cache 访问频率的思考

互联网的项目用户基数很大，有时候瞬间并发量非常大，这个时候对于数据访问来说是个灾难。为了应对这种场景，一般都会大量采用web服务器集群，缓存集群。采用集群后基本上就能解决大量并发的数据访问。当然这个时候内网的网速会成为缓存速度的瓶颈。

当然我们希望能有更好的缓存结构，比如一级缓存和二级缓存。一级缓存直接缓存在宿主主机内存上，二级缓存缓存在redis集群上，如果一个缓存实例被访问的频率非常高，我们希望这个缓存实例能缓存在宿主主机内存上，如果一个实例的访问频率非常低，我们甚至可能不会为此实例进行缓存处理。

基于这种设想，我们希望能够跟踪监视缓存实例，并根据监视结果，对实例的缓存级别进行动态调整，以达到最佳的缓存效果。（事实上dotNet4.0里面的System.Runtime.Caching.MemoryCache对此已经有很好的实现和支持了。当然我们的应用必须知道要缓存在宿主主机内存上，还是redis集群上，那就必须实现类似System.Runtime.Caching.MemoryCache的监视功能和动态调整功能）

首先我们需要附加一些监视信息到缓存实例上，

 public class CacheAttach
    {
        public CacheAttach(string key)
        {
            this.Key = key;
            this.InsertedTime = DateTime.Now;
        }
        public string Key { get; set; }
        public DateTime InsertedTime { get; private set; }
        public int QueryTimes { get; set; }
        public int AccessTimes { get; set; }
        public override bool Equals(object obj)
        {
            if (obj == null)
                return false;
            return obj.GetHashCode() == this.GetHashCode();
        }
        public override int GetHashCode()
        {
            return Key.GetHashCode();
        }
        public static implicit operator CacheAttach(string value)
        {
            return new CacheAttach(value);
        }
    }
    public class CacheAttachCollection : List<CacheAttach>, ICollection<CacheAttach>
    {
        public bool Contains(string Key)
        {
            return this.Find(i => i.Key == Key) == null;
        }
        public CacheAttach this[string key]
        {
            get
            {
                CacheAttach item =this.Find(i => i.Key == key);
                if (item == null)
                {
                    item = new CacheAttach(key);
                    this.Add(item);
                }
                return item;
            }
            set
            {
                CacheAttach item = this.Find(i => i.Key == key);
                if (item == null)
                {
                    item = new CacheAttach(key);
                    this.Add(item);
                }
                item = value;
            }
        }
    }

　　这里采用的是一种附加形式的监视，不去破坏原来的K/V缓存方式。这个时候我们可能需要重新包装一下原有的缓存访问，使得对缓存的操作能被监视。

public class MonitorCache: ICache
    {
        private ICache proxyCache;
        CacheAttachCollection cacheMonitor = new CacheAttachCollection();
        public MonitorCache(ICache cache)
        {
            this.proxyCache = cache;
        }
        #region ICache Implement
        public bool Set<T>(string key, T value)
        {
            cacheMonitor[key].QueryTimes++;
            cacheMonitor[key].AccessTimes++;
            return proxyCache.Set(key, value);
        }

        public bool Set<T>(string key, T value, DateTime absoluteTime, TimeSpan slidingTime, Action<string, T> removingHandler)
        {
            cacheMonitor[key].QueryTimes++;
            cacheMonitor[key].AccessTimes++;
            return this.proxyCache.Set(key, value, absoluteTime, slidingTime, removingHandler);
        }

        public object Get(string key)
        {
            cacheMonitor[key].QueryTimes++;
            cacheMonitor[key].AccessTimes++;
            return this.proxyCache.Get(key);
        }

        public T Get<T>(string key)
        {
            cacheMonitor[key].QueryTimes++;
            cacheMonitor[key].AccessTimes++;
            return this.proxyCache.Get<T>(key);
        }

        public bool Contains(string key)
        {
            cacheMonitor[key].QueryTimes++;
            return this.proxyCache.Contains(key);
        }

        public bool Remove(string key)
        {
            if (this.proxyCache.Remove(key))
            {
                cacheMonitor.Remove(key);
                return true;
            }
            return false;
        }
        #endregion

        public object this[string key]
        {
            get
            {
                return this.Get(key);
            }
            set
            {
                this.Set(key, value);
            }
        }

        public CacheAttachCollection Monitor
        {
            get
            {
                return this.cacheMonitor;
            }
        }

