Signalling Events in the Database with Alerters
This chapter describes how to use the DBMS_ALERT package to provide notification, or "alerts", of database events. Topics include the following:
See the Trusted Oracle7 Server Administrator's Guide for additional information about using alerts with Trusted Oracle.
Overview
The DBMS_ALERT package provides support for the asynchronous (as opposed to polling) notification of database events. By appropriate use of this package and database triggers, an application can cause itself to be notified whenever values of interest in the database are changed.
For example, 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 (DBMS_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 then performs a signal (DBMS_ALERT.SIGNAL) whenever the trigger is fired.
Alerts are transaction based. This means that the waiting session does not get alerted until the transaction signalling the alert commits.
There can be any number of concurrent signallers 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.
Note: Because database alerters issue COMMITS, they cannot be used with Oracle Forms. For more information on restrictions on calling stored procedures while Oracle Forms (Runform) is active, refer to your Oracle Forms documentation.
The following procedures are callable from the DBMS_ALERT package:
| Function/Procedure
| Description
| Refer to Page
|
| REGISTER
| Receive messages from an alert.
| 12 - 4
|
| REMOVE
| Disable notification from an alert.
| 12 - 5
|
| SIGNAL
| Signal an alert (send message to registered sessions).
| 12 - 5
|
| WAITANY
| Wait TIMEOUT seconds to receive alert message from an alert registered for session.
| 12 - 6
|
| WAITONE
| Wait TIMEOUT seconds to receive message from named alert.
| 12 - 6
|
| SET_DEFAULTS
| Set the polling interval.
| 12 - 8
|
Table 12 - 1. DBMS_ALERT Package Functions and Procedures
Creating the DBMS_ALERT Package
To create the DBMS_ALERT package, submit the DBMSALRT.SQL and PRVTALRT.PLB scripts when connected as the user SYS. These scripts are run automatically by the CATPROC.SQL script. See page 7 - 39 for information on granting the necessary privileges to users who will be executing this package.
Security
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.
Errors
DBMS_ALERT raises the application error -20000 on error conditions. Table 12 - 2 shows the messages, and the procedures that can raise them.
| 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
|
Table 12 - 2. DBMS_ALERT Error Messages
Using Alerts
The application can register for multiple events and can then wait for any of them to occur using the WAITANY call.
An application can also supply an optional TIMEOUT parameter to the WAITONE or WAITANY calls. A TIMEOUT of 0 returns immediately if there is no pending alert.
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. The application always gets the latest alert (based on transaction commit times).
If the application does not require transaction-based alerts, then the DBMS_PIPE package may provide a useful alternative; see page 8 - 2.
If the transaction is rolled back after the call to DBMS_ALERT.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.
REGISTER Procedure
The REGISTER procedure allows a session to register interest in an alert. The name of the alert is the IN parameter. 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.
Warning: Alert names beginning with 'ORA$' are reserved for use for products provided by Oracle Corporation.
Syntax
The syntax for the REGISTER procedure is
DBMS_ALERT.REGISTER(name IN VARCHAR2);
REMOVE Procedure
The REMOVE procedure allows 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.
If a session dies without removing the alert, that alert is eventually (but not immediately) cleaned up.
Syntax
The syntax for the REMOVE procedure is
DBMS_ALERT.REMOVE(name IN VARCHAR2);
SIGNAL Procedure
Call SIGNAL to signal an alert. The effect of the SIGNAL call only occurs when the transaction in which it is made commits. If the transaction rolls back, the SIGNAL call 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, then 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
The parameters for the SIGNAL procedure are described in Table 12 - 3. The syntax for this procedure is
DBMS_ALERT.SIGNAL(name IN VARCHAR2,
message IN VARCHAR2);
| Parameter
| Description
|
| name
| Specify the name of the alert to signal.
|
| message
| Specify the 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.
|
Table 12 - 3. DBMS_ALERT.SIGNAL Procedure Parameters
WAITANY Procedure
Call WAITANY to wait for an alert to occur for any of the alerts for which the current session is registered. 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.
Syntax
The parameters for the WAITANY procedure are described in Table 12 - 4. The syntax for this procedure is
DBMS_ALERT.WAITANY(name OUT VARCHAR2,
message OUT VARCHAR2,
status OUT INTEGER,
timeout IN NUMBER DEFAULT MAXWAIT);
| 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. Note that if multiple signals on this alert occurred before the WAITANY call, then the message corresponds to the most recent signal call. Messages from prior SIGNAL calls are discarded.
|
| status
| The values returned and their associated meanings are as follows:
0 - alert occurred
1 - timeout occurred
|
| timeout
| Specify the maximum time to wait for an alert. If no alert occurs before TIMEOUT seconds, this call returns with a status of 1.
|
Table 12 - 4. DBMS_ALERT.WAITANY Procedure Parameters
WAITONE Procedure
You call WAITONE to wait for a specific alert to occur. 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
The parameters for the WAITONE procedure are described in Table 12 - 5. The syntax for this procedure is
DBMS_ALERT.WAITONE(name IN VARCHAR2,
message OUT VARCHAR2,
status OUT INTEGER,
timeout IN NUMBER DEFAULT MAXWAIT);
| Parameter
| Description
|
| name
| Specify the name of the alert to wait for.
|
| message
| Returns the message associated with the alert. This is the message provided by the SIGNAL call. Note that if multiple signals on this alert occurred before the WAITONE call, then the message corresponds to the most recent signal call. Messages from prior SIGNAL calls are discarded.
|
| status
| The values returned and their associated meanings are as follows:
0 - alert occurred
1 - timeout occurred
|
| timeout
| Specify the maximum time to wait for an alert. If the named alert does not occurs before TIMEOUT seconds, this call returns with a status of 1.
|
Table 12 - 5. DBMS_ALERT.WAITONE Procedure Parameters
Checking for Alerts
Usually, Oracle is event-driven; that is, 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 call.
- WAITANY call. If you use the WAITANY call, and a signalling session does a signal but does not commit within one second of the signal, then 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.
SET_DEFAULTS Procedure
In case a polling loop is required, use the SET_DEFAULTS procedure to set the POLLING_INTERVAL. The POLLING_INTERVAL is the time, in seconds, to sleep between polls. The default interval is five seconds.
Syntax
The syntax for the SET_DEFAULTS procedure is
DBMS_ALERT.SET_DEFAULTS(polling_interval IN NUMBER);
Example of Using Alerts
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 the code below.
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 the following example:
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 the following 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, they are eventually cleaned up by future users of this package.
The above example guarantees that the application always sees the latest data, although it may not see every intermediate value.
