def save(self, record_key, record_data, overwrite=True, secret_key=''):
        
        ''' 
            a method to create a file in the collection folder on S3

        :param record_key: string with name to assign to record (see NOTES below)
        :param record_data: byte data for record body
        :param overwrite: [optional] boolean to overwrite records with same name
        :param secret_key: [optional] string with key to encrypt data
        :return: string with name of record

        NOTE:   record_key may only contain alphanumeric, /, _, . or -
                characters and may not begin with the . or / character.

        NOTE:   using one or more / characters splits the key into
                separate segments. these segments will appear as a
                sub directories inside the record collection and each
                segment is used as a separate index for that record
                when using the list method
                eg. lab/unittests/1473719695.2165067.json is indexed:
                [ 'lab', 'unittests', '1473719695.2165067', '.json' ]
        '''

        title = '%s.save' % self.__class__.__name__
    
    # validate inputs
        input_fields = {
            'record_key': record_key,
            'secret_key': secret_key
        }
        for key, value in input_fields.items():
            if value:
                object_title = '%s(%s=%s)' % (title, key, str(value))
                self.fields.validate(value, '.%s' % key, object_title)
    
    # validate byte data
        if not isinstance(record_data, bytes):
            raise ValueError('%s(record_data=b"...") must be byte data.' % title)
    
    # encrypt data
        if secret_key:
            from labpack.encryption import cryptolab
            record_data, secret_key = cryptolab.encrypt(record_data, secret_key)
            
    # define keyword arguments
        from time import time
        create_kwargs = {
            'bucket_name': self.bucket_name,
            'record_key': record_key,
            'record_data': record_data,
            'overwrite': overwrite,
            'record_metadata': { 'last_modified': str(time()) }
        }
    
    # add encryption metadata
        if secret_key:
            create_kwargs['record_metadata']['encryption'] = 'lab512'
    
    # add record mimetype and encoding
        import mimetypes
        guess_mimetype, guess_encoding = mimetypes.guess_type(record_key)
        if not guess_mimetype:
            if record_key.find('.yaml') or record_key.find('.yml'):
                guess_mimetype = 'application/x-yaml'
            if record_key.find('.drep'):
                guess_mimetype = 'application/x-drep'
        if guess_mimetype:
            create_kwargs['record_mimetype'] = guess_mimetype
        if guess_encoding:
            create_kwargs['record_encoding'] = guess_encoding
            
    # create record
        self.s3.create_record(**create_kwargs)

        return record_key
a method to create a file in the collection folder on S3

        :param record_key: string with name to assign to record (see NOTES below)
        :param record_data: byte data for record body
        :param overwrite: [optional] boolean to overwrite records with same name
        :param secret_key: [optional] string with key to encrypt data
        :return: string with name of record

        NOTE:   record_key may only contain alphanumeric, /, _, . or -
                characters and may not begin with the . or / character.

        NOTE:   using one or more / characters splits the key into
                separate segments. these segments will appear as a
                sub directories inside the record collection and each
                segment is used as a separate index for that record
                when using the list method
                eg. lab/unittests/1473719695.2165067.json is indexed:
                [ 'lab', 'unittests', '1473719695.2165067', '.json' ]