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CHAPTER 12 CACHING AND PERFORMANCE TUNING
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con.Open() ds = New DataSet Try ' Proceed to fill the dataset adExisting.Fill(ds, "Existing") 'adPlaces.Fill(ds, "Places") adPlaceTypes.Fill(ds, "Types") Finally con.Close() End Try End Sub Note that you ve deleted only the code that creates and initializes the SqlDataAdapter component for the Places data, and the code that fills the dataset with its contents. 2. You need to create a dataset for the Places data somewhere. You ll implement this functionally in a method called GetPlacesDataSet(). You ll create this method as a Protected method of the FriendsBase class, so that it will be available to AssignPlaces.aspx and any other page that needs to use the cached data. So, open FriendsBase.vb and add the following method to the FriendsBase class: Protected Function GetPlacesDataSet() As DataSet ' If it's already cached, return it Dim ds As DataSet = CType(Cache("Places"), DataSet) If Not (ds Is Nothing) Then Return ds End If ' Generate the new dataset Dim con As New SqlConnection( _ ConfigurationSettings.AppSettings("cnFriends.ConnectionString")) Dim adPlaces As SqlDataAdapter adPlaces = New SqlDataAdapter("SELECT * FROM Place ORDER BY TypeID", con) adPlaces.Fill(ds, "Places") ' Reset the dependency flag. Cache("PlacesChanged") = False ' Create a dependency based on the "PlacesChanged" cache key Dim dependencyKeys() As String = {"PlacesChanged"} Dim dependency As New CacheDependency(Nothing, dependencyKeys) ' Insert the dataset into the cache, ' with a dependency to the "PlacesChanged" key Cache.Insert("Places", ds, dependency) Return ds End Function
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CHAPTER 12 CACHING AND PERFORMANCE TUNING
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3. Import the following namespaces at the top of the FriendsBase.vb file: Imports Imports Imports Imports System.Data System.Data.SqlClient System.Configuration System.Web.Caching
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4. Now return to AssignPlaces.aspx.vb, and modify the InitForm() method so that it uses the cached dataset when populating the drop-down list: Private Sub InitForm() ' Initialize combo box If Not Page.IsPostBack Then ' Retrieve the dataset. ' If it's not already cached, ' it will be generated automatically and cached. Dim cachedDs As DataSet = MyBase.GetPlacesDataSet() ' Access the table by index Dim row As DataRow For Each row In ds.Tables(0).Rows ... Next End If End Sub 5. Finally, you need to invalidate the cached dataset when someone uses ViewPlace.aspx to edit the places-related information in the database. You must do this to ensure that no part of the application (such as AssignPlaces.aspx) uses outdated data. Open ViewPlace.aspx.vb and add the following lines to the end of the dlPlaces_UpdateCommand method: Private Sub dlPlaces_UpdateCommand(ByVal source As Object, _ ByVal e As System.Web.UI.WebControls.DataListCommandEventArgs) _ Handles dlPlaces.UpdateCommand ...etc... ' Invalidate the cached dataset Cache("PlacesChanged") = True End Sub This will ensure that, whenever the list of places is modified, the cached dataset will expire.
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How It Works The lines that you removed from the LoadDataSet() method are the lines that created, initialized, and filled the old dataset with its contents. After these changes, those lines of code reemerge in the GetPlacesDataSet() method, which is the place that now has the responsibility for setting up the new cached dataset:
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CHAPTER 12 CACHING AND PERFORMANCE TUNING
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Protected Function GetPlacesDataSet() As DataSet ' If it's already cached, return it Dim ds As DataSet = CType(Cache("Places"), DataSet) If Not (ds Is Nothing) Then Return ds End If ' Generate the new dataset Dim con As New SqlConnection( _ ConfigurationSettings.AppSettings("cnFriends.ConnectionString")) Dim adPlaces As SqlDataAdapter adPlaces = New SqlDataAdapter("SELECT * FROM Place ORDER BY TypeID", con) adPlaces.Fill(ds, "Places") ... Note that the method first checks for the existence of the DataSet, returning it if it s already cached. In the same method, you then create a dummy cache entry called PlacesChanged, which you will use for dependency purposes (essentially it acts like a flag; you change its value to True whenever the data in the database is changed, as you ll see in a moment): Cache("PlacesChanged") = True Then you create a dependency to tie the life of your dataset to the modification of the dummy key: Dim dependencyKeys() As String = {"PlacesChanged"} Dim dependency As New CacheDependency(Nothing, dependencyKeys) Finally, you use the Insert() method to insert the dataset in the cache. The Insert() method works in a similar way to the Add() method we discussed earlier, but also allows you to overwrite existing cache entries: Cache.Insert("Places", ds, dependency) You made just two changes to the InitForm() method, which is the one that populates the drop-down list. First, you added code to retrieve the cached dataset using the GetPlacesDataSet() method: Dim cachedDs As DataSet = MyBase.GetPlacesDataSet() Second, you changed the next bit of code so that it uses the cached dataset, rather than a freshly generated one as before: For Each row In ds.Tables(0).Rows ... The cached dataset should be regenerated when the database is updated (rendering the existing cached dataset outdated). So, you ve added a line of code to ViewPlace.aspx, to flag
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