If none of the previous methods seem suitable, a specific solution to meet specific needs can always be built. The following example could be used as a starting point for such a solution.
The sequence of jobs is protected using the job_chain_locks table. The RETRIES column specifies the number of times a task should check the locks before it gives up and reschedules itself. The RETRY_DELAY column specifies the number of minutes between retries. The TASK_NAME and LOCKED columns are self-explanatory.
CREATE TABLE job_chain_locks (
task_name VARCHAR2(20) NOT NULL,
locked VARCHAR2(1) DEFAULT 'Y' NOT NULL,
retries NUMBER(3) DEFAULT 0 NOT NULL,
retry_delay NUMBER(3) DEFAULT 1 NOT NULL,
CONSTRAINT job_chain_locks_pk PRIMARY KEY (task_name)
);
INSERT INTO job_chain_locks (task_name, locked, retries, retry_delay)
VALUES ('task_2', 'Y', 5, 1);
INSERT INTO job_chain_locks (task_name, locked, retries, retry_delay)
VALUES ('task_3', 'Y', 3, 1);
COMMIT;
The contents of the job_chain_locks table can be monitored using the job_chain_locks_query.sql script.
job_chain_locks_query.sql
select
*
from
job_chain_locks
order by
task_name;
The job_chain_custom.sql script creates a package specification and body which will do all the work for the example job chain.
CREATE OR REPLACE PACKAGE job_chain_custom AS
PROCEDURE task_1;
PROCEDURE task_2;
PROCEDURE task_3;
PROCEDURE lock_task (p_task_name IN job_chain_locks.task_name%TYPE,
p_lock IN BOOLEAN DEFAULT TRUE);
FUNCTION unlocked (p_task_name IN job_chain_locks.task_name%TYPE)
RETURN BOOLEAN;
END job_chain_custom;
/
SHOW ERRORS
CREATE OR REPLACE PACKAGE BODY job_chain_custom AS
-- -----------------------------------------------------------------
PROCEDURE task_1 AS
-- -----------------------------------------------------------------
BEGIN
DELETE FROM job_chain;
INSERT INTO job_chain (created_timestamp, task_name)
VALUES (systimestamp, 'TASK_1');
COMMIT;
-- Uncomment the following line to force a failure.
--RAISE_APPLICATION_ERROR(-20000,
-- 'This is a fake error to prevent task_2 being executed');
-- The work has comleted successfully so unlock task_2
lock_task ('task_2', FALSE);
EXCEPTION
WHEN OTHERS THEN
-- Don't unlock task_2.
NULL;
END task_1;
-- -----------------------------------------------------------------
-- -----------------------------------------------------------------
PROCEDURE task_2 AS
-- -----------------------------------------------------------------
BEGIN
IF unlocked('task_2') THEN
lock_task ('task_2');
INSERT INTO job_chain (created_timestamp, task_name)
VALUES (systimestamp, 'TASK_2');
COMMIT;
-- Uncomment the following line to force a failure.
--RAISE_APPLICATION_ERROR(-20000,
-- 'This is a fake error to prevent task_3 being executed');
-- The work has comleted successfully so unlock task_3
lock_task ('task_3', FALSE);
END IF;
EXCEPTION
WHEN OTHERS THEN
-- Don't unlock task_3.
NULL;
END task_2;
-- -----------------------------------------------------------------
-- -----------------------------------------------------------------
PROCEDURE task_3 AS
-- -----------------------------------------------------------------
BEGIN
IF unlocked('task_3') THEN
lock_task ('task_3');
INSERT INTO job_chain (created_timestamp, task_name)
VALUES (systimestamp, 'TASK_3');
COMMIT;
END IF;
END task_3;
-- -----------------------------------------------------------------
-- -----------------------------------------------------------------
PROCEDURE lock_task (p_task_name IN job_chain_locks.task_name%TYPE,
p_lock IN BOOLEAN DEFAULT TRUE) AS
-- -----------------------------------------------------------------
PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
UPDATE job_chain_locks
SET locked = 'Y'
WHERE task_name = p_task_name;
COMMIT;
END lock_task;
-- -----------------------------------------------------------------
-- -----------------------------------------------------------------
FUNCTION unlocked (p_task_name IN job_chain_locks.task_name%TYPE)
RETURN BOOLEAN AS
-- -----------------------------------------------------------------
l_jcl_row job_chain_locks%ROWTYPE;
BEGIN
SELECT *
INTO l_jcl_row
FROM job_chain_locks
WHERE task_name = p_task_name;
IF l_jcl_row.locked != 'Y' THEN
RETURN TRUE;
END IF;
FOR i IN 1 .. l_jcl_row.retries LOOP
DBMS_LOCK.sleep(60 * l_jcl_row.retry_delay);
SELECT locked
INTO l_jcl_row.locked
FROM job_chain_locks
WHERE task_name = p_task_name;
IF l_jcl_row.locked != 'Y' THEN
RETURN TRUE;
END IF;
END LOOP;
RETURN FALSE;
EXCEPTION
WHEN NO_DATA_FOUND THEN
RETURN FALSE;
END unlocked;
-- -----------------------------------------------------------------
END job_chain_custom;
/
SHOW ERRORS
Next, the jobs associated with each task are scheduled.
