public static Set<String> getAvailableZones(
            Map<String, ZoneSnapshot> snapshot, double triggeringLoad,
            double triggeringBlackoutPercentage) {
        if (snapshot.isEmpty()) {
            return null;
        }
        Set<String> availableZones = new HashSet<String>(snapshot.keySet());
        if (availableZones.size() == 1) {
            return availableZones;
        }
        Set<String> worstZones = new HashSet<String>();
        double maxLoadPerServer = 0;
        boolean limitedZoneAvailability = false;

        for (Map.Entry<String, ZoneSnapshot> zoneEntry : snapshot.entrySet()) {
            String zone = zoneEntry.getKey();
            ZoneSnapshot zoneSnapshot = zoneEntry.getValue();
            int instanceCount = zoneSnapshot.getInstanceCount();
            if (instanceCount == 0) {
                availableZones.remove(zone);
                limitedZoneAvailability = true;
            } else {
                double loadPerServer = zoneSnapshot.getLoadPerServer();
                if (((double) zoneSnapshot.getCircuitTrippedCount())
                        / instanceCount >= triggeringBlackoutPercentage
                        || loadPerServer < 0) {
                    availableZones.remove(zone);
                    limitedZoneAvailability = true;
                } else {
                    if (Math.abs(loadPerServer - maxLoadPerServer) < 0.000001d) {
                        // they are the same considering double calculation
                        // round error
                        worstZones.add(zone);
                    } else if (loadPerServer > maxLoadPerServer) {
                        maxLoadPerServer = loadPerServer;
                        worstZones.clear();
                        worstZones.add(zone);
                    }
                }
            }
        }

        if (maxLoadPerServer < triggeringLoad && !limitedZoneAvailability) {
            // zone override is not needed here
            return availableZones;
        }
        String zoneToAvoid = randomChooseZone(snapshot, worstZones);
        if (zoneToAvoid != null) {
            availableZones.remove(zoneToAvoid);
        }
        return availableZones;

    }
A rule that uses the a {@link CompositePredicate} to filter servers based on zone and availability. The primary predicate is composed of
a {@link ZoneAvoidancePredicate} and {@link AvailabilityPredicate}, with the fallbacks to {@link AvailabilityPredicate}
and an "always true" predicate returned from {@link AbstractServerPredicate#alwaysTrue()} 

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