asp.net cache multithreading locks webparts

问题:

I have following scenario:

Lets say we have two different webparts operating on the same data - one is a pie chart, another is a data table. in their Page_Load they asynchronously load data from the database and when loaded place it in application cache for further use or use by other web parts. So each o the web parts has code similar to this:

protected void Page_Load(object sender, EventArgs e)
    { 
       if (Cache["dtMine" + "_" + Session["UserID"].ToString()]==null)
       {
         ...
         Page.RegisterAsyncTask(new PageAsyncTask(
             new BeginEventHandler(BeginGetByUserID),
             new EndEventHandler(EndGetByUserID), 
             null, args, true));
       }
      else 
       {
         get data from cache and bind to controls of the webpart
       }
    }

Since both webparts operate on the same data it does not make sense for me to execute the code twice.

What is the best approach to have one web part communicate to the other "i am already fetching data so just wait until i place it in cache"?

I have been considering mutex, lock, assigning temporary value to the cache item and waiting until that temporary value changes... many options - which one should I use.

回答1:

You will want to take advantage of the lock keyword to make sure that the data is loaded and added to the cache in an atomic manner.

Update:

I modified the example to hold the lock accessing Cache for as short as possible. Instead of storing the data directly in the cache a proxy will be stored instead. The proxy will be created and added to the cache in an atomic manner. The proxy will then use its own locking to make sure that the data is only loaded once.

protected void Page_Load(object sender, EventArgs e)
{ 
   string key = "dtMine" + "_" + Session["UserID"].ToString();

   DataProxy proxy = null;

   lock (Cache)
   {
     proxy = Cache[key];
     if (proxy == null)
     {
       proxy = new DataProxy();
       Cache[key] = proxy;
     }
   }

   object data = proxy.GetData();
}

private class DataProxy
{
  private object data = null;

  public object GetData()
  {
    lock (this)
    {
      if (data == null)
      {
        data = LoadData(); // This is what actually loads the data.
      }
      return data;
    }
  }
}

回答2:

Why don't you load the data and put it in the cache in the Application_Start in Global.asax, then no lock will be needed since locking Cache is a serious thing.

回答3:

You could use a mutex around the test Cache["key"]==null:

The first thread acquires the lock, tests and sees that there's nothing in the cache and goes off to fetch the data. The second thread has to wait for the first to release the mutex. Once the second thread gets in the mutex, it tests, sees the data is there and then continues.

But this would lock up the thread that is running the Page_Load() method - probably a bad thing.

Perhaps a better solution would be to also test if the PageAsyncTask to fetch the data has been started? If not, start it. If so, you shouldn't start another so you may want to register your own event handler to catch when it completes...