20 DBMS_ALERT

Overview

Suppose a graphics tool is displaying a graph of some data from a database table. The graphics tool can, after reading and graphing the data, wait on a database alert (WAITONE) covering the data just read. The tool automatically wakes up when the data is changed by any other user. All that is required is that a trigger be placed on the database table, which performs a signal (SIGNAL) whenever the trigger is fired.

Security Model

Security on this package can be controlled by granting EXECUTE on this package to selected users or roles. You might want to write a cover package on top of this one that restricts the alert names used. EXECUTE privilege on this cover package can then be granted rather than on this package.

Constants

The DBMS_ALERT package uses the constants shown in Table 20-1:

Restrictions

Because database alerters issue commits, they cannot be used with Oracle Forms. For more information on restrictions on calling stored procedures while Oracle Forms is active, refer to your Oracle Forms documentation.

Exceptions

DBMS_ALERT raises the application error -20000 on error conditions. Table 20-2 shows the messages and the procedures that can raise them.

Operational Notes

The following notes relate to general and specific applications:

• Alerts are transaction-based. This means that the waiting session is not alerted until the transaction signalling the alert commits. There can be any number of concurrent signalers of a given alert, and there can be any number of concurrent waiters on a given alert.

• A waiting application is blocked in the database and cannot do any other work.

• An application can register for multiple events and can then wait for any of them to occur using the WAITANY procedure.

• An application can also supply an optional timeout parameter to the WAITONE or WAITANY procedures. A timeout of 0 returns immediately if there is no pending alert.

• The signalling session can optionally pass a message that is 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. The application always gets the latest alert (based on transaction commit times).

• If the application does not require transaction-based alerts, the DBMS_PIPE package may provide a useful alternative.

  See Also:

  Chapter 112, "DBMS_PIPE"

• If the transaction is rolled back after the call to SIGNAL, no alert occurs.

• 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.

• Usually, Oracle is event-driven; this means that there are no polling loops. There are two cases where polling loops can occur:

  • Shared mode. If your database is running in shared mode, a polling loop is required to check for alerts from another instance. The polling loop defaults to one second and can be set by the SET_DEFAULTS procedure.

  • WAITANY procedure. If you use the WAITANY procedure, and if a signalling session does a signal but does not commit within one second of the signal, a polling loop is required so that this uncommitted alert does not camouflage other alerts. The polling loop begins at a one second interval and exponentially backs off to 30-second intervals.

Examples

Suppose you want to graph average salaries by department, for all employees. Your application needs to know whenever EMP is changed. Your application would look similar to this code:

DBMS_ALERT.REGISTER('emp_table_alert');
    <<readagain>>: 
   /* ... read the emp table and graph it */ 
      DBMS_ALERT.WAITONE('emp_table_alert', :message, :status); 
      if status = 0 then goto <<readagain>>; else 
      /* ... error condition */ 

The EMP table would have a trigger similar to this:

CREATE TRIGGER emptrig AFTER INSERT OR UPDATE OR DELETE ON emp
    BEGIN 
      DBMS_ALERT.SIGNAL('emp_table_alert', 'message_text'); 
   END;

When the application is no longer interested in the alert, it makes this request:

DBMS_ALERT.REMOVE('emp_table_alert');

This reduces the amount of work required by the alert signaller. If a session exits (or dies) while registered alerts exist, the alerts are eventually cleaned up by future users of this package.

The example guarantees that the application always sees the latest data, although it may not see every intermediate value.

REGISTER Procedure

This procedure lets a session register interest in an alert.

DBMS_ALERT.REGISTER (
   name      IN  VARCHAR2,
   cleanup   IN  BOOLEAN DEFAULT TRUE);

REMOVE Procedure

This procedure enables a session that is no longer interested in an alert to remove that alert from its registration list. Removing an alert reduces the amount of work done by signalers of the alert.

DBMS_ALERT.REMOVE (
   name  IN  VARCHAR2);

REMOVEALL Procedure

This procedure removes all alerts for this session from the registration list. You should do this when the session is no longer interested in any alerts.

This procedure is called automatically upon first reference to this package during a session. Therefore, no alerts from prior sessions which may have terminated abnormally can affect this session.

This procedure always performs a commit.

DBMS_ALERT.REMOVEALL;

SET_DEFAULTS Procedure

In case a polling loop is required, use the SET_DEFAULTS procedure to set the polling interval.

DBMS_ALERT.SET_DEFAULTS (
   sensitivity  IN  NUMBER);

SIGNAL Procedure

This procedure signals an alert. The effect of the SIGNAL call only occurs when the transaction in which it is made commits. If the transaction rolls back, SIGNAL has no effect.

All sessions that have registered interest in this alert are notified. If the interested sessions are currently waiting, they are awakened. If the interested sessions are not currently waiting, they are notified the next time they do a wait call.

Multiple sessions can concurrently perform signals on the same alert. Each session, as it signals the alert, blocks all other concurrent sessions until it commits. This has the effect of serializing the transactions.

DBMS_ALERT.SIGNAL (
   name     IN  VARCHAR2,
   message  IN  VARCHAR2);

WAITANY Procedure

Call this procedure to wait for an alert to occur for any of the alerts for which the current session is registered.

DBMS_ALERT.WAITANY (
   name      OUT  VARCHAR2,
   message   OUT  VARCHAR2,
   status    OUT  INTEGER,
   timeout   IN   NUMBER DEFAULT MAXWAIT);

-20000, ORU-10024: there are no alerts registered.

WAITONE Procedure

This procedure waits for a specific alert to occur. An implicit COMMIT is issued before this procedure is executed. A session that is the first to signal an alert can also wait for the alert in a subsequent transaction. In this case, remember to commit after the signal and before the wait; otherwise, DBMS_LOCK.REQUEST (which is called by DBMS_ALERT) returns status 4.

DBMS_ALERT.WAITONE (
   name      IN   VARCHAR2,
   message   OUT  VARCHAR2,
   status    OUT  INTEGER,
   timeout   IN   NUMBER DEFAULT MAXWAIT);