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();
});
}
}
