System.Web.Caching.Cache doesn't seem to refresh itself on time ?

Question

I am using System.Web.Caching.Cache in an assembly used by my website. I have set some key expiration (absolute expiration) to be 10 seconds (just for debugging). I have also set a callback upon key removal.

The problem is that I see that the cache is getting refreshed after something like 20 seconds and not 10.

I am using HttpRuntime.Cache for this.

Any suggestion for why is that happening ?

I would like to show a code sample, which can shed more light:

public  void OnUpdate(string key
                      , CacheItemUpdateReason reason
                      , out object expensiveObject
                      , out CacheDependency dependency
                      , out DateTime absoluteExpiration
                      , out TimeSpan slidingExpiration)
{
    using (StreamWriter sw = new StreamWriter(@"C:\temp\foo.txt",true))
    {
        sw.WriteLine("Updated Cache at " + DateTime.UtcNow); 
    }
    expensiveObject = 11;
    dependency = null;
    absoluteExpiration = DateTime.UtcNow.AddSeconds(3);
    slidingExpiration = Cache.NoSlidingExpiration;
}
protected void Page_Load(object sender, EventArgs e)
{
    log.WriteInfo("Updated Cache", MethodBase.GetCurrentMethod());
    Page.Cache.Insert("foo", (object)11, null, DateTime.UtcNow.AddSeconds(10), Cache.NoSlidingExpiration, new CacheItemUpdateCallback(OnUpdate));
}

Here, i used Page.Cache. The update should be every 3 seconds. Actually it is performed every 40 seconds, as the below printout shows:

Updated Cache at 1/28/2011 1:38:20 AM Updated Cache at 1/28/2011 1:38:40 AM Updated Cache at 1/28/2011 1:39:00 AM Updated Cache at 1/28/2011 1:39:20 AM Updated Cache at 1/28/2011 1:39:40 AM Updated Cache at 1/28/2011 1:40:00 AM Updated Cache at 1/28/2011 1:40:20 AM Updated Cache at 1/28/2011 1:40:40 AM Updated Cache at 1/28/2011 1:41:00 AM Updated Cache at 1/28/2011 1:41:20 AM Updated Cache at 1/28/2011 1:41:40 AM Updated Cache at 1/28/2011 1:42:00 AM Updated Cache at 1/28/2011 1:42:20 AM Updated Cache at 1/28/2011 1:42:40 AM

What could be the problem ?

Answer 1

The internal cache expire timer only fires every 20 seconds.

I reflected into System.Web.Caching.CacheExpires

But then found it already is on SO

Changing frequency of ASP.NET cache item expiration?

Answer 2

You may use PCache class which is implemented inside PokeIn library. The very good thing is, there is no limitation regarding to this class on FREE edition. It has many functionalities and far better than System.Web.Caching.Cache class..

Additionally, there is a sample project on the link. You may change TimerInterval down to 6 sec with PCache.