Backend load balancing delegation in order to support sticky sessions

Bayron Jose Guevara Calderon Bayron.Guevara at
Mon Sep 27 20:04:28 CEST 2010

Hello community,

Currently I've installed Varnish 2.0.6 and I'm using a cluster of Web Servers. In order to find a work around the session clustering issue, I thought implement session stickiness as part of the load balancing configuration, so all subsequent requests from the same user would go to the same webserver and so ensure the session information consistency. But as I know, Varnish don't have integrated support to Stickiness, and therefore I have to delegate this function to a Load Balancer. Through this infrastructure the client requests would follow the below path:

User Agent --> Load Balancer Public VIP --> One of the Varnish Servers --> (IF cache hit --> deliver cached content to Client ) ELSE ( Load Balancer Private VIP --> One of the Web Servers)

To support sticky sessions my Load Balancer uses the Arrow Point cookie mechanism which, to be brief, add a cookie named ARPT that identified the user session. And according to the above path, Varnish sends requests and receive responses from the Load Balancer private VIP which needs that the cookie in question be passed transparently to and from the client. So, Varnish should take care of this cookie in two ways: the Cookie header in the incoming requests and the Set-Cookie in the responses from the Load Balancer.

What do you recommend me to do?. I have ideas but I don't sure. For example, to extract the desired cookie from Cookie header I'm doing the following:

### Code Start ####
vcl_receive {
if (req.http.Cookie ~ "ARPT=") {
       set req.http.LB-Cookie = regsub(req.http.Cookie, ".*(ARPT=[^;]+).*", "\1");
unset req.http.Cookie;

sub vcl_miss {
        set bereq.http.Cookie = req.http.LB-Cookie;

sub vcl_pass {
        set bereq.http.Cookie = req.http.LB-Cookie;
###  Code End ####

To keep it in the http response, I guess:

###  Code Start ###
sub vcl_fetch {
if (obj.http.Set-Cookie ~ "ARPT=") {
                #Save Set-Cookie, so it can be used in vcl_deliver
                set obj.http.X-Set-Cookie = obj.http.Set-Cookie;

sub vcl_deliver {
if (obj.hits > 0) {
        } else {
        #It was a cache miss
                #Set back Set-Cookie header in case it's set
                if (resp.http.X-Set-Cookie){
                        set resp.http.Set-Cookie = resp.http.X-Set-Cookie;
                        unset resp.http.X-Set-Cookie;
### Code End ###

I don't know how to extract a cookie from a Set-Cookie header, but I tried with this  code:
        obj.http.LB-Cookie = regsub(obj.http.Set-Cookie, ".*(ARPT=[^;]+(;[^=]+=[^;]+)*).*", "\1");
Nevertheless, it appears don't work fine.

Am I in the right way? Is there a better solution?

Thank you in advance.

Bayron Guevara
Web Programmer
Bayron.Guevara at

Grupo OPSA
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San Pedro Sula, Honduras, C.A.

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