Postgres Plus Advanced Server Oracle Compatibility Developer's Guide : 7.1 DBMS_ALERT

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Postgres Plus Advanced Server Oracle Compatibility Developer's Guide

 

7.1 DBMS_ALERT

The DBMS_ALERT package provides the capability to register for, send, and receive alerts.

The procedures and functions available in the DBMS_ALERT package are listed in the following table.

Table 7-7-1 DBMS_ALERT Functions/Procedures

Function/Procedure

Return Type

Description

REGISTER(name)

n/a

Register to be able to receive alerts named, name.

REMOVE(name)

n/a

Remove registration for the alert named, name.

REMOVEALL

n/a

Remove registration for all alerts.

SIGNAL(name, message)

n/a

Signals the alert named, name, with message.

WAITANY(name OUT, message OUT, status OUT, timeout)

n/a

Wait for any registered alert to occur.

WAITONE(name, message OUT, status OUT, timeout)

n/a

Wait for the specified alert, name, to occur.

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 (located by default in /opt/PostgresPlus/9.0AS/data) with your choice of editor, and 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.

7.1.1 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 be registered.

Examples

The following anonymous block registers for an alert named, alert_test, 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...

7.1.2 REMOVE

The REMOVE procedure unregisters the session for the named alert.

REMOVE(name VARCHAR2)

Parameters

name

Name of the alert to be unregistered.

7.1.3 REMOVEALL

The REMOVEALL procedure unregisters the session for all alerts.

REMOVEALL

7.1.4 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

The following 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

7.1.5 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

The following 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

7.1.6 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

The following 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

7.1.7 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;

The following 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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