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20 DBMS_ALERT

DBMS_ALERT supports asynchronous notification of database events (alerts). By appropriate use of this package and database triggers, an application can notify itself whenever values of interest in the database are changed.

This chapter contains the following topics:


Using 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:

Table 20-1 DBMS_ALERT Constants

Name Type Value Description

MAXWAIT

INTEGER

86400000

The maximum time to wait for an alert (1000 days which is essentially forever).



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.

  • 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.

Table 20-2 DBMS_ALERT Error Messages

Error Message Procedure

ORU-10001 lock request error, status: N

SIGNAL

ORU-10015 error: N waiting for pipe status

WAITANY

ORU-10016 error: N sending on pipe 'X'

SIGNAL

ORU-10017 error: N receiving on pipe 'X'

SIGNAL

ORU-10019 error: N on lock request

WAIT

ORU-10020 error: N on lock request

WAITANY

ORU-10021 lock request error; status: N

REGISTER

ORU-10022 lock request error, status: N

SIGNAL

ORU-10023 lock request error; status N

WAITONE

ORU-10024 there are no alerts registered

WAITANY

ORU-10025 lock request error; status N

REGISTER

ORU-10037 attempting to wait on uncommitted signal from same session

WAITONE



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.


Summary of DBMS_ALERT Subprograms

Table 20-3 DBMS_ALERT Package Subprograms

Subprogram Description

REGISTER Procedure

Receives messages from an alert

REMOVE Procedure

Disables notification from an alert

REMOVEALL Procedure

Removes all alerts for this session from the registration list

SET_DEFAULTS Procedure

Sets the polling interval

SIGNAL Procedure

Signals an alert (send message to registered sessions)

WAITANY Procedure

Waits timeout seconds to receive alert message from an alert registered for session

WAITONE Procedure

Waits timeout seconds to receive message from named alert



REGISTER Procedure

This procedure lets a session register interest in an alert.

Syntax

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

Parameters

Table 20-4 REGISTER Procedure Parameters

Parameter Description

name

Name of the alert in which this session is interested

cleanup

Specifies whether to perform cleanup of any extant orphaned pipes used by the DBMS_ALERT package. This cleanup is only performed on the first call to REGISTER for each package instantiation. The default for the parameter is TRUE.


Caution:

Alert names beginning with 'ORA$' are reserved for use for products provided by Oracle. Names must be 30 bytes or less. The name is case insensitive.

Usage Notes

A session can register interest in an unlimited number of alerts. Alerts should be deregistered when the session no longer has any interest, by calling REMOVE.


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.

Syntax

DBMS_ALERT.REMOVE (
   name  IN  VARCHAR2);

Parameters

Table 20-5 REMOVE Procedure Parameters

Parameter Description

name

Name of the alert (case-insensitive) to be removed from registration list.


Usage Notes

Removing alerts is important because it reduces the amount of work done by signalers of the alert. If a session dies without removing the alert, that alert is eventually (but not immediately) cleaned up.


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.

Syntax

DBMS_ALERT.REMOVEALL;

SET_DEFAULTS Procedure

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

Syntax

DBMS_ALERT.SET_DEFAULTS (
   sensitivity  IN  NUMBER);

Parameters

Table 20-6 SET_DEFAULTS Procedure Parameters

Parameter Description

sensitivity

Polling interval, in seconds, to sleep between polls. The default interval is five seconds.



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.

Syntax

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

Parameters

Table 20-7 SIGNAL Procedure Parameters

Parameter Description

name

Name of the alert to signal.

message

Message, of 1800 bytes or less, to associate with this alert.

This message is passed to the waiting session. The waiting session might be able to avoid reading the database after the alert occurs by using the information in the message.



WAITANY Procedure

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

Syntax

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

Parameters

Table 20-8 WAITANY Procedure Parameters

Parameter Description

name

Returns the name of the alert that occurred.

message

Returns the message associated with the alert.

This is the message provided by the SIGNAL call. If multiple signals on this alert occurred before WAITANY, the message corresponds to the most recent SIGNAL call. Messages from prior SIGNAL calls are discarded.

status

Values returned:

0 - alert occurred

1 - timeout occurred

timeout

Maximum time to wait for an alert.

If no alert occurs before timeout seconds, this returns a status of 1.


Usage Notes

An implicit COMMIT is issued before this procedure is executed. The same session that waits for the alert may also first signal the alert. 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.

Exceptions

-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.

Syntax

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

Parameters

Table 20-9 WAITONE Procedure Parameters

Parameter Description

name

Name of the alert to wait for.

message

Returns the message associated with the alert.

This is the message provided by the SIGNAL call. If multiple signals on this alert occurred before WAITONE, the message corresponds to the most recent SIGNAL call. Messages from prior SIGNAL calls are discarded.

status

Values returned:

0 - alert occurred

1 - timeout occurred

timeout

Maximum time to wait for an alert.

If the named alert does not occurs before timeout seconds, this returns a status of 1.