// private methods
 private void AddOverfow(BsonBinaryWriter bsonBinaryWriter, IByteBuffer overflow)
 {
     _lastRequestPosition = (int)bsonBinaryWriter.Stream.Position;
     bsonBinaryWriter.WriteRawBsonDocument(overflow);
     _batchCount++;
     _batchLength = (int)bsonBinaryWriter.Stream.Position - _batchStartPosition;
 }