code: public boolean track(GlobalStreamId stream, long ts) {
          Long currentVal = streamToTs.get(stream);
          if (currentVal == null || ts > currentVal) {
              streamToTs.put(stream, ts);
          }
          checkFailures();
          return ts >= lastWaterMarkTs;
      }
docs: Tracks the timestamp of the event in the stream, returns
  true if the event can be considered for processing or
  false if its a late event.