Question

如果查看RedisLock.cs类的代码，您会看到它正在读取锁值以验证Redis外部的超时本身，并且如果没有人触摸它，它还使用Watch和Unwatch覆盖超时值。

换句话说，当我们可以使用更可靠的Redis内部设置超时时，使用监视，取消监视和提交的确切含义是什么？

ExecUtils.RetryUntilTrue(() =>{
    //This pattern is taken from the redis command for SETNX http://redis.io/commands/setnx

    //Calculate a unix time for when the lock should expire
    var realSpan = timeOut ?? new TimeSpan(365, 0, 0, 0); //if nothing is passed in the timeout hold for a year
    var expireTime = DateTime.UtcNow.Add(realSpan);
    var lockString = (expireTime.ToUnixTimeMs() + 1).ToString();

    //Try to set the lock, if it does not exist this will succeed and the lock is obtained
    var nx = redisClient.SetValueIfNotExists(key, lockString);
    if (nx)
        return true;

    //If we've gotten here then a key for the lock is present. This could be because the lock is
    //correctly acquired or it could be because a client that had acquired the lock crashed (or didn't release it properly).
    //Therefore we need to get the value of the lock to see when it should expire

    redisClient.Watch(key);
    var lockExpireString = redisClient.Get<string>(key);
    if (!long.TryParse(lockExpireString, out var lockExpireTime))
    {
        redisClient.UnWatch();  // since the client is scoped externally
        return false;
    }

    //If the expire time is greater than the current time then we can't let the lock go yet
    if (lockExpireTime > DateTime.UtcNow.ToUnixTimeMs())
    {
        redisClient.UnWatch();  // since the client is scoped externally
        return false;
    }

    //If the expire time is less than the current time then it wasn't released properly and we can attempt to 
    //acquire the lock. The above call to Watch(_lockKey) enrolled the key in monitoring, so if it changes
    //before we call Commit() below, the Commit will fail and return false, which means that another thread 
    //was able to acquire the lock before we finished processing.
    using (var trans = redisClient.CreateTransaction()) // we started the "Watch" above; this tx will succeed if the value has not moved 
    {
        trans.QueueCommand(r => r.Set(key, lockString));
        return trans.Commit(); //returns false if Transaction failed
    }
},timeOut);

因为StackExchange.Redis库等效项仅使用string.set功能。尽管ServiceStackRedis锁定机制更加先进，并且会等待重试直到获得锁定，但仍不使用REDIS SET命令本身的超时是没有道理的。

public bool LockTake(RedisKey key, RedisValue value, TimeSpan expiry, CommandFlags flags = CommandFlags.None)            {
    if (value.IsNull) throw new ArgumentNullException(nameof(value));
    return StringSet(key, value, expiry, When.NotExists, flags);
}

为什么ServiceStack.Redis不使用SET Timeout获取锁？

0 个答案: