20. Writing 4GL Statements : How Database Events Are Handled
 
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How Database Events Are Handled
4GL database event statements allow an application to notify other applications that a specific event has occurred. The other application then can use that information to perform some action that you designate.
The sequence for handling a database event is:
1. A sending application or database procedure raises the event.
2. The Ingres DBMS Server notifies other applications, called receiving applications, that the event has occurred.
3. A receiving application takes some action based upon the event. (The receiving application can be the same as the sending application.) The receiving application must:
Be registered to receive notification when the event is raised
The server performs this notification by placing the event into an event queue within the receiving application.
Extract the event from the queue
Retrieve any desired information about the event
You can designate any program code within an application as a database event. For example, within an inventory control application, you can raise an event when an update causes the supply of a part to fall below a specified amount. A second application then is notified to reorder the part.