DBMS_ALERT v15

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The DBMS_ALERT package lets you register for, send, and receive alerts. The following table lists the supported procedures:

Function/procedureReturn typeDescription
REGISTER(name)n/aRegister to be able to receive alerts named name.
REMOVE(name)n/aRemove registration for the alert named name.
REMOVEALLn/aRemove registration for all alerts.
SIGNAL(name, message)n/aSignal the alert named name with message.
WAITANY(name OUT, message OUT, status OUT, timeout)n/aWait for any registered alert to occur.
WAITONE(name, message OUT, status OUT, timeout)n/aWait for the specified alert, name, to occur.

EDB Postgres Advanced Server's implementation of DBMS_ALERT is a partial implementation when compared to Oracle's version. Only those functions and procedures listed in the table are supported.

EDB Postgres Advanced Server allows a maximum of 500 concurrent alerts. You can use the dbms_alert.max_alerts GUC variable, located in the postgresql.conf file, to specify the maximum number of concurrent alerts allowed on a system.

To set a value for the dbms_alert.max_alerts variable, open the postgresql.conf file, which is located by default in /opt/PostgresPlus/14AS/data, with your choice of editor. Edit the dbms_alert.max_alerts parameter as shown:

dbms_alert.max_alerts = alert_count

alert_count

alert_count specifies the maximum number of concurrent alerts. By default, the value of dbms_alert.max_alerts is 100. To disable this feature, set dbms_alert.max_alerts to 0.

For the dbms_alert.max_alerts GUC to function correctly, the custom_variable_classes parameter must contain dbms_alerts:

custom_variable_classes = 'dbms_alert, …'

After editing the postgresql.conf file parameters, you must restart the server for the changes to take effect.

REGISTER

The REGISTER procedure enables the current session to be notified of the specified alert.

REGISTER(<name> VARCHAR2)

Parameters

name

Name of the alert to register.

Examples

This anonymous block registers for an alert named alert_test and then waits for the signal.

DECLARE
    v_name           VARCHAR2(30) := 'alert_test';
    v_msg            VARCHAR2(80);
    v_status         INTEGER;
    v_timeout        NUMBER(3) := 120;
BEGIN
    DBMS_ALERT.REGISTER(v_name);
    DBMS_OUTPUT.PUT_LINE('Registered for alert ' || v_name);
    DBMS_OUTPUT.PUT_LINE('Waiting for signal...');
    DBMS_ALERT.WAITONE(v_name,v_msg,v_status,v_timeout);
    DBMS_OUTPUT.PUT_LINE('Alert name   : ' || v_name);
    DBMS_OUTPUT.PUT_LINE('Alert msg    : ' || v_msg);
    DBMS_OUTPUT.PUT_LINE('Alert status : ' || v_status);
    DBMS_OUTPUT.PUT_LINE('Alert timeout: ' || v_timeout || ' seconds');
    DBMS_ALERT.REMOVE(v_name);
END;

Registered for alert alert_test
Waiting for signal...

REMOVE

The REMOVE procedure unregisters the session for the named alert.

REMOVE(<name> VARCHAR2)

Parameters

name

Name of the alert to unregister.

REMOVEALL

The REMOVEALL procedure unregisters the session for all alerts.

REMOVEALL

SIGNAL

The SIGNAL procedure signals the occurrence of the named alert.

SIGNAL(<name> VARCHAR2, <message> VARCHAR2)

Parameters

name

Name of the alert.

message

Information to pass with this alert.

Examples

This anonymous block signals an alert for alert_test.

DECLARE
    v_name   VARCHAR2(30) := 'alert_test';
BEGIN
    DBMS_ALERT.SIGNAL(v_name,'This is the message from ' || v_name);
    DBMS_OUTPUT.PUT_LINE('Issued alert for ' || v_name);
END;
Issued alert for alert_test

WAITANY

The WAITANY procedure waits for any of the registered alerts to occur.

