'Returns True if there is at least 1 active worker to run scheduled tasks
- run from the main request
NB Can\'t run this 1/request at the beginning since the tables
only get defined in zz_last'

def _is_alive(self):

    db = current.db
    cache = current.response.s3.cache
    now = datetime.datetime.now()
    offset = datetime.timedelta(minutes=1)
    table = db.scheduler_worker
    query = (table.last_heartbeat > (now - offset))
    worker_alive = db(query).select(table.id, limitby=(0, 1), cache=cache).first()
    if worker_alive:
        return True
    else:
        return False
