The event notification mechanism includes server and client components. An event can be posted by using the server component. The server component broadcasts event messages to interested client applications. An application can get messages using a 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 Database Server repeatedly. The Database Server will notify an application when the data came or event happened.