One of the nicest features of ASP.NET is the ability to cache page content. This can be used to substantially reduce load on a website's database - which is an obvious attraction if the site uses Microsoft's Access to store data rather than SQL Server.

Unfortunately there is no built in caching system in classic ASP, but it is easy to build one by using the Application object to store data. As such the techniques described in this article can be used to bring useful performance enhancements to legacy websites where upgrading the database or porting the code to ASP.NET is not a viable option.

An Example of ASP Data Caching

In this example, an e-commerce website contains a list of popular products visible in a box displayed on all of the pages within the site. Since the website only takes 2 - 3 orders a day, the list of popular products does not need to be constantly updated. There are three application variables required in the Application_OnStart subroutine of the global.asa file:

Application("CachingDuration") = 120 
Application("PopularProductsHTML") = "" 
Application("PopularProductsHTML.LastModified") = Now()

The CachingDuration application variable specifies the number of minutes that the website's cached content should be stored for. The PopularProductsHTML application variable stores the actual HTML for the popular products table. Finally, the PopularProductsHTML.LastModified application variable stores the date and time the popular products HTML was last retrieved from the database. The ASP code for retrieving the popular products from the database is containing within the function GetPopularProductsHTML. There is a single argument - the number of popular products to return:

<% 
Function GetPopularProductsHTML(NumberOfProducts) 


    On Error Resume Next 
    Dim DataConnection 
    Dim RecordSet 
    Dim SQL 
    Dim ReturnString 
    Dim ProductName 
    Dim NumberOfProductsDisplayed 
     
    'Access Query for returning popular products 
    SQL = "sp_GetPopularProducts" 


    Set DataConnection = Server.CreateObject("ADODB.connection") 
    DataConnection.Open Application("ConnectionString") 
     
    Set RecordSet = Server.CreateObject("ADODB.Recordset") 


    RecordSet.Open SQL, DataConnection 
     
    NumberOfProductsDisplayed = 0 
     
    Do While Not RecordSet.EOF 
     
            If NumberOfProductsDisplayed < NumberOfProducts Then 


                ProductName = Replace(RecordSet("ProductName"), Chr(34), "'") 
                ReturnString = ReturnString & "" & ProductName & "
" 


            End If 
         
            NumberOfProductsDisplayed = NumberOfProductsDisplayed + 1 
     
        RecordSet.MoveNext 
         
    Loop 
     
    RecordSet.Close 
    Set RecordSet = nothing 
    DataConnection.Close 
    Set DataConnection = nothing 
     
    GetPopularProductsHTML = ReturnString 


End Function 
%> 

This function is called from the ASP VBScript shown below that builds the popular products table. This code also checks that the cached HTML has not expired; if it has expired then the data is retrieved from the database.

<% 
Dim PopularProductsBoxTitleText 
Dim PopularProductsBoxContentsText 
Dim NumberOfProducts 


PopularProductsBoxTitleText = "Popular Products" 
NumberOfProducts = 25 


'If cache is empty then retrieve data from the database 
If Application("PopularProductsHTML") = "" Then 


    PopularProductsBoxContentsText = GetPopularProductsHTML(NumberOfProducts) 
     
    Application.Lock() 
    Application("PopularProductsHTML") = PopularProductsBoxContentsText 
    Application("PopularProductsHTML.LastModified") = Now() 
    Application.UnLock() 
     
'Retrieve content directly from the cache instead of the database 
Else 
    PopularProductsBoxContentsText = Application("PopularProductsHTML") 
     
    'Is the cache out of date? If so retrieve the content from the database and store in the cache 
    'The VBScript DateDiff function can be used to compare dates (n = comparisons of minutes) 
    If DateDiff("n", Application("PopularProductsHTML.LastModified"), Now()) > Application("CachingDuration") Then 
        Response.Write("") 'Useful debugging line - remove from production code 
        PopularProductsBoxContentsText = GetPopularProductsHTML(NumberOfProducts) 


        'Store the content in the Application variables. 
        Application.Lock() 
            Application("PopularProductsHTML") = PopularProductsBoxContentsText 
            Application("PopularProductsHTML.LastModified") = Now() 
        Application.UnLock() 


    End If     


End If 


'Display the popular products box 
If PopularProductsBoxContentsText <> "" Then 
%> 
    
 
        
 
            <%=PopularProductsBoxTitleText%> 
        
 
        
 
             
                <%=PopularProductsBoxContentsText%> 
             
        
 
    
 
<% 
End If 
%> 

Don't forget that content stored in Application variables are shared by all users of the site, so if updating Application variables the Lock() method should be called to prevent more than one client from modifying the same Application variable simultaneously. Remember to use the Unlock() method afterwards!