websocket-driver

websocket-driver

This module provides a complete implementation of the WebSocket protocols that can be hooked up to any TCP library. It aims to simplify things by decoupling the protocol details from the I/O layer, such that users only need to implement code to stream data in and out of it without needing to know anything about how the protocol actually works. Think of it as a complete WebSocket system with pluggable I/O.

Due to this design, you get a lot of things for free. In particular, if you hook this module up to some I/O object, it will do all of this for you:

• Select the correct server-side driver to talk to the client
• Generate and send both server- and client-side handshakes
• Recognize when the handshake phase completes and the WS protocol begins
• Negotiate subprotocol selection based on Sec-WebSocket-Protocol
• Negotiate and use extensions via the websocket-extensions module
• Buffer sent messages until the handshake process is finished
• Deal with proxies that defer delivery of the draft-76 handshake body
• Notify you when the socket is open and closed and when messages arrive
• Recombine fragmented messages
• Dispatch text, binary, ping, pong and close frames
• Manage the socket-closing handshake process
• Automatically reply to ping frames with a matching pong
• Apply masking to messages sent by the client

This library was originally extracted from the Faye project but now aims to provide simple WebSocket support for any Ruby server or I/O system.

Installation

$ gem install websocket-driver

Usage

To build either a server-side or client-side socket, the only requirement is that you supply a socket object with these methods:

• socket.url - returns the full URL of the socket as a string.
• socket.write(string) - writes the given string to a TCP stream.

Server-side sockets require one additional method:

• socket.env - returns a Rack-style env hash that will contain some of the following fields. Their values are strings containing the value of the named header, unless stated otherwise.
  • HTTP_CONNECTION
  • HTTP_HOST
  • HTTP_ORIGIN
  • HTTP_SEC_WEBSOCKET_EXTENSIONS
  • HTTP_SEC_WEBSOCKET_KEY
  • HTTP_SEC_WEBSOCKET_KEY1
  • HTTP_SEC_WEBSOCKET_KEY2
  • HTTP_SEC_WEBSOCKET_PROTOCOL
  • HTTP_SEC_WEBSOCKET_VERSION
  • HTTP_UPGRADE
  • rack.input, an IO object representing the request body
  • REQUEST_METHOD, the request's HTTP verb

Server-side with Rack

To handle a server-side WebSocket connection, you need to check whether the request is a WebSocket handshake, and if so create a protocol driver for it. You must give the driver an object with the env, url and write methods. A simple example might be:

require 'websocket/driver'
require 'eventmachine'

class WS
  attr_reader :env, :url

  def initialize(env)
    @env = env

    secure = Rack::Request.new(env).ssl?
    scheme = secure ? 'wss:' : 'ws:'
    @url = scheme + '//' + env['HTTP_HOST'] + env['REQUEST_URI']

    @driver = WebSocket::Driver.rack(self)

    env['rack.hijack'].call
    @io = env['rack.hijack_io']

    EM.attach(@io, Reader) { |conn| conn.driver = @driver }

    @driver.start
  end

  def write(string)
    @io.write(string)
  end

  module Reader
    attr_writer :driver

    def receive_data(string)
      @driver.parse(string)
    end
  end
end

To explain what's going on here: the WS class implements the env, url and write(string) methods as required. When instantiated with a Rack environment, it stores the environment and infers the complete URL from it. Having set up the env and url, it asks WebSocket::Driver for a server-side driver for the socket. Then it uses the Rack hijack API to gain access to the TCP stream, and uses EventMachine to stream in incoming data from the client, handing incoming data off to the driver for parsing. Finally, we tell the driver to start, which will begin sending the handshake response. This will invoke the WS#write method, which will send the response out over the TCP socket.

Having defined this class we could use it like this when handling a request:

if WebSocket::Driver.websocket?(env)
  socket = WS.new(env)
end

The driver API is described in full below.

Server-side with TCP

You can also handle WebSocket connections in a bare TCP server, if you're not using Rack and don't want to implement HTTP parsing yourself. For this, your socket object only needs a write method.

The driver will emit a :connect event when a request is received, and at this point you can detect whether it's a WebSocket and handle it as such. Here's an example using an EventMachine TCP server.

module Connection
  def initialize
    @driver = WebSocket::Driver.server(self)

    @driver.on :connect, -> (event) do
      if WebSocket::Driver.websocket?(@driver.env)
        @driver.start
      else
        # handle other HTTP requests, for example
        body = '<h1>hello</h1>'
        response = [
          'HTTP/1.1 200 OK',
          'Content-Type: text/plain',
          "Content-Length: #{body.bytesize}",
          '',
          body
        ]
        send_data response.join("\r\n")
      end
    end

    @driver.on :message, -> (e) { @driver.text(e.data) }
    @driver.on :close,   -> (e) { close_connection_after_writing }
  end

  def receive_data(data)
    @driver.parse(data)
  end

  def write(data)
    send_data(data)
  end
end

EM.run {
  EM.start_server('127.0.0.1', 4180, Connection)
}

In the :connect event, @driver.env is a Rack env representing the request. If the request has a body, it will be in the @driver.env['rack.input'] stream, but only as much of the body as you have so far routed to it using the parse method.

Client-side

Similarly, to implement a WebSocket client you need an object with url and write methods. Once you have one such object, you ask for a driver for it:

driver = WebSocket::Driver.client(socket)

After this you use the driver API as described below to process incoming data and send outgoing data.

