A waiting application is blocked in the database and cannot do any other work. Set the polling interval. You might want to write a cover package on top of this one that restricts the alert names used. Table  16-2 shows the messages, and the procedures that can raise them. The signalling session can optionally pass a message that will be received by the waiting session.
Alerts can be signalled more often than the corresponding application WAIT calls.
In such cases, the older alerts are discarded. It is possible to receive an alert, read the data, and find that no data has changed. This is because the data changed after the prior alert, but before the data was read for that prior alert. The name of the alert is the IN parameter. A session can register interest in an unlimited number of alerts.