code: private void writeFixedFields(long millis, Level level, int threadId, long seq, DataOutput writer)
  		throws IOException {
  		// Write time first for ease of time filtering.
  		writer.writeLong(millis);
  		
  		// Write our own sequence number for merging purposes.
  		long seqNumber;
  		synchronized(BinaryLogRecordSerializerImpl.class) {
  			seqNumber = counter++;
  		}
  		writer.writeLong(seqNumber);
  		
  		// Write data always present in a log record.
  		writer.writeShort(level.intValue());
  		writer.writeInt(threadId);
  		writer.writeLong(seq);
  	}
docs: sequence number of message.