docs: public static MongoDS getDS(Setting setting, String... groups) {
  		final String key = setting.getSettingPath() + GROUP_SEPRATER + ArrayUtil.join(groups, GROUP_SEPRATER);
  		MongoDS ds = dsMap.get(key);
  		if (null == ds) {
  			// 没有在池中加入之
  			ds = new MongoDS(setting, groups);
  			dsMap.put(key, ds);
  		}
  
  		return ds;
  	}