sockjs

What is sockjs?

SockJS is a JavaScript library that provides a WebSocket-like object in environments where true WebSockets are not available or not fully supported. It is designed to work both on the client-side and server-side to enable seamless communication between the two. SockJS provides a consistent API and attempts to provide a low-latency, full duplex, cross-domain communication channel that can work even behind restrictive corporate proxies.

What are sockjs's main functionalities?

Server-side WebSocket emulation

This code sample demonstrates how to create a SockJS server that emulates a WebSocket. It listens for connections on the '/echo' path and simply echoes back any messages it receives.

const http = require('http');
const sockjs = require('sockjs');
const echo = sockjs.createServer({ prefix: '/echo' });
echo.on('connection', function(conn) {
  conn.on('data', function(message) {
    conn.write(message);
  });
});
const server = http.createServer();
echo.installHandlers(server, { prefix: '/echo' });
server.listen(9999, '0.0.0.0');

Client-side WebSocket emulation

This code sample shows how to create a SockJS client that connects to a SockJS server. It sends a message when the connection is opened and logs any messages it receives before closing the connection.

const SockJS = require('sockjs-client');
let sock = new SockJS('http://localhost:9999/echo');
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');
};

Other packages similar to sockjs

websocket

The 'websocket' package provides both client and server-side components for working with WebSockets. It is a more direct implementation of the WebSocket protocol and does not include the fallback mechanisms that SockJS offers for environments where WebSockets are not supported.

socket.io

Socket.IO is a popular real-time communication library that abstracts WebSockets and other methods to provide a consistent API across different browser and server environments. It offers features like auto-reconnection, broadcasting, and rooms, which can be more feature-rich compared to SockJS, but it also requires using the Socket.IO client to communicate with a Socket.IO server.

engine.io

Engine.IO is the lower-level component of Socket.IO that provides the core real-time transport functionality. It is designed to be as minimal and flexible as possible, focusing on the essentials of real-time communication. Engine.IO is similar to SockJS in that it provides WebSocket-like functionality with fallbacks, but it is part of the broader Socket.IO ecosystem.

faye-websocket

Faye is a set of tools for simple publish-subscribe messaging between web clients. Its 'faye-websocket' package is a WebSocket client and server implementation that allows for easy WebSocket communication. Unlike SockJS, it does not provide fallback options for non-WebSocket environments.

README

SockJS family:

• SockJS-client JavaScript client library
• SockJS-node Node.js server
• SockJS-erlang Erlang server
• SockJS-tornado Python/Tornado server
• vert.x Java/vert.x server

Work in progress:

• SockJS-ruby
• SockJS-netty
• SockJS-gevent (and a fork)
• pyramid-SockJS
• wildcloud-websockets
• SockJS-cyclone
• SockJS-twisted
• wai-SockJS
• SockJS-perl
• SockJS-go

What is SockJS?

SockJS is a JavaScript library (for browsers) 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, with WebSockets or without. This necessitates the use of a server, which this is one version of, for Node.js.

SockJS-node server

SockJS-node is a Node.js server side counterpart of SockJS-client browser library written in CoffeeScript.

To install sockjs-node run:

npm install sockjs

A simplified echo SockJS server could look more or less like:

var http = require('http');
var sockjs = require('sockjs');

var echo = sockjs.createServer();
echo.on('connection', function(conn) {
    conn.on('data', function(message) {
        conn.write(message);
    });
    conn.on('close', function() {});
});

var server = http.createServer();
echo.installHandlers(server, {prefix:'/echo'});
server.listen(9999, '0.0.0.0');

(Take look at examples directory for a complete version.)

Subscribe to SockJS mailing list for discussions and support.

SockJS-node API

The API design is based on common Node APIs like the Streams API or the Http.Server API.

