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try { cn.Open(); gvTables.DataSource = cm.ExecuteReader(); gvTables.DataBind(); btnSave.Visible = true; } catch { gvTables.Visible = false; btnSave.Visible = false; Button1.Visible = true; } finally { cn.Close(); } } The query in use in this method also does on outer join to the polling table (AspNet_ SqlCacheTablesForChangeNotification). When this table is not present, the execution of the query throws an exception. From this, you infer that the database is not configured for cache dependencies, and display a button to let the user configure the database (see Figure 11-8).
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Figure 11-8. The admin display when the selected database is not configured When the user clicks this button, you configure the database for cache dependencies and render the grid. protected void Button1_Click(object sender, EventArgs e) { string sConn = BuildConnStr(ddlDatabase.SelectedValue); SqlCacheDependencyAdmin.EnableNotifications(sConn); Button1.Visible = false; gvTables.Visible = true; btnSave.Visible = true; BindGrid(); } In the case where the database is configured (and so the polling table is present), the outer join attempts to link the object name with the TableName from the polling table.
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...FROM sysobjects LEFT OUTER JOIN AspNet_SqlCacheTablesForChangeNotification ON sysobjects.name = AspNet_SqlCacheTablesForChangeNotification.tableName If the table name is present in the polling table, it s configured, and the outer join will succeed. If the table is not configured, it won t be present in the polling table, and the column value will be null. The query uses a case and a coalesce to translate these possible values into true or false, which you use to bind the checked value of the CheckBox on the grid. case coalesce (AspNet_SqlCacheTablesForChangeNotification.tableName ,'0') when '0' then 'false' else 'true' end AS Configured <asp:TemplateField HeaderText='Configured'> <ItemTemplate> <asp:checkbox runat='server' TableName='<%# Eval("name") %>' Checked='<%# Convert.ToBoolean(Eval("Configured")) %>' OnCheckedChanged='FlipBit' /> </ItemTemplate> </asp:TemplateField> The TableName attribute is not actually a property of the CheckBox control, but the rendering engine is smart enough to pass this value through into the markup and the ViewState, making it available for your use when the postback occurs. As the user makes changes to these CheckBox controls, the server-side OnCheckChanged events get queued up on the client, until the user clicks the submit button. Then a postback occurs, and the CheckChanged trap fires once for each CheckBox that has had its value altered. After all of the check-changed events are processed, the onclick of the button finally fires. protected void FlipBit(Object sender, EventArgs e) { CheckBox c = (CheckBox)sender; string sConn = BuildConnStr(ddlDatabase.SelectedValue); string tableName = c.Attributes["TableName"]; if (c.Checked) SqlCacheDependencyAdmin.EnableTableForNotifications (sConn, tableName); else SqlCacheDependencyAdmin.DisableTableForNotifications (sConn, tableName);
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} void btnSave_Click(object sender, EventArgs e) { BindGrid(); } In the check-changed trap, you use the TableName attribute and the event sender to determine what table should be enabled or disabled. By the time the button click event fires, the only work left to do is to refresh the grid to ensure it matches the current caching configuration in the database. SQL cache dependencies are a powerful new feature in ASP .NET 2.0. It s one people have been clamoring for for years, and is best suited for read-mostly data, or data where some latency is acceptable between changing the data and seeing the changes on the site. If the data is highly volatile and no latency is acceptable, then there s no sense in caching it, as it will constantly be invalidated as the data changes, and/or frequent polling will be required to get the latest version. For situations where the data is in high demand and it does not change constantly within the database, a caching strategy can increase the performance of your application by several orders of magnitude. SQL Server 2005 has a notification infrastructure, which enables the database to call out to other processes as events occur. This eliminates the need for polling from the cache dependency, and results in less latency between changes and cache invalidations. See the last section of this chapter (on SQL Server 2005) for details.
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