Understanding the Event Mechanism (Embedded SQL Guide)
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In InterBase, an event is a message passed by a trigger or a stored procedure to the InterBase event manager to announce the occurrence of a specified condition or action, usually a database change such as an INSERT, UPDATE, or DELETE. Events are passed by triggers or stored procedures only when the transaction under which they occur is committed.
The event manager maintains a list of events posted to it by triggers and stored procedures. It also maintains a list of applications that have registered an interest in events. Each time a new event is posted to it, the event manager notifies interested applications that the event has occurred.
Applications can respond to specific events that might be posted by a trigger or stored procedure, by:
- Indicating an interest in the events to the event manager.
- Waiting for an event notification.
- Determining which event occurred (if an application is waiting for more than one event to occur).
The InterBase event mechanism, then, consists of three parts:
- A trigger or stored procedure that posts an event to the event manager.
- The event manager that maintains an event queue and notifies applications when an event occurs.
- An application that registers interest in the event and waits for it to occur.
A second application that uses the event-posting stored procedure (or that fires the trigger) causes the event manager to notify the waiting application so that it can resume processing.