Varnish and Multiple Backend

Multiple Backends
backend default {
  .host = "127.0.0.1";
  .port = "8080";
}
 
backend stylesheets {
  .host = "10.0.0.10";
  .port = "80";
}
 
sub vcl_recv {
  if (req.url ~ "^/stylesheets") {
    # set stylesheets backend
    set req.backend = stylesheets;
    return(lookup);
  }
 
  # set default backend
  set req.backend = default;
  return(pass);
}

Round Robin And Random Multiple Server Backend

http://highscalability.com/blog/2011/2/28/a-practical-guide-to-varnish-why-varnish-matters.html