socket.io/README.md

# Engine.IO: the realtime engine

[![Build Status](https://secure.travis-ci.org/LearnBoost/engine.io.png)](http://travis-ci.org/LearnBoost/engine.io)

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

## Hello World

### Server

#### (A) Listening on a port

```js
var engine = require('engine.io')
  , server = engine.listen(80)

server.on('connection', function (socket) {
  socket.send('utf 8 string');
});
```

#### (B) Intercepting requests for a http.Server

```js
var engine = require('engine.io')
  , http = require('http').createServer().listen(3000)
  , server = engine.attach(http)

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

#### (C) Passing in requests

```js
var engine = require('engine.io')
  , 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

```html
<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](http://github.com/learnboost/engine.io-client) repository.

## What features does it have?

- **Isomorphic with WebSocket.IO**. You can switch between a WebSocket server
  and a multi-transport server by chaning the `require`.
- **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

<hr><br>

#### 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

- `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.
      - `Function`: callback for `listen`.
    - **Returns** `Server`
- `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**
      - `resource` (`String`): name of resource for this server (`default`).
        Setting a resource allows you to initialize multiple engine.io
        endpoints on the same host without them interfering.
      - `policyFile` (`Boolean`): whether to handle policy file requests (`true`)
      - `destroyUpgrade` (`Boolean`): destroy unhandled upgrade requests (`true`)
      - **See Server options below for additional options you can pass**
    - **Returns** `Server`

<hr><br>

#### 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 (`60000`)
      - `pingInterval` (`Number`): how many ms before sending a new ping
        packet (`25000`)
      - `transports` (`<Array> String`): transports to allow connections
        to (`['polling', 'websocket', 'flashsocket']`)
      - `allowUpgrades` (`Boolean`): whether to allow tranport upgrades
        (`true`)
      - `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`)
- `close`
    - Closes all clients
    - **Returns** `Server` for chaining
- `handleRequest`
    - Called internally when a `Engine` request is intercepted.
    - **Parameters**
      - `http.ServerRequest`: 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.ServerRequest`: a node request object
      - `net.Stream`: TCP socket for the request
      - `Buffer`: legacy tail bytes
    - **Returns** `Server` for chaining
- `handleSocket`
    - Called with raw TCP sockets from http requests to intercept flash policy
      file requests
    - **Parameters**
      - `net.Stream`: TCP socket on which requests are listened
    - **Returns** `Server` for chaining

<hr><br>

#### 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`: unicode string
- `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

##### Properties

- `server` _(Server)_: engine parent reference
- `request` _(http.ServerRequest)_: 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])`.
    - **Parameters**
      - `String`: a string or any object implementing `toString()`, with outgoing data
      - `Function`: optional, a callback executed when the message gets flushed out by the transport
    - **Returns** `Socket` for chaining
- `close`
    - Disconnects the client
    - **Returns** `Socket` for chaining

### Client

<hr><br>

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 env 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.
- `flashsocket`: WebSocket transport backed by flash.

## 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

### Unit/Integration

```
$ make test
```

### Acceptance

```
# make test-acceptance
```

And point browser/s to `http://localhost:3000`.

### Server

## Benchmarks

### Server

```
$ make bench
```

### Client

```
$ make bench-server
```

And point browser/s to `http://localhost:3000`.

## 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/FlashSocket 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 [SPEC](https://github.com/LearnBoost/engine.io-client/blob/master/SPEC.md)
file contains the most up to date description of the implementation specification
at all times. If you're targeting the latest stable release of `Engine`, make sure
to look at the file in the appropriate git branch/tag.

The Java/NIO implementation will be officially supported, and is being worked
on by the author.

## License 

(The MIT License)

Copyright (c) 2011 Guillermo Rauch &lt;guillermo@learnboost.com&gt;

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.