FROM http://code.google.com/p/nginx-sticky-module/source/browse/tags/1_0
Nginx Sticky Module 
--
Description:
	A nginx module to add a sticky cookie to be always forwarded the the same
	upstream server.
	When dealing with several backend servers, it's sometimes useful that one
	client (browser) is always served by the same backend server
	(for session persistance for example).
	Using a persistance by IP (with the ip_hash upstream module) is maybe not
	a good idea because there could be situations where a lot of different
	browsers are coming with the same IP address (behind proxies)and the load
	balancing system won't be fair.
	Using a cookie to track the upstream server makes each browser unique.
	When the sticky module can't apply, it switchs back to the classic Round Robin
	Upstream or returns a "Bad Gateway" (depending on the no_fallback flag).
	
	Sticky module can't apply when cookies are not supported by the browser
	* Sticky module is based on a "best effort" algorithm. Its aim is not to handle
	* security somehow. It's been made to ensure that normal users are always
	* redirected to the same backend server: that's all!
Installation
	You'll need to re-compile Nginx from source to include this module.
	Modify your compile of Nginx by adding the following directive
	(modified to suit your path of course):
	./configure ... --add-module=/absolute/path/to/nginx-sticky-module
	make
	make install
Usage
	upstream {
		sticky;
		server 127.0.0.1:9000;
		server 127.0.0.1:9001;
		server 127.0.0.1:9002;
	}
	sticky [name=route] [domain=.foo.bar] [path=/] [expires=1h] [hash=index|md5|sha1] [no_fallback];
	 - name: the name of the cookies used to track the persistant upstream srv
	 default: route
	 - domain: the domain in which the cookie will be valid
	 default: nothing. Let the browser handle this.
	 - path: the path in which the cookie will be valid
	 default: nothing. Let the browser handle this.
	 - expires: the validity duration of the cookie
	 default: nothing. It's a session cookie.
	 restriction: must be a duration greater than one second
	 - hash: the hash mechanism to encode upstream server. It cant' be used
	 with hmac.
	 md5|sha1: well known hash
	 index: it's not hashed, an in-memory index is used instead
	 it's quicker and the overhead is shorter
	 Warning: the matching against upstream servers list
	 is inconsistent. So, at reload, if upstreams servers
	 has changed, index values are not guaranted to
	 correspond to the same server as before!
	 USE IT WITH CAUTION and only if you need to!
	 default: md5
	 - hmac: the HMAC hash mechanism to encode upstream server
	 It's like the hash mechanism but it uses hmac_key
	 to secure the hashing. It can't be used with hash.
	 md5|sha1: well known hash
	 default: none. see hash.
	 -hmac_key: the key to use with hmac. It's mandatory when hmac is set
	 default: nothing.
	 -no_fallback: when this flag is set, nginx will return a 502 (Bad Gateway or
		 Proxy Error) if a request comes with a cookie and the
		 corresponding backend is unavailable.
Detail Mechanism
	see docs/sticky.{vsd,pdf}	
Contributing
	http://code.google.com/p/nginx-sticky-module/
TODO
	Stress
	Code review
Author
	Jerome Loyet <jerome at loyet dot net>
Copyright & License
	This module is licenced under the BSD license.
	Copyright (C) 2010 Jerome Loyet (jerome at loyet dot net)
	Redistribution and use in source and binary forms, with or without
	modification, are permitted provided that the following conditions
	are met:
	1. Redistributions of source code must retain the above copyright
	notice, this list of conditions and the following disclaimer.
	2. Redistributions in binary form must reproduce the above copyright
	notice, this list of conditions and the following disclaimer in the
	documentation and/or other materials provided with the distribution.
	THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
	ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	ARE DISCLAIMED. IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
	FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	SUCH DAMAGE.