Client drivers have two additional methods for reading the HTTP data that was sent back by the server:

• driver.status - the integer value of the HTTP status code
• driver.headers - a hash-like object containing the response headers

HTTP Proxies

The client driver supports connections via HTTP proxies using the CONNECT method. Instead of sending the WebSocket handshake immediately, it will send a CONNECT request, wait for a 200 response, and then proceed as normal.

To use this feature, call proxy = driver.proxy(url) where url is the origin of the proxy, including a username and password if required. This produces an object that manages the process of connecting via the proxy. You should call proxy.start to begin the connection process, and pass data you receive via the socket to proxy.parse(data). When the proxy emits :connect, you should then start sending incoming data to driver.parse(data) as normal, and call driver.start.

proxy = driver.proxy('http://username:[e-mail]')

proxy.on :connect, -> (event) do
  driver.start
end

The proxy's :connect event is also where you should perform a TLS handshake on your TCP stream, if you are connecting to a wss: endpoint.

In the event that proxy connection fails, proxy will emit an :error. You can inspect the proxy's response via proxy.status and proxy.headers.

proxy.on :error, -> (error) do
  puts error.message
  puts proxy.status
  puts proxy.headers.inspect
end

Before calling proxy.start you can set custom headers using proxy.set_header:

proxy.set_header('User-Agent', 'ruby')
proxy.start

Driver API

Drivers are created using one of the following methods:

driver = WebSocket::Driver.rack(socket, options)
driver = WebSocket::Driver.server(socket, options)
driver = WebSocket::Driver.client(socket, options)

The rack method returns a driver chosen using the socket's env. The server method returns a driver that will parse an HTTP request and then decide which driver to use for it using the rack method. The client method always returns a driver for the RFC version of the protocol with masking enabled on outgoing frames.

The options argument is optional, and is a hash. It may contain the following keys:

• :max_length - the maximum allowed size of incoming message frames, in bytes. The default value is 2^26 - 1, or 1 byte short of 64 MiB.
• :protocols - an array of strings representing acceptable subprotocols for use over the socket. The driver will negotiate one of these to use via the Sec-WebSocket-Protocol header if supported by the other peer.

All drivers respond to the following API methods, but some of them are no-ops depending on whether the client supports the behaviour.

Note that most of these methods are commands: if they produce data that should be sent over the socket, they will give this to you by calling socket.write(string).

driver.on :open, -> (event) {}

Adds a callback block to execute when the socket becomes open.

driver.on :message, -> (event) {}

Adds a callback block to execute when a message is received. event will have a data attribute containing either a string in the case of a text message or an array of integers in the case of a binary message.

driver.on :error, -> (event) {}

Adds a callback to execute when a protocol error occurs due to the other peer sending an invalid byte sequence. event will have a message attribute describing the error.

driver.on :close, -> (event) {}

Adds a callback block to execute when the socket becomes closed. The event object has code and reason attributes.

driver.on :ping, -> (event) {}

Adds a callback block to execute when a ping is received. You do not need to handle this by sending a pong frame yourself; the driver handles this for you.

driver.on :pong, -> (event) {}

Adds a callback block to execute when a pong is received. If this was in response to a ping you sent, you can also handle this event via the driver.ping(message) { ... } callback.

driver.add_extension(extension)

Registers a protocol extension whose operation will be negotiated via the Sec-WebSocket-Extensions header. extension is any extension compatible with the websocket-extensions framework.

driver.set_header(name, value)

Sets a custom header to be sent as part of the handshake response, either from the server or from the client. Must be called before start, since this is when the headers are serialized and sent.

driver.start

Initiates the protocol by sending the handshake - either the response for a server-side driver or the request for a client-side one. This should be the first method you invoke. Returns true if and only if a handshake was sent.

driver.parse(string)

Takes a string and parses it, potentially resulting in message events being emitted (see on('message') above) or in data being sent to socket.write. You should send all data you receive via I/O to this method.

driver.text(string)

Sends a text message over the socket. If the socket handshake is not yet complete, the message will be queued until it is. Returns true if the message was sent or queued, and false if the socket can no longer send messages.

driver.binary(array)

Takes an array of byte-sized integers and sends them as a binary message. Will queue and return true or false the same way as the text method. It will also return false if the driver does not support binary messages.

driver.ping(string = '', &callback)

Sends a ping frame over the socket, queueing it if necessary. string and the callback block are both optional. If a callback is given, it will be invoked when the socket receives a pong frame whose content matches string. Returns false if frames can no longer be sent, or if the driver does not support ping/pong.

driver.pong(string = '')

Sends a pong frame over the socket, queueing it if necessary. string is optional. Returns false if frames can no longer be sent, or if the driver does not support ping/pong.

You don't need to call this when a ping frame is received; pings are replied to automatically by the driver. This method is for sending unsolicited pongs.

driver.close

Initiates the closing handshake if the socket is still open. For drivers with no closing handshake, this will result in the immediate execution of the on('close') callback. For drivers with a closing handshake, this sends a closing frame and emit('close') will execute when a response is received or a protocol error occurs.

driver.version

Returns the WebSocket version in use as a string. Will either be hixie-75, hixie-76 or hybi-$version.

driver.protocol

Returns a string containing the selected subprotocol, if any was agreed upon using the Sec-WebSocket-Protocol mechanism. This value becomes available after emit('open') has fired.