def _set(self, key, value, handle=None):
        """
        Sets a permanent value

        :param key: name of the value
        :type key: str

        :param value: actual value
        :type value: any serializable type is accepted

        :param handle: an optional instance of a Sqlite database
        :type handle: a connection

        This functions stores or updates a value in the back-end storage
        system.

        Example::

            store._set('parameter_123', 'George')

        """
        handle = handle if handle else self.get_db()

        cursor = handle.cursor()
        cursor.execute("DELETE FROM store WHERE context=? AND key=?",
                       (self.id, key))
        cursor.execute("INSERT INTO store (context,key,value) VALUES (?,?,?)",
                       (self.id, key, value))
        handle.commit()
        cursor.close()