'Invoked prior to each group join or rejoin.
This is typically used to perform any cleanup from the previous
generation (such as committing offsets for the consumer)
Arguments:
generation (int): The previous generation or -1 if there was none
member_id (str): The identifier of this member in the previous group
or \'\' if there was none'

@abc.abstractmethod
def _on_join_prepare(self, generation, member_id):

    pass
