StackExchange.Redis - LockTake / LockRelease Usage

Question

I am using Redis with StackExchange.Redis. I have multiple threads that will at some point access and edit the value of the same key, so I need to synchronize the manipulati

Answer 1

There is my part of code for lock->get->modify(if required)->unlock actions with comments.

    public static T GetCachedAndModifyWithLock(string key, Func retrieveDataFunc, TimeSpan timeExpiration, Func 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(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;
    }

and usage :

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

var cachedAndModifiedItem = GetCachedAndModifyWithLock( "MyAwesomeKey", () =>
        {
            //return from db or kind of that
            return new User() { ViewCount = 0 };
        }, TimeSpan.FromMinutes(10), user=>
        {
            if (user.ViewCount< 3)
            {
                user.ViewCount++;
                return true; //save it to cache
            }
            return false; //do not update it in cache
        }, TimeSpan.FromSeconds(10),true);

That code can be improved (for example, you can add transactions for less count call to cache and etc), but i glad it will be helpfull for you.