Server class

SockJS module is generating a Server class, similar to Node.js http.createServer module.

var sockjs_server = sockjs.createServer(options);

Where options is a hash which can contain:

sockjs_url (string)

Transports which don't support cross-domain communication natively ('eventsource' to name one) use an iframe trick. A simple page is served from the SockJS server (using its foreign domain) and is placed in an invisible iframe. Code run from this iframe doesn't need to worry about cross-domain issues, as it's being run from domain local to the SockJS server. This iframe also does need to load SockJS javascript client library, and this option lets you specify its url (if you're unsure, point it to the latest minified SockJS client release, this is the default). You must explicitly specify this url on the server side for security reasons - we don't want the possibility of running any foreign javascript within the SockJS domain (aka cross site scripting attack). Also, sockjs javascript library is probably already cached by the browser - it makes sense to reuse the sockjs url you're using in normally.

prefix (string regex)

A url prefix for the server. All http requests which paths begins with selected prefix will be handled by SockJS. All other requests will be passed through, to previously registered handlers.

response_limit (integer)

Most streaming transports save responses on the client side and don't free memory used by delivered messages. Such transports need to be garbage-collected once in a while. `response_limit` sets a minimum number of bytes that can be send over a single http streaming request before it will be closed. After that client needs to open new request. Setting this value to one effectively disables streaming and will make streaming transports to behave like polling transports. The default value is 128K.

websocket (boolean)

Some load balancers don't support websockets. This option can be used to disable websockets support by the server. By default websockets are enabled.

jsessionid (boolean or function)

Some hosting providers enable sticky sessions only to requests that have JSESSIONID cookie set. This setting controls if the server should set this cookie to a dummy value. By default setting JSESSIONID cookie is disabled. More sophisticated behaviour can be achieved by supplying a function.

log (function(severity, message))

It's quite useful, especially for debugging, to see some messages printed by a SockJS-node library. This is done using this `log` function, which is by default set to `console.log`. If this behaviour annoys you for some reason, override `log` setting with a custom handler. The following `severities` are used: `debug` (miscellaneous logs), `info` (requests logs), `error` (serious errors, consider filing an issue).

heartbeat_delay (milliseconds)

In order to keep proxies and load balancers from closing long running http requests we need to pretend that the connection is active and send a heartbeat packet once in a while. This setting controls how often this is done. By default a heartbeat packet is sent every 25 seconds.

disconnect_delay (milliseconds)

The server sends a `close` event when a client receiving connection have not been seen for a while. This delay is configured by this setting. By default the `close` event will be emitted when a receiving connection wasn't seen for 5 seconds.

disable_cors (boolean)

Enabling this option will prevent CORS headers from being included in the HTTP response. Can be used when the sockjs client is known to be connecting from the same origin as the sockjs server.

Server instance

Once you have create Server instance you can hook it to the http.Server instance.

var http_server = http.createServer();
sockjs_server.installHandlers(http_server, options);
http_server.listen(...);

Where options can overshadow options given when creating Server instance.

Server instance is an EventEmitter, and emits following event:

Event: connection (connection)

A new connection has been successfully opened.

All http requests that don't go under the path selected by prefix will remain unanswered and will be passed to previously registered handlers. You must install your custom http handlers before calling installHandlers.

Connection instance

A Connection instance supports Node Stream API and has following methods and properties:

Property: readable (boolean)

Is the stream readable?

Property: writable (boolean)

Is the stream writable?

Property: remoteAddress (string)

Last known IP address of the client.

Property: remotePort (number)

Last known port number of the client.

Property: address (object)

Hash with 'address' and 'port' fields.

Property: headers (object)

Hash containing various headers copied from last receiving request on that connection. Exposed headers include: `origin`, `referer` and `x-forwarded-for` (and friends). We explicitly do not grant access to `cookie` header, as using it may easily lead to security issues (for details read the section "Authorisation").

Property: url (string)

Url property copied from last request.

Property: pathname (string)

`pathname` from parsed url, for convenience.

Property: prefix (string)

Prefix of the url on which the request was handled.

Property: protocol (string)

Protocol used by the connection. Keep in mind that some protocols are indistinguishable - for example "xhr-polling" and "xdr-polling".

Property: readyState (integer)

Current state of the connection: 0-connecting, 1-open, 2-closing, 3-closed.

write(message)

Sends a message over opened connection. A message must be a non-empty string. It's illegal to send a message after the connection was closed (either after 'close' or 'end' method or 'close' event).

close([code], [reason])

Asks the remote client to disconnect. 'code' and 'reason' parameters are optional and can be used to share the reason of disconnection.

end()

Asks the remote client to disconnect with default 'code' and 'reason' values.

A Connection instance emits the following events:

Event: data (message)

A message arrived on the connection. Message is a unicode string.

