问题描述

我正在使用 SQLDependency 类探索查询通知.构建一个简单的工作示例很容易，但我觉得我错过了一些东西.一旦我跳过了一个简单的单表/单依赖示例，我就想知道如何确定哪个依赖触发了我的回调?

I'm exploring query notifications with the SQLDependency class. Building a simple working example is easy, but I feel like I'm missing something. Once I step past a simple one-table/one-dependency example I'm left wondering how can I figure out which dependency triggered my callback?

我在解释时遇到了一些麻烦，所以我在下面提供了一个简单的示例.当调用 AChange() 时，我无法查看依赖项中的 sql，并且我没有对关联缓存对象的引用.

I'm having a bit of trouble explaining, so I included the simple example below. When AChange() is called I cannot look at the sql inside the dependency, and i don't have a reference to the associated cache object.

那么男孩该怎么办?

• 选项 1 - 为我要跟踪的每个对象创建一个不同的函数，并在回调中硬编码缓存键(或相关信息).这感觉很脏&消除了在不部署新代码的情况下添加新缓存项的可能性——ewww.
• 选项 2 - 使用 Dependency Id 属性和并行跟踪结构

• Option 1 - create a distinct function for each object i want to track and hard code the cache-key (or relevant information) in the callback. This feels dirty & eliminates the posibility of adding new cache items without deploying new code--ewww.
• Option 2 - Use the Dependency Id property and a parallel tracking structure

我只是错过了什么吗?这是 SQLDependency 结构的缺陷吗?我已经查看了 20 篇关于该主题的不同文章，所有文章似乎都有相同的漏洞.有什么建议吗?

Am I just missing something? Is this a deficiency in the SQLDependency structure? I've I've looked at 20 different articles on the topic and all of them seem to have the same hole. Suggestions?

代码示例

public class DependencyCache{
   public static  string       cacheName  = "Client1";
   public static  MemoryCache  memCache   = new MemoryCache(cacheName);

   public DependencyCache() {
      SqlDependency.Start(connString);
   }

   private static string GetSQL() {
      return "select  someString FROM dbo.TestTable";
   }

   public void DoTest() {
      if (memCache["TEST_KEY"] != null ) {
         Debug.WriteLine("resources found in cache");
         return;
      }
      Cache_GetData();
   }

   private void Cache_GetData() {
      SqlConnection         oConn;
      SqlCommand            oCmd;
      SqlDependency         oDep;
      SqlDataReader         oRS;
      List<string>          stuff    = new List<string>();
      CacheItemPolicy       policy   = new CacheItemPolicy();

      SqlDependency.Start(connString);
      using (oConn = new SqlConnection(connString) ) {
         using (oCmd = new SqlCommand(GetSQL(), oConn) ) {
            oDep = new SqlDependency(oCmd);
            oConn.Open();
            oRS = oCmd.ExecuteReader();

            while(oRS.Read() ) {
                  resources.Add( oRS.GetString(0) );
            }

            oDep.OnChange += new OnChangeEventHandler (AChange);
         }
      }
      memCache.Set("TEST_KEY", stuff, policy);
   }

   private void AChange(  object sender, SqlNotificationEventArgs e) {
      string msg= "Dependency Change 
INFO: {0} : SOURCE {1} :TYPE: {2}";
      Debug.WriteLine(String.Format(msg, e.Info, e.Source, e.Type));

      // If multiple queries use this as a callback how can i figure 
      // out WHAT QUERY TRIGGERED the change?
      // I can't figure out how to tell multiple dependency objects apart

      ((SqlDependency)sender).OnChange -= Cache_SqlDependency_OnChange; 
      Cache_GetData(); //reload data
   }
}

推荐答案

首先:必须在在命令执行之前设置处理程序:

First and foremost: the handler has to be set up before the command is executed:

 oDep = new SqlDependency(oCmd);
 oConn.Open();
 oDep.OnChange += new OnChangeEventHandler (AChange);
 oRS = oCmd.ExecuteReader();
 while(oRS.Read() ) {
     resources.Add( oRS.GetString(0) );
 }

否则，当通知可能丢失并且您的回调永远不会被调用时，您会有一个窗口.

Otherwise you have a window when the notification may be lost and your callback never invoked.

现在关于您的问题:您应该为每个查询使用单独的回调.虽然这可能看起来很麻烦，但使用 lambda 实际上是微不足道的.类似于以下内容:

Now about your question: you should use a separate callback for each query. While this may seem cumbersome, is actually trivial by using a lambda. Something like the following:

oDep = new SqlDependency(oCmd);
oConn.Open();
oDep.OnChange += (sender, e) =>
{
   string msg = "Dependency Change 
INFO: {0} : SOURCE {1} :TYPE: {2}";
   Debug.WriteLine(String.Format(msg, e.Info, e.Source, e.Type));

   // The command that trigger the notification is captured in the context:
   //  is oCmd
   //
   // You can now call a handler passing in the relevant info:
   //
   Reload_Data(oCmd, ...);
};
oRS = oCmd.ExecuteReader();
...

记得始终检查通知来源、信息和类型.否则，当您因数据更改以外的原因(如无效查询) 收到通知时，您可能会冒着头晕目眩的风险.作为旁注，我要补充一点，一个好的缓存设计不会在失效时刷新缓存，而只是使缓存的项目失效，并让 下一个请求 实际上获取一个新的项目.使用您的主动"方法，即使不需要，您也可以刷新缓存的项目，在访问它们之前多次刷新等等.我忽略了示例错误处理和正确的线程同步(两者都是必需的).

And remember to always check the notification source, info and type. Otherwise you run the risk of spinning ad-nauseam when you are notified for reasons other than data change, like invalid query. As a side comment I would add that a good cache design does not refresh the cache on invalidation, but simply invalidates the cached item and lets the next request actually fetch a fresh item. With your 'proactive' approach you are refreshing cached items even when not needed, refresh multiple times before they are accessed etc etc. I left out from the example error handling and proper thread synchronization (both required).

最后，看看 LinqtoCache 这几乎可以满足您的要求做，但对于 LINQ 查询.

Finally, have a look at LinqtoCache which does pretty much what you're trying to do, but for LINQ queries.

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