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Oracle provides several packages that allow you to accomplish a wide range of tasks, from interprocess communication to file I/O to dynamically creating and executing SQL statements inside a PL/SQL block. All of these packages are owned by the SYS userone of the two users that exist when Oracle is first installed. The most important of these packages include:
This chapter discusses these packages in detail and describes some common uses for each package.
The DBMS_Alert package is used to implement synchronous, event-driven interprocess communication via signals. This package is often used in conjunction with database triggers in systems that process data based on individual transactions.
The basic processing of an implementation using signals is fairly simple, as you can see in the following steps:
The importance of the COMMIT in this scheme cant be overlooked. A COMMIT must be issued for the signal to be sent.
The processes that receive the signal must first register for the signal by calling the DBMS_Alert.Register() procedure.
The DBMS_Alert package contains the following procedures:
These procedures and the parameters needed to call each procedure are explained in the following sections.
The Register() procedure is used by a PL/SQL block to indicate that it would like to receive a particular signal. This procedure accepts a single parameter, as follows:
PROCEDURE Register (name IN varchar2)
The name parameter is the name of the signal for which the PL/SQL block is registering. This parameter must have a length of 30 characters or less.
The Remove() procedure is used by a PL/SQL block when receiving a registered signal is no longer appropriate. This procedure accepts a single parameter, as follows:
PROCEDURE Remove (name IN varchar2)
The name parameter is the name of the signal for which the PL/SQL block has no further need.
The RemoveAll() procedure is used by a PL/SQL block when no further signals should be received. This procedure does not accept any parameters and appears as follows:
The Set_Defaults() procedure is used by a PL/SQL block to determine the time that will pass between checks to see if a signal has occurred. This procedure accepts a single parameter, as follows:
PROCEDURE Set_Defaults (sensitivity IN number)
The sensitivity parameter indicates the number of seconds that should pass between checks for the signal. This value can be specified to a precision of hundredths of a second (two decimal positions).
The Signal() procedure is used by a PL/SQL block to send a signal. The procedure accepts two parameters:
PROCEDURE Signal (name IN varchar2, message IN varchar2)
The name parameter is the name of a specific signal. The message parameter is a string of text that is received by all objects that receive the specific signal.
Its worth noting that consecutive signals will overwrite the message from a previous signal. Thus, unless you can guarantee that a signal will be processed immediately, it is unwise to pass data to a routine via the message parameter because the data could be overwritten by a later alert. Figure 9.1 illustrates how this could happen.
Figure 9.1 Conflicting alerts.
If you need to pass data when an event occurs, you should consider the use of the DBMS_Pipe package instead of the DBMS_Alert package (more about DBMS_Pipe later in this chapter). Alternately, you can mark data that has been affected by an event so that the code on the receiving end of the signal can identify rows that need to be processed.
The WaitAny() procedure is used by a PL/SQL block to wait for any of its registered signals to occur. The procedure has four parameters and appears as follows:
PROCEDURE WaitAny (name OUT varchar2, message OUT varchar2, status OUT integer, timeout IN number DEFAULT MAXWAIT)
The name parameter for this procedure is the name of the signal that has occurred while waiting for an event. Its possible for a signal to have already occurred at the time of the call to the WaitAny() procedure. If this is the case, the procedure will return the name of the first signal that is found to have occurred.
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