Regarding how sql cache dependency works

Question

here i go through a article on sql cache dependency from this site http://www.dotnetcurry.com/showarticle.aspx?ID=263 . specially a routine was called which cache the table data first time and when underlying table data will be changed then routine will load data again from table. i just do not understand when data change in db then how automatically cache will be null or invalidated. so here i am pasting that routine where i just do not understand how cache data is getting invalidated when data change occur in db?

just see the routine and discuss this point : how cache data is getting invalidated when data change occur in db

public static class MyExtensions
{
    public static List<T> LinqCache<T>(this Table<T> query) where T : class
    {
        string tableName = query.Context.Mapping.GetTable(typeof(T)).TableName;
        List<T> result = HttpContext.Current.Cache[tableName] as List<T>;

        if (result == null)
        {
            using (SqlConnection cn = new SqlConnection(query.Context.Connection.ConnectionString))
            {
                cn.Open();
                SqlCommand cmd = new SqlCommand(query.Context.GetCommand(query).CommandText, cn);
                cmd.Notification = null;
                cmd.NotificationAutoEnlist = true;
                                        SqlCacheDependencyAdmin.EnableNotifications(query.Context.Connection.ConnectionString);
                if (!SqlCacheDependencyAdmin.GetTablesEnabledForNotifications(query.Context.Connection.ConnectionString).Contains(tableName))
                {
                        SqlCacheDependencyAdmin.EnableTableForNotifications(query.Context.Connection.ConnectionString, tableName);
                }                   

                SqlCacheDependency dependency = new SqlCacheDependency(cmd);
                cmd.ExecuteNonQuery();

                result = query.ToList();
                HttpContext.Current.Cache.Insert(tableName, result, dependency);
            }
        }
        return result;
    }
}

who will create this table AspNet_SqlCacheTablesForChangeNotification ? what is the importance of this table AspNet_SqlCacheTablesForChangeNotification ? suppose data change in the my employee table then what will happen in this table AspNet_SqlCacheTablesForChangeNotification

please discuss all my points as a result my all doubts will be clear about how sql dependency works and how automatically cache will be invalidated ?

thanks

Answer 1

Since SQL Server 2005 the database implements notification mechanism to inform applications about changes - http://en.wikipedia.org/wiki/SQL_Server_Notification_Services

Before that they simply polled the database for changes periodically.

More information here - http://www.asp.net/web-forms/tutorials/data-access/caching-data/using-sql-cache-dependencies-cs