.NET4.0的可扩展缓存系统

.NET Framework中，叫做System.Runtime.Caching，这不仅是个缓存库，还是个框架，可以在上面开发自己的库。ObjectCache定义了所有缓存都要实现的通用操作。与之搭配的是个内存缓存实现，叫做MemoryCache。这个缓存系统的结构如下：

上图大家可以看出来对应那些产品了吗？

下面我给大家介绍一个实现这样一个架构的代码示例，代码的核心就是ObjectCache：

定义一个抽象的Provider接口：

public interface ICacheBuilder
{
    ObjectCache GetInstance();
    string DefaultRegionName { get; }
}

In-memory提供者的实现使用MemoryCache：

public class MemoryCacheBuilder : ICacheBuilder
{
    public MemoryCacheBuilder() { }

    public ObjectCache GetInstance()
    {
        return MemoryCache.Default;
    }

    public string DefaultRegionName
    {
        get { return null; }
    }
}

分布式缓存提供者Memcached：

public class MemcachedCache : ObjectCache, ICacheBuilder
{
    private long _lDefaultExpireTime = 3600; // default Expire Time
    private MemcachedClient _client = null;
    #region ICache Members

    public MemcachedCache()
    {
        this._client = MemcachedClientService.Instance.Client;
    }

    public override void Set(string key, object value, System.DateTimeOffset absoluteExpiration, string regionName = null)
    {
        Enforce.NotNull(key, "key");
        CacheItem item = new CacheItem(key, value, regionName);
        CacheItemPolicy policy = new CacheItemPolicy();
        policy.AbsoluteExpiration = absoluteExpiration;

        Set(item, policy);
    }

    public override void Set(CacheItem item, CacheItemPolicy policy)
    {
        if (item == null || item.Value == null)
            return;

        item.Key = item.Key.ToLower();

        if (policy != null && policy.ChangeMonitors != null && policy.ChangeMonitors.Count > 0)
            throw new NotSupportedException("Change monitors are not supported");

        // max timeout in scaleout = 65535
        TimeSpan expire = (policy.AbsoluteExpiration.Equals(null)) ?
                            policy.SlidingExpiration :
                            (policy.AbsoluteExpiration - DateTimeOffset.Now);

        double timeout = expire.TotalMinutes;
        if (timeout > 65535)
            timeout = 65535;
        else if (timeout > 0 && timeout < 1)
            timeout = 1;

        this._client.Store(Enyim.Caching.Memcached.StoreMode.Set, item.Key.ToString(), item.Value);

    }

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

    public override object AddOrGetExisting(string key, object value, CacheItemPolicy policy, string regionName = null)
    {
        CacheItem item = GetCacheItem(key, regionName);
        if (item == null)
        {
            Set(new CacheItem(key, value, regionName), policy);
            return value;
        }

        return item.Value;
    }

    public override CacheItem AddOrGetExisting(CacheItem value, CacheItemPolicy policy)
    {
        CacheItem item = GetCacheItem(value.Key, value.RegionName);
        if (item == null)
        {
            Set(value, policy);
            return value;
        }

        return item;
    }

    public override object AddOrGetExisting(string key, object value, System.DateTimeOffset absoluteExpiration, string regionName = null)
    {
        CacheItem item = new CacheItem(key, value, regionName);
        CacheItemPolicy policy = new CacheItemPolicy();
        policy.AbsoluteExpiration = absoluteExpiration;

        return AddOrGetExisting(item, policy);
    }

    public override bool Contains(string key, string regionName = null)
    {
        return false;
    }

    public override CacheEntryChangeMonitor CreateCacheEntryChangeMonitor(System.Collections.Generic.IEnumerable<string> keys, string regionName = null)
    {
        throw new System.NotImplementedException();
    }

    public override DefaultCacheCapabilities DefaultCacheCapabilities
    {
        get
        {
            return
                DefaultCacheCapabilities.OutOfProcessProvider |
                DefaultCacheCapabilities.AbsoluteExpirations |
                DefaultCacheCapabilities.SlidingExpirations |
                DefaultCacheCapabilities.CacheRegions;
        }
    }

    public override object Get(string key, string regionName = null)
    {
        key = key.ToLower();