WAITANY(<name> OUT VARCHAR2, <message> OUT VARCHAR2,
  <status> OUT INTEGER, <timeout> NUMBER)

Parameters

name

Variable receiving the name of the alert.

message

Variable receiving the message sent by the SIGNAL procedure.

status

Status code returned by the operation. Possible values are: 0 – alert occurred; 1 – timeout occurred.

timeout

Time to wait for an alert in seconds.

Examples

This anonymous block uses the WAITANY procedure to receive an alert named alert_test or any_alert:

DECLARE
    v_name           VARCHAR2(30);
    v_msg            VARCHAR2(80);
    v_status         INTEGER;
    v_timeout        NUMBER(3) := 120;
BEGIN
    DBMS_ALERT.REGISTER('alert_test');
    DBMS_ALERT.REGISTER('any_alert');
    DBMS_OUTPUT.PUT_LINE('Registered for alert alert_test and any_alert');
    DBMS_OUTPUT.PUT_LINE('Waiting for signal...');
    DBMS_ALERT.WAITANY(v_name,v_msg,v_status,v_timeout);
    DBMS_OUTPUT.PUT_LINE('Alert name   : ' || v_name);
    DBMS_OUTPUT.PUT_LINE('Alert msg    : ' || v_msg);
    DBMS_OUTPUT.PUT_LINE('Alert status : ' || v_status);
    DBMS_OUTPUT.PUT_LINE('Alert timeout: ' || v_timeout || ' seconds');
    DBMS_ALERT.REMOVEALL;
END;

Registered for alert alert_test and any_alert
Waiting for signal...

An anonymous block in a second session issues a signal for any_alert:

DECLARE
    v_name   VARCHAR2(30) := 'any_alert';
BEGIN
    DBMS_ALERT.SIGNAL(v_name,'This is the message from ' || v_name);
    DBMS_OUTPUT.PUT_LINE('Issued alert for ' || v_name);
END;

Issued alert for any_alert

Control returns to the first anonymous block and the remainder of the code is executed:

Registered for alert alert_test and any_alert
Waiting for signal...
Alert name   : any_alert
Alert msg    : This is the message from any_alert
Alert status : 0
Alert timeout: 120 seconds

WAITONE

The WAITONE procedure waits for the specified registered alert to occur.

WAITONE(<name> VARCHAR2, <message> OUT VARCHAR2,
  <status> OUT INTEGER, <timeout> NUMBER)

Parameters

name

Name of the alert.

message

Variable receiving the message sent by the SIGNAL procedure.

status

Status code returned by the operation. Possible values are: 0 – alert occurred; 1 – timeout occurred.

timeout

Time to wait for an alert in seconds.

Examples

This anonymous block is similar to the one used in the WAITANY example except the WAITONE procedure is used to receive the alert named alert_test.

DECLARE
    v_name           VARCHAR2(30) := 'alert_test';
    v_msg            VARCHAR2(80);
    v_status         INTEGER;
    v_timeout        NUMBER(3) := 120;
BEGIN
    DBMS_ALERT.REGISTER(v_name);
    DBMS_OUTPUT.PUT_LINE('Registered for alert ' || v_name);
    DBMS_OUTPUT.PUT_LINE('Waiting for signal...');
    DBMS_ALERT.WAITONE(v_name,v_msg,v_status,v_timeout);
    DBMS_OUTPUT.PUT_LINE('Alert name   : ' || v_name);
    DBMS_OUTPUT.PUT_LINE('Alert msg    : ' || v_msg);
    DBMS_OUTPUT.PUT_LINE('Alert status : ' || v_status);
    DBMS_OUTPUT.PUT_LINE('Alert timeout: ' || v_timeout || ' seconds');
    DBMS_ALERT.REMOVE(v_name);
END; 

Registered for alert alert_test
Waiting for signal...

Signal sent for alert_test sent by an anonymous block in a second session:

DECLARE
    v_name   VARCHAR2(30) := 'alert_test';
BEGIN
    DBMS_ALERT.SIGNAL(v_name,'This is the message from ' || v_name);
    DBMS_OUTPUT.PUT_LINE('Issued alert for ' || v_name);
END;

Issued alert for alert_test

First session is alerted, control returns to the anonymous block, and the remainder of the code is executed:

Registered for alert alert_test
Waiting for signal...
Alert name   : alert_test
Alert msg    : This is the message from alert_test
Alert status : 0
Alert timeout: 120 seconds

Comprehensive example

The following example uses two triggers to send alerts when the dept table or the emp table is changed. An anonymous block listens for these alerts and displays messages when an alert is received.