job_chain_custom_jobs.sql
-- Oracle
BEGIN
DBMS_SCHEDULER.create_job (
job_name => 'job_chain_custom_task_1',
job_type => 'STORED_PROCEDURE',
job_action => 'job_chain_custom.task_1',
start_date => SYSTIMESTAMP,
repeat_interval => 'freq=daily; byhour=6; byminute=0; bysecond=0;',
end_date => NULL,
enabled => TRUE,
comments => 'First task in the AQ chain.');
END;
/
BEGIN
DBMS_SCHEDULER.create_job (
job_name => 'job_chain_custom_task_2',
job_type => 'STORED_PROCEDURE',
job_action => 'job_chain_custom.task_2',
start_date => SYSTIMESTAMP,
repeat_interval => 'freq=daily; byhour=12; byminute=0; bysecond=0;',
end_date => NULL,
enabled => TRUE,
comments => 'Second task in the AQ chain.');
END;
/
BEGIN
DBMS_SCHEDULER.create_job (
job_name => 'job_chain_custom_task_3',
job_type => 'STORED_PROCEDURE',
job_action => 'job_chain_custom.task_3',
start_date => SYSTIMESTAMP,
repeat_interval => 'freq=daily; byhour=18; byminute=0; bysecond=0;',
end_date => NULL,
enabled => TRUE,
comments => 'Third task in the AQ chain.');
END;
/
At this point, the tasks are scheduled but have not been executed; hence, no results in the job_chain table. Rather than waiting until 6:00, the first job can be forced to run immediately. The results below show that the first task has run and second task has been unlocked.
SQL> exec dbms_scheduler.run_job ('job_chain_custom_task_1');
PL/SQL procedure successfully completed.
SQL> @job_chain_query.sql
CREATED_TIMESTAMP TASK_NAME
--------------------------- --------------------
07-AUG-2004 19:54:51.010000 TASK_1
SQL> @job_chain_locks_query.sql
TASK_NAME L RETRIES RETRY_DELAY
-------------------- - ---------- -----------
task_2 N 5 1
task_3 Y 3 1
Running the second job manually results in the second task being relocked and the third task being unlocked.
SQL> exec dbms_scheduler.run_job ('job_chain_custom_task_2');
PL/SQL procedure successfully completed.
SQL> @job_chain_query.sql
CREATED_TIMESTAMP TASK_NAME
--------------------------- --------------------
07-AUG-2004 19:54:51.010000 TASK_1
07-AUG-2004 19:57:29.636000 TASK_2
SQL> @job_chain_locks_query.sql
TASK_NAME L RETRIES RETRY_DELAY
-------------------- - ---------- -----------
task_2 Y 5 1
task_3 N 3 1
Running the third job manually results in the second task being relocked.
SQL> exec dbms_scheduler.run_job ('job_chain_custom_task_3');
PL/SQL procedure successfully completed.
SQL> @job_chain_query.sql
CREATED_TIMESTAMP TASK_NAME
--------------------------- --------------------
07-AUG-2004 19:54:51.010000 TASK_1
07-AUG-2004 19:57:29.636000 TASK_2
07-AUG-2004 19:59:11.184000 TASK_3
3 rows selected.
SQL> @job_chain_locks_query.sql
TASK_NAME L RETRIES RETRY_DELAY
-------------------- - ---------- -----------
task_2 Y 5 1
task_3 Y 3 1
2 rows selected.
Attempting to run a job out of order will result in the session hanging until the task is unlocked or the appropriate number of retries has been attempted, at which point, the job is rescheduled.
SQL> set timing on
SQL> exec dbms_scheduler.run_job ('job_chain_custom_task_3');
PL/SQL procedure successfully completed.
Elapsed: 00:03:04.50
The DBA now has a variety of tools available with which to build job chains. The next section will introduce the error handling requirements associated with job scheduling.
and include the URL for the page.