Event: close ()

Connection was closed. This event is triggered exactly once for every connection.

For example:

sockjs_server.on('connection', function(conn) {
    console.log('connection' + conn);
    conn.on('close', function() {
        console.log('close ' + conn);
    });
    conn.on('data', function(message) {
        console.log('message ' + conn,
                    message);
    });
});

Footnote

A fully working echo server does need a bit more boilerplate (to handle requests unanswered by SockJS), see the echo example for a complete code.

Examples

If you want to see samples of running code, take a look at:

• ./examples/echo directory, which contains a full example of a echo server.
• ./examples/test_server a standard SockJS test server.

Connecting to SockJS-node without the client

Although the main point of SockJS it 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).

Note: This endpoint will not send any heartbeat packets.

Deployment and load balancing

There are two issues that need to be considered when planning a non-trivial SockJS-node deployment: WebSocket-compatible load balancer and sticky sessions (aka session affinity).

WebSocket compatible load balancer

Often WebSockets don't play nicely with proxies and load balancers. Deploying a SockJS server behind Nginx or Apache could be painful.

Fortunately recent versions of an excellent load balancer HAProxy are able to proxy WebSocket connections. We propose to put HAProxy as a front line load balancer and use it to split SockJS traffic from normal HTTP data. Take a look at the sample SockJS HAProxy configuration.

The config also shows how to use HAproxy balancing to split traffic between multiple Node.js servers. You can also do balancing using dns names.

Sticky sessions

If you plan deploying more than one SockJS server, you must make sure that all HTTP requests for a single session will hit the same server. SockJS has two mechanisms that can be useful to achieve that:

• Urls are prefixed with server and session id numbers, like: /resource/<server_number>/<session_id>/transport. This is useful for load balancers that support prefix-based affinity (HAProxy does).
• JSESSIONID cookie is being set by SockJS-node. Many load balancers turn on sticky sessions if that cookie is set. This technique is derived from Java applications, where sticky sessions are often necessary. HAProxy does support this method, as well as some hosting providers, for example CloudFoundry. In order to enable this method on the client side, please supply a cookie:true option to SockJS constructor.

Development and testing

If you want to work on SockJS-node source code, you need to clone the git repo and follow these steps. First you need to install dependencies:

cd sockjs-node
npm install
npm install --dev
ln -s .. node_modules/sockjs

You're ready to compile CoffeeScript:

make build

If compilation succeeds you may want to test if your changes pass all the tests. Currently, there are two separate test suites. For both of them you need to start a SockJS-node test server (by default listening on port 8081):

make test_server

SockJS-protocol Python tests

To run it run something like:

cd sockjs-protocol
make test_deps
./venv/bin/python sockjs-protocol.py

For details see SockJS-protocol README.

SockJS-client QUnit tests

You need to start a second web server (by default listening on 8080) that is serving various static html and javascript files:

cd sockjs-client
make test

At that point you should have two web servers running: sockjs-node on 8081 and sockjs-client on 8080. When you open the browser on http://localhost:8080/ you should be able run the QUnit tests against your sockjs-node server.

For details see SockJS-client README.

Additionally, if you're doing more serious development consider using make serve, which will automatically the server when you modify the source code.

Various issues and design considerations

Authorisation

SockJS-node does not expose cookies to the application. This is done deliberately as using cookie-based authorisation with SockJS simply doesn't make sense and will lead to security issues.

Cookies are a contract between a browser and an http server, and are identified by a domain name. If a browser has a cookie set for particular domain, it will pass it as a part of all http requests to the host. But to get various transports working, SockJS uses a middleman

• an iframe hosted from target SockJS domain. That means the server will receive requests from the iframe, and not from the real domain. The domain of an iframe is the same as the SockJS domain. The problem is that any website can embed the iframe and communicate with it - and request establishing SockJS connection. Using cookies for authorisation in this scenario will result in granting full access to SockJS communication with your website from any website. This is a classic CSRF attack.

Basically - cookies are not suited for SockJS model. If you want to authorise a session - provide a unique token on a page, send it as a first thing over SockJS connection and validate it on the server side. In essence, this is how cookies work.

Deploying SockJS on Heroku

Long polling is known to cause problems on Heroku, but workaround for SockJS is available.