Complex Backend Selection through VCL?

[Joe Chen]

Would someone here help me with using VCL for a complex situation with
Varnish?

I was ask to configure Varnish within using VCL to do a very dynamic backend
selection process, but unable to achieve that. The task I was given is:

1.  Using VCL to have a list of 3 backend address:
           m.com
           s01.com
           s02.com

2. Configure Varnish so that:
            a.  it relays each of incoming GET requests to the master
backend (m.com, in this case), and the master backend (m.com) will always
respond with a status 302 for redirect.

            b.  for each of fetched 302 from master backend, Varnish will
check the http header and to select a corresponding slave backend from the
the list( either s01.com or s02.com), and restart the fetch to let Varnish
fetch the request from the slave backend.


I have tried to construct a VCL (included at the bottom)  but I am unable to
make Varnish work the exactly as described above.

FYI, I am using Varnish 2.1.3.

Any help would appreciated very much,

--Joe

backend b_m {
         .host = "m.com";
         .port = "80";
}

backend b1 {
         .host = "s01.com";
         .port = "80";
}

backend b2 {
         .host = "s02.com";
         .port = "80";
}


sub vcl_recv {
    if (!(req.url ~ "(.*s0.*)")  && req.url ~ "(.*e\.html)") {
        set req.backend = b_m;
        set req.http.host = "m.com";
     }
}

sub vcl_fetch {
    if (beresp.status == 302) {
        if (beresp.http.location ~ "(.*s01.*)" ) {
           set req.backend = b1;
           set req.url = beresp.http.location;
           set req.http.host = "s01.a.com";
        }
        else if (beresp.http.location ~ "(.*s02.*)" ) {
           set req.backend = b2;
           set req.url = beresp.http.location;
           set req.http.host = "s02.com";
        }
    }

    restart;
}
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