'Fails over replication for a group and volumes in the group.
:param group: group object
:param volumes: list of volume objects in the group
:param secondary_backend_id: backend_id of the secondary site
:returns: model_update - dict of group updates
:returns: volume_model_updates - list of dicts of volume updates'

def failover_replication(self, context, group, volumes, secondary_backend_id=None):

    raise NotImplementedError()
