public void set(String groupName, String counterName, long value) {
    Map<String, Long> counterMap = _pubCounters.get(groupName);
    if (counterMap == null) {
      counterMap = new HashMap<String, Long>(4);
      _pubCounters.put(groupName, counterMap);
    }
    counterMap.put(counterName, value);
  }
Tez Counter Data defining data structure.
app/com/linkedin/drelephant/tez/data/TezCounterData.java