I don't know if that will work (see http://wiki.nginx.org/IfIsEvil)

This will almost certainly work:

[code]
location / {
set_by_lua $backend_ip '
if ngx.var.cookie_memberid ~= "" and ngx.var.cookie_memberid ~= nil then
return "10.10.10.10"
elseif ngx.var.cookie_clientid ~= "" and ngx.var.cookie_clientid ~= nil then
return "10.10.10.15"
else
return "10.10.10.20"
end
';
proxy_pass http://$backend_ip;
}
[/code]

Needs 3rd party lua module.

I only just discovered it myself and starting to learn lua. Seems worth it as it opens up a lot of possibilities without getting into the if is evil issues.