private void checkWorkerLiveness(){
        //System.out.println("monitor timer expire!");
        
        Set<Integer> workerIdSet = workerStatusMap.keySet();
        Iterator<Integer> idIterator = workerIdSet.iterator();
        while(idIterator.hasNext()){
            int id = idIterator.next();
            if(workerStatusMap.get(id).intValue() == -1)
                continue;
            else if(workerStatusMap.get(id).intValue() > 1){
                System.out.println("worker "+id+" is not alive. remove it");
                removeNode(id);
            }
            else{
                workerStatusMap.put(id, workerStatusMap.get(id).intValue()+1);
            }
                
        }
    }