SockJS-client

SockJS for enterprise

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Summary

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
• SockJS-aiohttp Python/Aiohttp server
• Spring Framework Java client & server
• vert.x Java/vert.x server
• Xitrum Scala server
• Atmosphere Framework JavaEE Server, Play Framework, Netty, Vert.x
• Actix SockJS Rust Server, Actix Framework

Work in progress:

• SockJS-ruby
• SockJS-netty
• SockJS-gevent (SockJS-gevent fork)
• pyramid-SockJS
• wildcloud-websockets
• wai-SockJS
• SockJS-perl
• SockJS-go
• syp.biz.SockJS.NET - .NET port of the SockJS client

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="https://cdn.jsdelivr.net/npm/sockjs-client@1/dist/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('https://mydomain.com/my_prefix');
 sock.onopen = function() {
     console.log('open');
     sock.send('test');
 };

 sock.onmessage = function(e) {
     console.log('message', e.data);
     sock.close();
 };

 sock.onclose = function() {
     console.log('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.

• timeout (number)

  Specify a minimum timeout in milliseconds to use for the transport connections. By default this is dynamically calculated based on the measured RTT and the number of expected round trips. This setting will establish a minimum, but if the calculated timeout is higher, that will be used.

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://)

Browser	Websockets	Streaming	Polling
IE 6, 7	no	no	jsonp-polling
IE 8, 9 (cookies=no)	no	xdr-streaming †	xdr-polling †
IE 8, 9 (cookies=yes)	no	iframe-htmlfile	iframe-xhr-polling
IE 10	rfc6455	xhr-streaming	xhr-polling
Chrome 6-13	hixie-76	xhr-streaming	xhr-polling
Chrome 14+	hybi-10 / rfc6455	xhr-streaming	xhr-polling
Firefox <10	no ‡	xhr-streaming	xhr-polling
Firefox 10+	hybi-10 / rfc6455	xhr-streaming	xhr-polling
Safari 5.x	hixie-76	xhr-streaming	xhr-polling
Safari 6+	rfc6455	xhr-streaming	xhr-polling
Opera 10.70+	no ‡	iframe-eventsource	iframe-xhr-polling
Opera 12.10+	rfc6455	xhr-streaming	xhr-polling
Konqueror	no	no	jsonp-polling

• †: 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:

Browser	Websockets	Streaming	Polling
IE 8, 9	same as above	iframe-htmlfile	iframe-xhr-polling
Other	same as above	iframe-eventsource	iframe-xhr-polling

Supported transports, by name

Transport	References
websocket (rfc6455)	rfc 6455
websocket (hixie-76)	draft-hixie-thewebsocketprotocol-76
websocket (hybi-10)	draft-ietf-hybi-thewebsocketprotocol-10
xhr-streaming	Transport using Cross domain XHR streaming capability (readyState=3).
xdr-streaming	Transport using XDomainRequest streaming capability (readyState=3).
eventsource	EventSource/Server-sent events.
iframe-eventsource	EventSource/Server-sent events used from an iframe via postMessage.
htmlfile	HtmlFile.
iframe-htmlfile	HtmlFile used from an iframe via postMessage.
xhr-polling	Long-polling using cross domain XHR.
xdr-polling	Long-polling using XDomainRequest.
iframe-xhr-polling	Long-polling using normal AJAX from an iframe via postMessage.
jsonp-polling	Slow and old fashioned JSONP polling. This transport will show "busy indicator" (aka: "spinning wheel") when sending data.

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="https://cdn.jsdelivr.net/npm/sockjs-client@1/dist/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-c