Software Technologies

Introduction
Storing frequently accessed database data, like lookup lists, in ASP.NET’s cache is a great way to optimize a web application’s performance. But anytime you store database data in ASP.NET’s cache, you have to provide a way to update that cache. This ensures changes made in the database make it into the web application’s cache. This is handled using either notifications or polling.

Notifications are possible when using SQL Server 2005 with notification services and .NET’s SqlDependency object from the System.Data.SqlClient namespace. But notification services aren’t available in SQL Server 2008 or other databases. Even if you are using SQL Server 2005, employing this technology requires more server configuration and therefore complicates deployment.

Polling is often handled by setting an expiration of the cached item at some time interval such as hourly. That way the cache holds the data for an hour and then automatically dumps it. Then when the data is requested and the cache is empty, the data is read from the database and placed back into the cache. This approach works fairly well, but with two drawbacks. First, it requires the cache be refreshed even when it is not needed. Second, it runs on the same thread as the request so some unlucky user has to wait a little longer for their request to return while the data is read from the database and stored in the cache.

The BackgroundWorker object in .NET enables a better solution. When the web application starts, it spins off a background thread, and passes to it the application’s HttpContext object, enabling the thread, access to the application’s cache. When the web application ends, it stops the background thread. The background thread runs a continuous loop of sleeping for some time interval, and then polls the database to see if data has changed during that interval. If data has changed, the cache is refreshed. This solution is highly efficient because it runs on a background thread and refreshes the cache only when needed.