StackExchange.Redis - LockTake/LockRelease用法

Question

我正在使用Redis和StackExchange.Redis.我有多个线程,在某些时候会访问和编辑相同键的值,所以我需要同步数据的操作.

查看可用的函数,我看到有两个函数,TakeLock和ReleaseLock.但是,这些函数既可以使用键参数,也可以使用值参数,而不是锁定预期的单个键.GitHub上的intellisene文档和源代码没有解释如何使用LockTake和LockRelease函数或传递的键和值参数.

问:StackExchange.Redis中LockTake和LockRelease的正确用法是什么？

我想要做的伪代码示例:

//Add Items Before Parallel Execution
redis.StringSet("myJSONKey", myJSON);

//Parallel Execution
Parallel.For(0, 100, i =>
    {
        //Some work here
        //....

        //Lock
        redis.LockTake("myJSONKey");

        //Manipulate
        var myJSONObject = redis.StringGet("myJSONKey");
        myJSONObject.Total++;
        Console.WriteLine(myJSONObject.Total);
        redis.StringSet("myJSONKey", myNewJSON);

        //Unlock
        redis.LockRelease("myJSONKey");

        //More work here
        //...
    });

Answer 1

锁有3个部分:

• 密钥(数据库中锁的唯一名称)
• 值(一个调用者定义的标记,可用于指示谁"拥有"锁,并检查释放和扩展锁是否正确完成)
• 持续时间(锁定故意是有限持续时间的事情)

如果没有想到其他价值,guid可能会产生一个合适的"价值".我们倾向于使用机器名称(如果多个进程可以在同一台机器上竞争,则使用机器名称的munged版本).

另外,请注意,锁定是推测性的,而不是阻塞.您完全有可能无法获得锁定,因此您可能需要对此进行测试,并可能添加一些重试逻辑.

一个典型的例子可能是:

RedisValue token = Environment.MachineName;
if(db.LockTake(key, token, duration)) {
    try {
        // you have the lock do work
    } finally {
        db.LockRelease(key, token);
    }
}

请注意,如果工作很长(尤其是循环),您可能希望LockExtend在中间添加一些偶尔的呼叫 - 再次记住检查是否成功(如果它超时).

另请注意,所有单独的redis命令都是原子的,因此您无需担心两个谨慎的操作竞争.对于更复杂的多操作单元,事务和脚本是可选的.

Answer 2

我的部分代码用于锁定->获取->修改（如果需要）-> 带注释的解锁操作。

    public static T GetCachedAndModifyWithLock<T>(string key, Func<T> retrieveDataFunc, TimeSpan timeExpiration, Func<T, bool> modifyEntityFunc,
       TimeSpan? lockTimeout = null, bool isSlidingExpiration=false) where T : class
    {
        
        int lockCounter = 0;//for logging in case when too many locks per key
        Exception logException = null;

        var cache = Connection.GetDatabase();
        var lockToken = Guid.NewGuid().ToString(); //unique token for current part of code
        var lockName = key + "_lock"; //unique lock name. key-relative.
        T tResult = null;
        
        while ( lockCounter < 20)
        {
            //check for access to cache object, trying to lock it
            if (!cache.LockTake(lockName, lockToken, lockTimeout ?? TimeSpan.FromSeconds(10)))
            {
                lockCounter++;
                Thread.Sleep(100); //sleep for 100 milliseconds for next lock try. you can play with that
                continue;
            }

            try
            {
                RedisValue result = RedisValue.Null;

                if (isSlidingExpiration)
                {
                    //in case of sliding expiration - get object with expiry time
                    var exp = cache.StringGetWithExpiry(key);
                    
                    //check ttl.
                    if (exp.Expiry.HasValue && exp.Expiry.Value.TotalSeconds >= 0)
                    {
                        //get only if not expired
                        result = exp.Value;
                    }
                }
                else //in absolute expiration case simply get
                {
                    result = cache.StringGet(key);
                }

                //"REDIS_NULL" is for cases when our retrieveDataFunc function returning null (we cannot store null in redis, but can store pre-defined string :) )
                if (result.HasValue && result == "REDIS_NULL") return null;
                //in case when cache is epmty
                if (!result.HasValue)
                {
                    //retrieving data from caller function (from db from example)
                    tResult = retrieveDataFunc();

                    if (tResult != null)
                    {
                        //trying to modify that entity. if caller modifyEntityFunc returns true, it means that caller wants to resave modified entity.
                        if (modifyEntityFunc(tResult))
                        {
                            //json serialization
                            var json = JsonConvert.SerializeObject(tResult);
                            cache.StringSet(key, json, timeExpiration);
                        }
                    }
                    else
                    {
                        //save pre-defined string in case if source-value is null.
                        cache.StringSet(key, "REDIS_NULL", timeExpiration);
                    }
                }
                else
                {
                    //retrieve from cache and serialize to required object
                    tResult = JsonConvert.DeserializeObject<T>(result);
                    //trying to modify
                    if (modifyEntityFunc(tResult))
                    {
                        //and save if required
                        var json = JsonConvert.SerializeObject(tResult);
                        cache.StringSet(key, json,  timeExpiration);
                    }
                }

                //refresh exiration in case of sliding expiration flag
                if(isSlidingExpiration)
                    cache.KeyExpire(key, timeExpiration);
            }
            catch (Exception ex)
            {
                logException = ex;
            }
            finally
            {                    
                cache.LockRelease(lockName, lockToken);
            }
            break;
        }

        if (lockCounter >= 20 || logException!=null)
        {
            //log it
        }

        return tResult;
    }

和用法：

public class User
{
    public int ViewCount { get; set; }
}

var cachedAndModifiedItem = GetCachedAndModifyWithLock<User>( 
        "MyAwesomeKey", //your redis key
        () => // callback to get data from source in case if redis's store is empty
        {
            //return from db or kind of that
            return new User() { ViewCount = 0 };
        }, 
        TimeSpan.FromMinutes(10), //object expiration time to pass in Redis
        user=> //modify object callback. return true if you need to save it back to redis
        {
            if (user.ViewCount< 3)
            {
                user.ViewCount++;
                return true; //save it to cache
            }
            return false; //do not update it in cache
        },
        TimeSpan.FromSeconds(10), //lock redis timeout. if you will have race condition situation - it will be locked for 10 seconds and wait "get_from_db"/redis read/modify operations done.
        true //is expiration should be sliding.
        );

该代码可以改进（例如，您可以添加事务以减少对缓存等的调用次数），但我很高兴它对您有所帮助。