and computer that owns the `Engine`
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connection. This negatively impacts RAM and CPU usage.
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- _B: Network traffic_<br>
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WebSocket is designed around the premise that each message frame has to be
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surrounded by the least amount of data. In HTTP 1.1 transports, each message
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frame is surrounded by HTTP headers and chunked encoding frames. If you try to
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send the message _"Hello world"_ with xhr-polling, the message ultimately
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becomes larger than if you were to send it with WebSocket.
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- _C: Lightweight parser_<br>
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As an effect of **B**, the server has to do a lot more work to parse the network
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data and figure out the message when traditional HTTP requests are used
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(as in long polling). This means that another advantage of WebSocket is
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less server CPU usage.
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2. **Better user experience**
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Due to the reasons stated in point **1**, the most important effect of being able
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to establish a WebSocket connection is raw data transfer speed, which translates
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in _some_ cases in better user experience.
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Applications with heavy realtime interaction (such as games) will benefit greatly,
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whereas applications like realtime chat (Gmail/Facebook), newsfeeds (Facebook) or
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timelines (Twitter) will have negligible user experience improvements.
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Having said this, attempting to establish a WebSocket connection directly so far has
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proven problematic:
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1. **Proxies**<br>
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Many corporate proxies block WebSocket traffic.
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2. **Personal firewall and antivirus software**<br>
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As a result of our research, we've found that at least 3 personal security
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applications block WebSocket traffic.
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3. **Cloud application platforms**<br>
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Platforms like Heroku or No.de have had trouble keeping up with the fast-paced
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nature of the evolution of the WebSocket protocol. Applications therefore end up
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inevitably using long polling, but the seamless installation experience of
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Socket.IO we strive for (_"require() it and it just works"_) disappears.
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||
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)
|
||
|
||
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.
|
||
|
||
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.
|