shadow.py 文件源码

def become_leader(config, redis_client):
    """ tscached can be deployed on multiple servers. Only one of them should exert shadow load.
        We use RedLock (http://redis.io/topics/distlock) to achieve this. If we cannot acquire the
        shadow lock, fail fast. If our server (or this program) crashes, the leader key will expire and
        another server will take over eventually.

        RedLock is (debatably) imperfect, but that's okay with us: our worst case is that some work gets
        done twice -  because we are using Redis as a cache and *not* as a datastore. We're using one of
        the standard Python clientlibs: https://github.com/glasslion/redlock

        This implementation assumes a single-master Redis cluster.

        :param config: dict representing the top-level tscached config
        :param redis_client: redis.StrictRedis
        :return: redlock.RedLock or False
    """
    hostname = socket.gethostname()
    leader_expiration = config['shadow'].get('leader_expiration', 3600) * 1000  # ms expected
    deets = [redis_client]  # no need to reinitialize a redis connection.

    try:
        lock = redlock.RedLock(SHADOW_LOCK_KEY, ttl=leader_expiration, connection_details=deets)
        if lock.acquire():
            # mostly for debugging purposes
            redis_client.set(SHADOW_SERVER_KEY, hostname, px=leader_expiration)
            logging.info('Lock acquired; now held by %s' % hostname)
            return lock
        else:
            other_host = redis_client.get(SHADOW_SERVER_KEY)
            logging.info('Could not acquire lock; lock is held by %s' % other_host)
            return False
    except redis.exceptions.RedisError as e:
        logging.error('RedisError in acquire_leader: ' + e.message)
        return False
    except redlock.RedLockError as e:
        logging.error('RedLockError in acquire_leader: ' + e.message)
        return False