@Override
    public Set<String> checkConflict(Request request){
        if (!this.isRunning){
            throw new RuntimeException("access a down site");
        }        
        switch (request.requestType){
        case READ:
            return lockManager.checkConflict(request.resource, 
                    request.transaction, LockType.READ);            
        case WRITE:
            return lockManager.checkConflict(request.resource, 
                    request.transaction, LockType.WRITE);
        case ROREAD:
            return new HashSet<String>();
        default:
            throw new IllegalArgumentException("request type must be either read or write");
        }
    }