自长时间的操作开始以来就很难确定ASP.Net中的数据缓存是否已更改

Question

I am inserting a List into data cache in ASP.Net as shown in sample code below. I am using C# as the programming language.

I am not able to find a solution to following problem:

1. A long database operation which could last from .1 s to 20 s is started.
2. At the end of this operation, I need to determine if the Cache by the name of 'dSummary' has been re-inserted or refreshed from database since the long operation began (ie while the operation was in progress).

The code I use to insert cache item is as below:

      List<Doc> ds = GetDocSummary();
   System.Web.HttpContext.Current.Cache.Insert("dSummary", ds, null, 
           DateTime.Now.AddMinutes(15), System.Web.Caching.Cache.NoSlidingExpiration);

UPDATE 1:

After getting so many replies, I think a possible foolproof approach would be to store a guid string along with the List object in Cache. Then I could simply compare the guid string property of the object stored in Cache before and after the long operation. If they are the same then Cache["dSummary"] has not been re-inserted, else it has been re-inserted. So I would need to use the following extended class of 'Doc' class.

public class DocX : Doc
 { 
          public UniQueIdentifier { get;set; }
          public DocX ( Doc doc, string unqiueIdentifier)
             {
                  this.Doc = doc;
                  this.UniqueIdentifier = unqiueIdentifier;

            }
 }

I would then insert into cache using following code:

       List<Doc> ds = GetDocSummary();
       System.Web.HttpContext.Current.Cache.Insert("dSummary",new DocX( ds,  
                new Guid.Newguid().ToString() ), null, 
           DateTime.Now.AddMinutes(15), System.Web.Caching.Cache.NoSlidingExpiration);

Answer 1

I would just add another Cache property at the time that you add or update dSummary that contains the timestamp that you modified it.

You can record the original value at the start of the long-running operation and compare it to the current value in the cache at the end; if the values are different, the data was updated.

Answer 2

Suppose there are two or more threads that want to obtain docs summary, I guess you don't want all of them to compute the summary (as you said it is a long running operation that is compute intensive). In such case I go with following solution:

//declare lock object for implementing critical section
private static object docSummaryLock = new object();

//this is the body of GetDocSummary method
List<Doc> ds = (List<Doc>) System.Web.HttpContext.Current.Cache.Get("dSummary");
if (ds != null) return ds;
//summary not found in cache, computing the doc summary occurs in critical section
lock (docSummaryLock)
{
    //there is possibility that another thread waited for obtaining the lock and the summary may be already in cache
    ds = (List<Doc>) System.Web.HttpContext.Current.Cache.Get("dSummary");
    if (ds != null) return ds;

    //compute the summary
    ds = GetDocSummary();
    System.Web.HttpContext.Current.Cache.Insert("dSummary", summaries, null, 
    DateTime.Now.AddMinutes(15), System.Web.Caching.Cache.NoSlidingExpiration);
    return ds;
}

Answer 3

Before starting the operation GetDocSummary() you could get the current value from the cache and store that value (or a Hash from this value, if memory might be an issue) in a local variable. After the operation is finished, before adding the data to the cache, compare the local variable with the current cached value.

Adding some sample, based on the comments, using a Tuple :

var cacheEntry = new Tuple<List<Doc>, DateTime>(ds, DateTime.Now);

After executing GetDocSummary() , get your entry from the cache and see if the date falls between starting and ending this operation.