Friday, February 18, 2011

Cache Frequently-Used Data in the Application or Session Objects

The ASP Application and Session objects provide convenient containers for caching data in memory. You can assign data to both Application and Session objects, and this data will remain in memory between HTTP calls. Session data is stored per user, while Application data is shared between all users.

At what point do you load data into the Application or Session? Often, the data is loaded when an Application or Session starts. To load data during Application or Session startup, add appropriate code to Application_OnStart() or Session_OnStart(), respectively. These functions should be located in Global.asa; if they are not, you can add these functions. You

can also load the data when it's first needed. To do this, add some code (or write a reusable script function) to your ASP page that checks for the existence of the data and loads the data if it's not there. This is an example of the classic performance technique known as lazy evaluation-don't calculate something until you know you need it. An example:

Function GetEmploymentStatusList
Dim d
d = Application("EmploymentStatusList")
If d = "" Then
' FetchEmploymentStatusList function (not shown)
' fetches data from DB, returns an Array
d = FetchEmploymentStatusList()
Application("EmploymentStatusList") = d
End If
GetEmploymentStatusList = d
End Function

Similar functions could be written for each chunk of data needed.

In what format should the data be stored? Any variant type can be stored, since all script variables are variants. For instance, you can store strings, integers, or arrays. Often,you'll be storing the contents of an ADO recordset in one of these variable types. To get data out of an ADO recordset, you can manually copy the data into VBScript variables, one field at a time. It's faster and easier to use one of the ADO recordset persistence functions GetRows(), GetString() or Save() (ADO 2.5). here's a function that demonstrates using GetRows() to return an array of recordset data:

' Get Recordset, return as an Array
Function FetchEmploymentStatusList
Dim rs
Set rs = CreateObject("ADODB.Recordset")
rs.Open "select StatusName, StatusID from EmployeeStatus", _
FetchEmploymentStatusList = rs.GetRows() " Return data as an Array
Set rs = Nothing
End Function

When you store data in Application or Session scope, the data will remain there until you programmatically change it, the Session expires, or the Web application is restarted. What if the data needs to be updated? To manually force an update of Application data, you can call into an administrator-access-only ASP page that updates the data. Alternatively, you can automatically refresh your data periodically through a function.

Be aware that caching large arrays in Session or Application objects is not a good idea.Before you can access any element of the array, the semantics of the scripting languages require that a temporary copy of the entire array be made. For example, if you cache a 100,000-element array of strings that maps U.S. zip codes to local weather stations in the

Application object, ASP must first copy all 100,000 weather stations into a temporary array before it can extract just one string. In this case, it would be much better to build a custom component with a custom method to store the weather stations—or to use one of the dictionary components.

No comments:

Post a Comment