class LogicalClock:
public:
      void setClusterTime(const LogicalTime& newTime);
      const LogicalTime& getClusterTime() const;
     
     // increments clusterTime and return the result. this method should sync with the wall clock 
     LogicalTime addTicks(unsigned ticks);

    // increments clusterTime but returns the time before the increment therefor "reserving" ticks.
    // Needed for the batch writes oplog functionality. this method should sync with the wall clock  
    LogicalTime reserveTicks(unsigned ticks);
  
  private:
      mutable stdx::mutex _mutex; // unless there is a measurable performance degradation 
                                                     // serialize on mutex
      LogicalTime _clusterTime;
 };

LogicalTime LogicalClock::reserveTicks(unsigned ticks) {
  
      const unsigned wallClockSecs = durationCount<Seconds>(
          getGlobalServiceContext()->getFastClockSource()->now().toDurationSinceEpoch());
    
      stdx::lock_guard<stdx::mutex> lock(_mutex);
      // Optimistic assumption that seconds did not change
      LogicalTime currentTime = _clusterTime;
     _clusterTime.addTicks(ticks);
  
      unsigned currentSecs = _clusterTime.asTimestamp().getSecs();
      // If the seconds need to be updated, try to do it. This can happen at most once per second.
      if (MONGO_unlikely(currentSecs < wallClockSecs)) {
          // First fix the seconds portion. 
         _clusterTime = LogicalTime(Timestamp(wallClockSecs, 1));
          currentTime = _clusterTime;
          _clusterTime.addTicks(ticks);
      }
  
      return currentTime;
  }