        return this._client.Get(key);
    }

    public override CacheItem GetCacheItem(string key, string regionName = null)
    {
        object value = Get(key, regionName);
        if (value != null)
            return new CacheItem(key, value, regionName);

        return null;
    }

    public override long GetCount(string regionName = null)
    {
        return -1;
    }

    protected override System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<string, object>> GetEnumerator()
    {
          throw new System.NotImplementedException();
    }

    public override System.Collections.Generic.IDictionary<string, object> GetValues(System.Collections.Generic.IEnumerable<string> keys, string regionName = null)
    {
        throw new System.NotImplementedException();
    }

    public override string Name
    {
        get { return "MemcachedProvider"; }
    }

    public override object Remove(string key, string regionName = null)
    {
        key = key.ToLower();
        return this._client.Remove(key);
    }

    public override void Set(string key, object value, CacheItemPolicy policy, string regionName = null)
    {
        Set(new CacheItem(key, value, regionName), policy);
    }

    #endregion

    #region ICacheBuilder Members

    public ObjectCache GetInstance()
    {
        return this;
    }

    public string DefaultRegionName
    {
        get { throw new NotImplementedException(); }
    }

    #endregion
}

分布式缓存提供者Windows Server AppFabric Caching：

public class AppFabricCacheProvider : ObjectCache, ICacheBuilder
{
    public static DataCache factory = null;
    public static object syncObj = new object();

    public override object AddOrGetExisting(string key, object value, CacheItemPolicy policy, string regionName = null)
    {
        CacheItem item = GetCacheItem(key, regionName);
        if (item == null)
        {
            Set(new CacheItem(key, value, regionName), policy);
            return value;
        }

        return item.Value;
    }

    public override CacheItem AddOrGetExisting(CacheItem value, CacheItemPolicy policy)
    {
        CacheItem item = GetCacheItem(value.Key, value.RegionName);
        if (item == null)
        {
            Set(value, policy);
            return value;
        }

        return item;
    }

    public override object AddOrGetExisting(string key, object value, System.DateTimeOffset absoluteExpiration, string regionName = null)
    {
        CacheItem item = new CacheItem(key, value, regionName);
        CacheItemPolicy policy = new CacheItemPolicy();
        policy.AbsoluteExpiration = absoluteExpiration;

        return AddOrGetExisting(item, policy);
    }

    public override bool Contains(string key, string regionName = null)
    {
        return Get(key, regionName) != null;
    }

    public override CacheEntryChangeMonitor CreateCacheEntryChangeMonitor(System.Collections.Generic.IEnumerable<string> keys, string regionName = null)
    {
        throw new NotImplementedException();
    }

    public override DefaultCacheCapabilities DefaultCacheCapabilities
    {
        get
        {
            return
                DefaultCacheCapabilities.OutOfProcessProvider |
                DefaultCacheCapabilities.AbsoluteExpirations |
                DefaultCacheCapabilities.SlidingExpirations |
                DefaultCacheCapabilities.CacheRegions;
        }
    }

    public override object Get(string key, string regionName = null)
    {
        key = key.ToLower();
        CreateRegionIfNeeded();

        return (regionName == null) ?
            CacheFactory.Get(key) :
            CacheFactory.Get(key, regionName);
    }

    public override CacheItem GetCacheItem(string key, string regionName = null)
    {
        object value = Get(key, regionName);
        if (value != null)
            return new CacheItem(key, value, regionName);

        return null;
    }

    public override long GetCount(string regionName = null)
    {
        if (string.IsNullOrEmpty(regionName))
            throw new NotSupportedException();

        return CacheFactory.GetObjectsInRegion(regionName).LongCount();
    }

    protected override System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<string, object>> GetEnumerator()
    {
        throw new NotSupportedException();
    }

    public override System.Collections.Generic.IDictionary<string, object> GetValues(System.Collections.Generic.IEnumerable<string> keys, string regionName = null)
    {
        if (string.IsNullOrEmpty(regionName))
            throw new NotSupportedException();

        return CacheFactory.GetObjectsInRegion(regionName).ToDictionary(x => x.Key, x => x.Value);
    }

    public override string Name
    {
        get { return "AppFabric"; }
    }

    public override object Remove(string key, string regionName = null)
    {
        key = key.ToLower();
        CreateRegionIfNeeded();

