sockjs


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:

``javascript
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.

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

`javascript
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.

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

`javascript
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.

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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:

`javascript
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);
});
});
`

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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.

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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).

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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.

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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///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

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To run it run something like:

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

For details see
SockJS-protocol README.

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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
----------------------------------------

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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.

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