SockJS-client

SockJS is a browser JavaScript library that provides a WebSocket-like
object. SockJS gives you a coherent, cross-browser, Javascript API
which creates a low latency, full duplex, cross-domain communication
channel between the browser and the web server.

Under the hood SockJS tries to use native WebSockets first. If that
fails it can use a variety of browser-specific transport protocols and
presents them through WebSocket-like abstractions.

SockJS is intended to work for all modern browsers and in environments
which don't support the WebSocket protocol -- for example, behind restrictive
corporate proxies.

SockJS-client does require a server counterpart:

• SockJS-node is a SockJS
  server for Node.js.

Philosophy:

• The API should follow
  HTML5 Websockets API as
  closely as possible.
• All the transports must support cross domain connections out of the
  box. It's possible and recommended to host a SockJS server on a
  different server than your main web site.
• There is support for at least one streaming protocol for every
  major browser.
• Streaming transports should work cross-domain and
  should support cookies (for cookie-based sticky sessions).
• Polling transports are used as a fallback for old browsers and
  hosts behind restrictive proxies.
• Connection establishment should be fast and lightweight.
• No Flash inside (no need to open port 843 - which doesn't work
  through proxies, no need to host 'crossdomain.xml', no need
  to wait for 3 seconds
  in order to detect problems)

Subscribe to
SockJS mailing list for
discussions and support.

SockJS family:

• SockJS-client JavaScript client library
• SockJS-node Node.js server
• SockJS-erlang Erlang server
• SockJS-cyclone Python/Cyclone/Twisted server
• SockJS-tornado Python/Tornado server
• SockJS-twisted Python/Twisted server
• Spring Framework Java client & server
• vert.x Java/vert.x server
• Xitrum Scala server
• Atmosphere Framework JavaEE Server, Play Framework, Netty, Vert.x

Work in progress:

• SockJS-ruby
• SockJS-netty
• SockJS-gevent (SockJS-gevent fork)
• pyramid-SockJS
• wildcloud-websockets
• wai-SockJS
• SockJS-perl
• SockJS-go

Getting Started

SockJS mimics the WebSockets API,
but instead of WebSocket there is a SockJS Javascript object.

First, you need to load the SockJS JavaScript library. For example, you can
put that in your HTML head:

<script src="//cdn.jsdelivr.net/sockjs/1/sockjs.min.js"></script>

After the script is loaded you can establish a connection with the
SockJS server. Here's a simple example:

 var sock = new SockJS('http://mydomain.com/my_prefix');
 sock.onopen = function() {
     console.log('open');
 };
 sock.onmessage = function(e) {
     console.log('message', e.data);
 };
 sock.onclose = function() {
     console.log('close');
 };

 sock.send('test');
 sock.close();

SockJS-client API

SockJS class

Similar to the 'WebSocket' API, the 'SockJS' constructor takes one, or more arguments:

var sockjs = new SockJS(url, _reserved, options);

url may contain a query string, if one is desired.

Where options is a hash which can contain:

• server (string)

  String to append to url for actual data connection. Defaults to a random 4 digit number.

• transports (string OR array of strings)

  Sometimes it is useful to disable some fallback transports. This
  option allows you to supply a list transports that may be used by
  SockJS. By default all available transports will be used.

• sessionId (number OR function)

  Both client and server use session identifiers to distinguish connections.
  If you specify this option as a number, SockJS will use its random string
  generator function to generate session ids that are N-character long
  (where N corresponds to the number specified by sessionId).
  When you specify this option as a function, the function must return a
  randomly generated string. Every time SockJS needs to generate a session
  id it will call this function and use the returned string directly.
  If you don't specify this option, the default is to use the default random
  string generator to generate 8-character long session ids.

Although the 'SockJS' object tries to emulate the 'WebSocket'
behaviour, it's impossible to support all of its features. An
important SockJS limitation is the fact that you're not allowed to
open more than one SockJS connection to a single domain at a time.
This limitation is caused by an in-browser limit of outgoing
connections - usually browsers don't allow opening more than two
outgoing connections to a single domain. A single SockJS session
requires those two connections - one for downloading data, the other for
sending messages. Opening a second SockJS session at the same time
would most likely block, and can result in both sessions timing out.