The following are the triggers on the dept and emp tables:

CREATE OR REPLACE TRIGGER dept_alert_trig
    AFTER INSERT OR UPDATE OR DELETE ON dept
DECLARE
    v_action        VARCHAR2(25);
BEGIN
    IF INSERTING THEN
        v_action := ' added department(s) ';
    ELSIF UPDATING THEN
        v_action := ' updated department(s) ';
    ELSIF DELETING THEN
        v_action := ' deleted department(s) ';
    END IF;
    DBMS_ALERT.SIGNAL('dept_alert',USER || v_action || 'on ' ||
        SYSDATE);
END;

CREATE OR REPLACE TRIGGER emp_alert_trig
    AFTER INSERT OR UPDATE OR DELETE ON emp
DECLARE
    v_action        VARCHAR2(25);
BEGIN
    IF INSERTING THEN
        v_action := ' added employee(s) ';
    ELSIF UPDATING THEN
        v_action := ' updated employee(s) ';
    ELSIF DELETING THEN
        v_action := ' deleted employee(s) ';
    END IF;
    DBMS_ALERT.SIGNAL('emp_alert',USER || v_action || 'on ' ||
        SYSDATE);
END;

This anonymous block is executed in a session while updates to the dept and emp tables occur in other sessions:

DECLARE
    v_dept_alert     VARCHAR2(30) := 'dept_alert';
    v_emp_alert      VARCHAR2(30) := 'emp_alert';
    v_name           VARCHAR2(30);
    v_msg            VARCHAR2(80);
    v_status         INTEGER;
    v_timeout        NUMBER(3) := 60;
BEGIN
    DBMS_ALERT.REGISTER(v_dept_alert);
    DBMS_ALERT.REGISTER(v_emp_alert);
    DBMS_OUTPUT.PUT_LINE('Registered for alerts dept_alert and emp_alert');
    DBMS_OUTPUT.PUT_LINE('Waiting for signal...');
    LOOP
        DBMS_ALERT.WAITANY(v_name,v_msg,v_status,v_timeout);
        EXIT WHEN v_status != 0;
        DBMS_OUTPUT.PUT_LINE('Alert name   : ' || v_name);
        DBMS_OUTPUT.PUT_LINE('Alert msg    : ' || v_msg);
        DBMS_OUTPUT.PUT_LINE('Alert status : ' || v_status);
        DBMS_OUTPUT.PUT_LINE('------------------------------------' ||
            '-------------------------');
    END LOOP;
    DBMS_OUTPUT.PUT_LINE('Alert status : ' || v_status);
    DBMS_ALERT.REMOVEALL;
END;

Registered for alerts dept_alert and emp_alert
Waiting for signal...

The following changes are made by user, mary:

INSERT INTO dept VALUES (50,'FINANCE','CHICAGO');
INSERT INTO emp (empno,ename,deptno) VALUES (9001,'JONES',50);
INSERT INTO emp (empno,ename,deptno) VALUES (9002,'ALICE',50);

The following change is made by user, john:

INSERT INTO dept VALUES (60,'HR','LOS ANGELES');

The following is the output displayed by the anonymous block receiving the signals from the triggers:

Registered for alerts dept_alert and emp_alert
Waiting for signal...
Alert name   : dept_alert
Alert msg    : mary added department(s) on 25-OCT-07 16:41:01
Alert status : 0
-------------------------------------------------------------
Alert name   : emp_alert
Alert msg    : mary added employee(s) on 25-OCT-07 16:41:02
Alert status : 0
-------------------------------------------------------------
Alert name   : dept_alert
Alert msg    : john added department(s) on 25-OCT-07 16:41:22
Alert status : 0
-------------------------------------------------------------
Alert status : 1
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