public void markServerDown(String id) {
        boolean triggered = false;

        id = Server.normalizeId(id);

        if (id == null) {
            return;
        }

        Lock writeLock = upServerLock.writeLock();
    	writeLock.lock();
        try {
            final List<Server> changedServers = new ArrayList<Server>();

            for (Server svr : upServerList) {
                if (svr.isAlive() && (svr.getId().equals(id))) {
                    triggered = true;
                    svr.setAlive(false);
                    changedServers.add(svr);
                }
            }

            if (triggered) {
                logger.error("LoadBalancer [{}]:  markServerDown called for server [{}]", name, id);
                notifyServerStatusChangeListener(changedServers);
            }

        } finally {
            writeLock.unlock();
        }
    }
Class that contains the mechanism to "ping" all the instances

@author stonse
ribbon-loadbalancer/src/main/java/com/netflix/loadbalancer/BaseLoadBalancer.java