'Enables session persistence for a Balancer by setting the persistence
type to \'HTTP_COOKIE\'.  This method blocks until the enable request
has been processed and the balancer is in a RUNNING state again.
:param balancer: Balancer to enable session persistence on.
:type  balancer: :class:`LoadBalancer`
:return: Updated Balancer.
:rtype: :class:`LoadBalancer`'

def ex_enable_balancer_session_persistence(self, balancer):

    if (not self.ex_enable_balancer_session_persistence_no_poll(balancer)):
        msg = 'Enable    session    persistence    request    not    accepted'
        raise LibcloudError(msg, driver=self)
    return self._get_updated_balancer(balancer)
