code: private static boolean addCallback(String applicationId, DelayedCallback callback) {
  		boolean isFirst = false;
  		List<DelayedCallback> list = BACKLOG.get(applicationId);
  		if (null == list) {
  			list = new ArrayList<DelayedCallback>();
  			BACKLOG.put(applicationId, list);
  			isFirst = true;
  		}
  		list.add(callback);
  		return isFirst;
  	}
docs: Add a delayed callback for the given application id. Returns whether this is the first request for the
  application id.
  
  @param applicationId application id
  @param callback callback
  @return true when first request for that application id