@Override
    public boolean apply(PredicateKey input) {
        Server s = input.getServer();
        String az = s.getZone();
        if (az != null && zone != null && az.toLowerCase().equals(zone.toLowerCase())) {
            return true;
        } else {
            return false;
        }
    }
A predicate the filters out servers that are not in the same zone as the client's current
zone.

@author awang
ribbon-loadbalancer/src/main/java/com/netflix/loadbalancer/ZoneAffinityPredicate.java