ASP.NET Cache緩衝的使用 載）

轉載http://www.cnblogs.com/leochu2008/articles/1161772.html

 

 ASP.NET Cache是提升系統效能的重要方法，它使用了“最近使用”原則（a least-recently-used algorithm）。在資料庫訪問中經常會用到Cache儲存資料庫資料。

1.緩衝的添加：

Cache的添加方法有Add()或Insert（），兩種方法幾乎類似，只是Inser方法可以使用選擇性參數，即使用預設參數，來實現緩衝的添加：

Cache.Add(

       KeyName,//緩衝名

       KeyValue,//要緩衝的對象

       Dependencies,//依賴項

       AbsoluteExpiration,//絕對到期時間

       SlidingExpiration,//相對到期時間

       Priority,//優先順序

       CacheItemRemovedCallback);//緩衝到期引發事件

2. 緩衝依賴項：

       緩衝可以設定的時效性可以通過 檔案依賴，其他緩衝依賴，資料庫依賴和到期時間方法來設定，當檔案改變，依賴快取項目改變，資料庫改變或時間的到期時，緩衝會失效，並可以引發一定事件。

2.1 檔案依賴：

        CacheDependency fileDepends = new CacheDependency(Server.MapPath("Northwind.xml"));

        Cache.Insert("GridViewDataSet", dsGrid, fileDepends);

此例為通過Northiwind.xml檔案依賴出來緩衝的用法：

2.2 其他快取項目依賴：

string[] fileDependsArray = {Server.MapPath("Northwind.xml")};

string[] cacheDependsArray = {"Depend0", "Depend1", "Depend2"};

CacheDependency cacheDepends = new CacheDependency(fileDependsArray, cacheDependsArray);

Cache.Insert("GridViewDataSet", dsGrid, cacheDepends);

此例設定了Northwind.xml檔案依賴和 Depend0,depend1,Depend2快取項目

其中Depend0,depend1,Depend2為另外三個緩衝。

如果不需要檔案依賴可以設定為NULL。 

2.3 到期時間設定：

AbsoluteExpiration可以設定緩衝的絕對到期時間，如：

Cache.Insert("GridViewDataSet ", dsGrid, null, DateTime.Now.AddMinutes(30), Cache.NoSlidingExpiration);

緩衝會在添加起30分鐘後到期。

NoSlidingExpiration可以設定相對到期時間,如果緩衝在NoSlidingExpiration設定的時間內沒有被訪問，緩衝到期，如果在這段時間內有訪問，則緩衝到期時間將會重設為原始值，如NoSlidingExpiration=20

在20分鐘內如果沒有被訪問，緩衝到期，如果每次19分鐘訪問緩衝，緩衝將永遠不會到期。

Cache.Insert("DataGridDataSet", dsGrid, null,Cache.NoAbsoluteExpiration, TimeSpan.FromSeconds(30));

3. 優先順序：

        Priority屬性值和意義：

Priority value	Description
NotRemovable	Items with this priority will not be evicted.
High	Items with this priority level are the least likely to be evicted.
AboveNormal	Items with this priority level are less likely to be evicted than items assigned Normal priority.
Default	This is equivalent to Normal.
Normal	The default value.
BelowNormal	Items with this priority level are more likely to be evicted than items assigned Normal priority.
Low	Items with this priority level are the most likely to be evicted.

4. 緩衝失效事件處理：

using System;

using System.Data;

using System.Configuration;

using System.Web;

using System.Web.Security;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Web.UI.WebControls.WebParts;

using System.Web.UI.HtmlControls;

using System.Web.Caching;         // necessary for CacheDependency

using System.Xml;                  // necessary for Xml stuff

public partial class _Default : System.Web.UI.Page

{

   public static CacheItemRemovedCallback onRemove = null;

   protected void Page_Load(object sender, EventArgs e)

    {

       CreateGridView( );

    }

    private void CreateGridView( )

    {

       DataSet dsGrid;

       dsGrid = (DataSet)Cache["GridViewDataSet"];

       onRemove = new CacheItemRemovedCallback(this.RemovedCallback);

       if (dsGrid == null)

       {

          dsGrid = GetDataSet( );

          string[] fileDependsArray = {Server.MapPath("Northwind.xml")};

          string[] cacheDependsArray = {"Depend0", "Depend1", "Depend2"};

          CacheDependency cacheDepends = new CacheDependency

                                (fileDependsArray, cacheDependsArray);

          Cache.Insert("GridViewDataSet", dsGrid, cacheDepends,

                        DateTime.Now.AddSeconds(10),

                        Cache.NoSlidingExpiration,

                        CacheItemPriority.Default,

                        onRemove);

          lblMessage.Text = "Data from XML file.";

       }

       else

       {

          lblMessage.Text = "Data from cache.";

       }

       gv.DataSource = dsGrid.Tables[0];

       gv.DataBind( );

    }

    private DataSet GetDataSet( )

    {

       DataSet dsData = new DataSet( );

       XmlDataDocument doc = new XmlDataDocument( );

       doc.DataSet.ReadXml(Server.MapPath("Northwind.xml"));

       dsData = doc.DataSet;

       return dsData;

    }

   public void RemovedCallback(string cacheKey,

                                 Object cacheObject,

                                 CacheItemRemovedReason reasonToRemove)

   {

      WriteFile("Cache removed for following reason: " +

         reasonToRemove.ToString( ));

   }

   private void WriteFile(string strText)

   {

      System.IO.StreamWriter writer = new System.IO.StreamWriter(

                                                   @"C:"test.txt", true);

      string str;

      str = DateTime.Now.ToString( ) + " " + strText;

      writer.WriteLine(str);

      writer.Close( );

   }

   protected void btnClear_Click(object sender, EventArgs e)

    {

      Cache.Remove("GridViewDataSet");

      CreateGridView( );

    }

   protected void btnInit_Click(object sender, EventArgs e)

   {

      // Initialize caches to depend on.

      Cache["Depend0"] = "This is the first dependency.";

      Cache["Depend1"] = "This is the 2nd dependency.";

      Cache["Depend2"] = "This is the 3rd dependency.";

   }

   protected void btnKey0_Click(object sender, EventArgs e)

   {

      Cache["Depend0"] = "This is a changed first dependency.";

   }

}

Table 17-5. Members of the CacheItemRemovedReason enumeration
Reason	Description
DependencyChanged	A file or item key dependency has changed.
Expired	The cached item has expired.
Removed	The cached item has been explicitly removed by the Remove method or replaced by another item with the same key.
Underused	The cached item was removed to free up system memory.