    }

　　通过对原有的缓存访问进行包装，我们已经实现对原有缓存的重构，实现监视的意图。

 public class CacheHelper : ICache
    {
        private MonitorCache level1 = null;
        private MonitorCache level2 = null;

        private CacheHelper()
        {
            this.level1 = new MonitorCache(new MemoryCache());
            this.level2 = new MonitorCache(new RedisCache());
        }

        public bool Set<T>(string key, T value)
        {
            if (this.level1.Set(key, value))
                return true;
            if (this.level2.Set(key, value))
                return true;
            return false;
        }

        public bool Set<T>(string key, T value, DateTime absoluteTime, TimeSpan slidingTime, Action<string, T> removingHandler)
        {
            if (this.level1.Set(key, value, absoluteTime, slidingTime, removingHandler))
                return true;
            if (this.level2.Set(key, value, absoluteTime, slidingTime, removingHandler))
                return true;
            return false;
        }

        public object Get(string key)
        {
            return this.level1.Get(key) ?? this.level2.Get(key) ?? null;
        }

        public T Get<T>(string key)
        {
            if (this.level1.Contains(key))
                return this.level1.Get<T>(key);
            if (this.level2.Contains(key))
                return this.level2.Get<T>(key);
            return default(T);
        }

        public T Get<T>(string key, Func<T> valueGetter)
        {
            var result = default(T);
            if (this.level1.Contains(key))
                result = this.level1.Get<T>(key);
            else if (this.level2.Contains(key))
                result = this.level2.Get<T>(key);

            if (result == null && valueGetter != null)
                result = valueGetter();
            return result;
        }

        public bool Contains(string key)
        {
            if (this.level1.Contains(key))
                return true;
            if (this.level2.Contains(key))
                return true;
            return false;
        }

        public bool Remove(string key)
        {
            if (this.level1.Contains(key))
                this.level1.Remove(key);
            if (this.level2.Contains(key))
                this.level2.Remove(key);
            return true;
        }

        public object this[string key]
        {
            get
            {
                return this.Get(key);
            }
            set
            {
                this.Set(key, value);
            }
        }
        public void Trim()
        {
            //对一级缓存进行整理
            for (int i = 0, lengh = this.level1.KeyMonitor.Count; i < lengh; i++)
            {
                CacheAttach item = this.level1.KeyMonitor[i];

                //频率小于10次/秒的缓存需要移除一级缓存
                if (item.AccessRate < 10)
                {
                    //频率大于1次/秒的缓存移到二级缓存
                    if (item.AccessRate >= 1)
                    {
                        this.level2.Set(item.Key, this.level1[item.Key]);
                        this.level2.KeyMonitor[item.Key] = item;
                    }
                    this.level1.Remove(item.Key);
                }
            }

            //对二级缓存进行整理
            for (int i = 0, lengh = this.level2.KeyMonitor.Count; i < lengh; i++)
            {
                CacheAttach item = this.level1.KeyMonitor[i];

                //频率大于等于10次/秒的缓存需要移至一级缓存
                if (item.AccessRate >= 10)
                {
                    this.level1.Set(item.Key, this.level2[item.Key]);
                    this.level1.KeyMonitor[item.Key] = item;
                    this.level1.Remove(item.Key);
                    continue;
                }
                if (item.AccessRate < 1)
                {
                    this.level2.Remove(item.Key);
                    continue;
                }
            }
        }

        private static CacheHelper _Current = new CacheHelper();
        public static CacheHelper Current
        {
            get { return _Current; }
        }
        public static CacheHelper()
        {
            System.Threading.Timer timer = new System.Threading.Timer(delegate
            {
                Current.Trim();
            });
        }
    }