        return (regionName == null) ?
            CacheFactory.Remove(key) :
            CacheFactory.Remove(key, regionName);
    }

    public override void Set(string key, object value, CacheItemPolicy policy, string regionName = null)
    {
        Set(new CacheItem(key, value, regionName), policy);
    }

    public override void Set(CacheItem item, CacheItemPolicy policy)
    {
        if (item == null || item.Value == null)
            return;

        if (policy != null && policy.ChangeMonitors != null && policy.ChangeMonitors.Count > 0)
            throw new NotSupportedException("Change monitors are not supported");

        item.Key = item.Key.ToLower();
        CreateRegionIfNeeded();

        TimeSpan expire = (policy.AbsoluteExpiration.Equals(null)) ?
            policy.SlidingExpiration :
            (policy.AbsoluteExpiration - DateTimeOffset.Now);

        if (string.IsNullOrEmpty(item.RegionName))
            CacheFactory.Put(item.Key, item.Value, expire);
        else
            CacheFactory.Put(item.Key, item.Value, expire, item.RegionName);
    }

    private static DataCache CacheFactory
    {
        get
        {
            if (factory == null)
            {
                lock (syncObj)
                {
                    if (factory == null)
                    {
                        DataCacheFactory cacheFactory = new DataCacheFactory();
                        factory = cacheFactory.GetDefaultCache();
                    }
                }
            }

            return factory;
        }
    }

    private void CreateRegionIfNeeded()
    {
        try
        {
            CacheFactory.CreateRegion(DefaultRegionName);
        }
        catch (DataCacheException ex)
        {
            if (!ex.ErrorCode.Equals(DataCacheErrorCode.RegionAlreadyExists))
                throw ex;
        }
    }

    public override void Set(string key, object value, System.DateTimeOffset absoluteExpiration, string regionName = null)
    {
        CacheItem item = new CacheItem(key, value, regionName);
        CacheItemPolicy policy = new CacheItemPolicy();
        policy.AbsoluteExpiration = absoluteExpiration;

        Set(item, policy);
    }

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

    public ObjectCache GetInstance()
    {
        return this;
    }

    public string DefaultRegionName
    {
        get
        {
            string defaultRegion= FrameworkConfiguationManager.GetConfiguration().GetAppVariable("AppFabricCacheDefaultRegion");
            if (string.IsNullOrEmpty(defaultRegion))
            {
                defaultRegion = "Default";
            }
            return defaultRegion;
        }
    }
}

输出缓存对于改善性能有很大好处，在ASP.NET 4.0中可以自定义输出缓存的策略，比如把输出保存在磁盘中，外部的memcached服务中等等。甚至还可以定义一些高级规则，比如为A页面使用A输出缓存策略来把数据保存于内存中，为B页面使用B输出缓存策略来把数据保存于磁盘中。

代码例子可以参看文章http://www.buraksenyurt.com/post/AspNet-40-Custom-Cache-Provider.aspx，在web.config中配置

<caching>
        <outputCache defaultProvider="AspNetInternalProvider">
          <providers>
            <add name="DiskBasedCacheProvider" type="CustomCaching.DiskCacheProvider,CustomCaching"/>
          </providers>
        </outputCache>
      </caching>

在ASP.NET 4 的默认输出缓存策略中。所有的HTTP响应、所呈现的页面和控件缓存均使用上例所示的默认输出缓存提供程序(其中defaultProvider属性值为AspNetInternalProvider)。通过为defaultProvider指定不同的提供程序。就可以更改web应用程序的默认输出缓存提供程序。

另外，还可以针对每个用户控件和各个请求选择不同的输出缓存提供程序。要为不同的Web用户控件选择不同的输出缓存提供程序，最简便的方法是设置页面或控件指令中新增加的providerName属性，如下面的示例所示：

<%@ OutputCache Duration="60" VaryByParam="None"  providerName="DiskBasedCacheProvider" %>

若要为某个HTTP请求指定不同的输出缓存提供程序，可以覆盖Global.asax文件中新增加的GetOutputCacheProviderName方法，以编程的方式指定要用于特定请求的提供程序。

参看文章：.NET 4.0新增可扩展缓存