如何使用 python 在 redis 中实现原子 get 或 set&get 键？

Question

我有一个 redis 服务器，我想实现一个原子（或伪原子）方法，它将执行以下操作（注意：我有一个与 redis 服务器有多个会话的系统）：

1. 如果某个键 K 存在，则获取它的值
2. 否则，使用某个函数F（生成salts）生成的随机值调用SETNX function
3. 向 redis 询问当前会话刚刚生成的键 K 的值（或由另一个会话“同时”生成 - 在当前会话生成之前不久）

我不想使用 F 函数预先生成（在检查值是否存在之前）一个值，并在键不存在时使用它的原因是：

1. 我不想毫无理由地调用 F（这可能会导致 CPU 密集型行为（
2. 我想避免下一个有问题的情况： T1 : 会话 1 生成一个随机值 VAL1 T2：会话 1 询问密钥 K 是否存在并得到“假” T3：会话 2 生成一个随机值 VAL2 T4：会话 2 询问密钥 K 是否存在并得到“假” T5：会话 2 调用 SETNX 并使用值 VAL2 并从现在开始使用 VAL2 T6：会话 1 调用 SETNX 并使用值 VAL1 并从现在开始使用 VAL1，其中键 K 的实际值为 VAL2

我创建的python伪代码是：

    import redis
    r = redis.StrictRedis(host='localhost', port=6379, db=0)
    ''' gets the value of key K if exists (r.get(K) if r.exists(K)), 
    otherwise gets the value of key K if calling SETNX function returned TRUE 
    (else r.get(K) if r.setnx(K,F())), meaning this the sent value is really the value,
    otherwise, get the value of key K, that was generated by another session a         
    short moment ago (else r.get(K))
    The last one is redundant and I can write "**r.setnx(K,F()) or True**" to get the 
    latest value instead, but the syntax requires an "else" clause at the end '''
    r.get(K) if r.exists(K) else r.get(K) if r.setnx(K,F()) else r.get(K)

还有其他解决方案吗？

Answer 1

是的，您可以为此使用WATCH。这是redis-py 的修改示例：

def atomic_get_set(some_key):
    with r.pipeline() as pipe:
        try:
            # put a WATCH on the key that holds our sequence value
            pipe.watch(some_key)
            # after WATCHing, the pipeline is put into immediate execution
            # mode until we tell it to start buffering commands again.
            # this allows us to get the current value of our sequence
            if pipe.exists(some_key):
                return pipe.get(some_key)
            # now we can put the pipeline back into buffered mode with MULTI
            pipe.multi()
            pipe.set(some_key, F())
            pipe.get(some_key)
            # and finally, execute the pipeline (the set and get commands)
            return pipe.execute()[-1]
            # if a WatchError wasn't raised during execution, everything
            # we just did happened atomically.
        except WatchError:
            # another client must have changed some_key between
            # the time we started WATCHing it and the pipeline's execution.
            # Let's just get the value they changed it to.
            return pipe.get(some_key)