Opening more than one SockJS connection at a time is generally a
bad practice. If you absolutely must do it, you can use
multiple subdomains, using a different subdomain for every
SockJS connection.

Supported transports, by browser (html served from http:// or https://)

• †: IE 8+ supports XDomainRequest, which is
  essentially a modified AJAX/XHR that can do requests across
  domains. But unfortunately it doesn't send any cookies, which
  makes it inappropriate for deployments when the load balancer uses
  JSESSIONID cookie to do sticky sessions.

• ‡: Firefox 4.0 and Opera 11.00 and shipped with disabled
  Websockets "hixie-76". They can still be enabled by manually
  changing a browser setting.

Supported transports, by browser (html served from file://)

Sometimes you may want to serve your html from "file://" address - for
development or if you're using PhoneGap or similar technologies. But
due to the Cross Origin Policy files served from "file://" have no
Origin, and that means some of SockJS transports won't work. For this
reason the SockJS transport table is different than usually, major
differences are:

Supported transports, by name

Connecting to SockJS without the client

Although the main point of SockJS is to enable browser-to-server
connectivity, it is possible to connect to SockJS from an external
application. Any SockJS server complying with 0.3 protocol does
support a raw WebSocket url. The raw WebSocket url for the test server
looks like:

• ws://localhost:8081/echo/websocket

You can connect any WebSocket RFC 6455 compliant WebSocket client to
this url. This can be a command line client, external application,
third party code or even a browser (though I don't know why you would
want to do so).

Deployment

You should use a version of sockjs-client
that supports the protocol used by your server. For example:

<script src="//cdn.jsdelivr.net/sockjs/1/sockjs.min.js"></script>

For server-side deployment tricks, especially about load balancing and
session stickiness, take a look at the
SockJS-node readme.

Development and testing

SockJS-client needs node.js for running a test
server and JavaScript minification. If you want to work on
SockJS-client source code, checkout the git repo and follow these
steps:

cd sockjs-client
npm install

To generate JavaScript, run:

gulp browserify

To generate minified JavaScript, run:

gulp browserify:min

Both commands output into the build directory.

Testing

Once you've compiled the SockJS-client you may want to check if your changes
pass all the tests.

make test-local

This will start zuul and a test support server. Open the browser to http://localhost:9090/_zuul and watch the tests run.

Browser Quirks

There are various browser quirks which we don't intend to address:

• Pressing ESC in Firefox, before Firefox 20, closes the SockJS connection. For a workaround
  and discussion see #18.
• jsonp-polling transport will show a "spinning wheel" (aka. "busy indicator")
  when sending data.
• You can't open more than one SockJS connection to one domain at the
  same time due to the browser's limit of concurrent connections
  (this limit is not counting native WebSocket connections).
• Although SockJS is trying to escape any strange Unicode characters
  (even invalid ones - like surrogates \xD800-\xDBFF or \xFFFE and \xFFFF)
  it's advisable to use only valid characters. Using invalid
  characters is a bit slower, and may not work with SockJS servers
  that have proper Unicode support.
• Having a global function called onmessage or such is probably a
  bad idea, as it could be called by the built-in postMessage API.
• From SockJS' point of view there is nothing special about
  SSL/HTTPS. Connecting between unencrypted and encrypted sites
  should work just fine.
• Although SockJS does its best to support both prefix and cookie based
  sticky sessions, the latter may not work well cross-domain with
  browsers that don't accept third-party cookies by default (Safari).
  In order to get around this make sure you're connecting to SockJS
  from the same parent domain as the main site. For example
  'sockjs.a.com' is able to set cookies if you're connecting from
  'www.a.com' or 'a.com'.
• Trying to connect from secure "https://" to insecure "http://" is
  not a good idea. The other way around should be fine.
• Long polling is known to cause problems on Heroku, but a
  workaround for SockJS is available.
• SockJS websocket transport is more stable over SSL. If
  you're a serious SockJS user then consider using SSL
  (more info).