The event notification mechanism contains server and client components. An event can be posted by MS SQL Server using the server component the xp_messenger extended stored procedure. The extended stored procedure to broadcast event messages to interested client applications. A client application can get messages using client component. This component is the Sql_messenger ActiveX control. The control allows client application to receive messages for specific event. You can easy build much more sophisticated applications or services using these components. Now, in awaiting some critical data or events an application does not need to query SQL Server repeatedly. The SQL Server will notify an application when the data came or event happend.