Question

We have a website that runs on two load balanced servers. We use the ASP.Net caching to help improve performance by caching high usage data. BUT, occasionally that data changes. When it does, we need to clear the relevant cache items on BOTH the load balanced servers. Does anyone have some easy to implement suggestions for how this can be done?

I know that there is software out there to manage this for you (Microsoft Velocity for one). I also know that there are other options for having separate state servers etc. However, for what we want they all seem like overkill. Just a simple mechanism for clearing out specific cache items across servers is all we need for now.

Thanks for any suggestions.

Answer 1

We use a simple web service approach. Our cache clear mechanism checks a web config setting to see if any other servers exist and calls the web service on those server(s) asynchronously.

We store data with specific naming convetions to make it easy to clear what we want. So we pass in either a prefix or postfix for the item to be removed as sometimes it might be user specific (ex. the userid is appended to the name of the item) or application specific (ex. the prefix of the item is the application name).

Here is a VB example of the ClearItem routine that would be called on either one of your nodes:

Public Shared Sub ClearItem(ByVal strPrefix As String, ByVal strPostfix As String)      If WebConfig.Caching_Enabled() Then          ' Exit if no criteria specified '         If IsNothing(strPrefix) AndAlso IsNothing(strPostfix) Then             Exit Sub         End If          ' At the very least we need a Postfix '         If Not IsNothing(strPostfix) AndAlso Not strPostfix.Length.Equals(0) Then             _ClearItem(strPrefix, strPostfix)         End If          If WebConfig.Caching_WebFarmEnabled() Then             ' Now clear the cache across the rest of the server farm '             _ClearItem_WebFarm(strPrefix, strPostfix)         End If      End If  End Sub  Private Shared Sub _ClearItem_WebFarm(ByVal strPrefix As String, ByVal strPostfix As String)      If WebConfig.Caching_WebFarmEnabled() Then          ' Use a web service on each server in the farm to clear the '         ' requested item from the Cache '          ' Determine which servers need to remove cache items '         Dim arrServers As String()         arrServers = Split(WebConfig.Caching_WebFarmServers(), '|')          Dim strServer As String ' Holds which server we are currently contacting '           ' Loop through all the servers and call their web services '         For Each strServer In arrServers              Dim WS As New WebServiceAsyncCall             WS.StartCallBack(strServer, strPrefix, strPostfix)          Next      End If  End Sub  Private Shared Sub _ClearItem(ByVal strPrefix As String, ByVal strPostfix As String)      If WebConfig.Caching_Enabled() Then          ' Determine how we are comparing keys '         Dim blnPrefix, blnPostfix As Boolean          If strPrefix.Length.Equals(0) Then             blnPrefix = False         Else             blnPrefix = True         End If          If strPostfix.Length.Equals(0) Then             blnPostfix = False         Else             blnPostfix = True         End If          ' Reference the Cache collection '         Dim objCache As System.Web.Caching.Cache = HttpContext.Current.Cache          ' Exit if the cache is empty '         If objCache.Count.Equals(0) Then             Exit Sub         End If          ' Clear out the cache for all items matching the input(s) (on this local server) '         Dim objCacheEnum As IEnumerator = objCache.GetEnumerator()         Dim objCacheItem As Object         Dim objCurrentKey As System.Collections.DictionaryEntry         Dim strCurrentKey As String          ' Enumerate through the cache '         While objCacheEnum.MoveNext()              objCurrentKey = CType(objCacheEnum.Current, DictionaryEntry)             strCurrentKey = objCurrentKey.Key.ToString()              ' How are we comparing the key? '             If blnPrefix AndAlso Not (blnPostfix) Then ' Only by PREFIX '                  If strCurrentKey.StartsWith(strPrefix) Then                     ' Remove it from the cache '                     objCacheItem = objCache.Remove(strCurrentKey) ' Returns a reference to the item '                     objCacheItem = Nothing ' Need to explicitly nuke this because the objCache.Remove() above doesn t destroy '                 End If              ElseIf Not (blnPrefix) AndAlso blnPostfix Then ' Only by POSTFIX '                  If strCurrentKey.EndsWith(strPostfix) Then                     ' Remove it from the cache '                     objCacheItem = objCache.Remove(strCurrentKey) ' Returns a reference to the item '                     objCacheItem = Nothing ' Need to explicitly nuke this because the objCache.Remove() above doesn t destroy '                 End If              ElseIf blnPrefix AndAlso blnPostfix Then ' By both PREFIX and POSTFIX'                  If strCurrentKey.StartsWith(strPrefix) AndAlso strCurrentKey.EndsWith(strPostfix) Then                     ' Remove it from the cache '                     objCacheItem = objCache.Remove(strCurrentKey) ' Returns a reference to the item '                     objCacheItem = Nothing ' Need to explicitly nuke this because the objCache.Remove() above doesn t destroy '                 End If              Else                 ' Not comparing prefix OR postfix? Why bother continuing then! '                 Exit Sub             End If          End While      End If  End Sub 

You can see that the code above calls other server(s) by using this helper class:

Private Class WebServiceAsyncCall      Public Sub StartCallBack(ByVal strService As String, ByVal strPrefix As String, ByVal strPostfix As String)          ActiveWebServiceCounter += 1          Dim clearCacheProxy As New CacheClearService.CacheClear ' This is the web service which of course will exist on the other node as well '         clearCacheProxy.Url = strService          AddHandler clearCacheProxy.ClearItemCompleted, AddressOf DoneCallBack          clearCacheProxy.ClearItemAsync(strPrefix, strPostfix)      End Sub      Public Sub DoneCallBack(ByVal sender As Object, ByVal e As CacheClearService.ClearItemCompletedEventArgs)          ActiveWebServiceCounter -= 1          If e.Result.Length > 0 Then ' Something failed '             ' Log the error '         End If      End Sub  End Class 

The web service on the remote server then calls the same code that the _ClearItem called.