8
Managing Job Queues

This chapter describes how to use job queues to schedule periodic execution of
PL/SQL code, and includes the following topics:

• SNP Background Processes

• Managing Job Queues

• Viewing Job Queue Information

SNP Background Processes

This section describes SNP background processes and their role in managing job queues, and includes the following topics:

• Multiple SNP processes

• Starting up SNP processes

You can schedule routines to be performed periodically using the job queue. A routine is any PL/SQL code. To schedule a job, you submit it to the job queue and specify the frequency at which the job is to be run. You can also alter, disable, or delete jobs you have submitted.

To maximize performance and accommodate many users, a multi-process Oracle system uses some additional processes called background processes. Background processes consolidate functions that would otherwise be handled by multiple Oracle programs running for each user process. Background processes asynchronously perform I/O and monitor other Oracle processes to provide increased parallelism for better performance and reliability.

SNP background processes execute job queues. SNP processes periodically wake up and execute any queued jobs that are due to be run. You must have at least one SNP process running to execute your queued jobs in the background.

SNP background processes differ from other Oracle background processes, in that the failure of an SNP process does not cause the instance to fail. If an SNP process fails, Oracle restarts it.

SNP background processes will not execute jobs if the system has been started in restricted mode. However, you can use the ALTER SYSTEM command to turn this behavior on and off as follows:

ALTER SYSTEM ENABLE RESTRICTED SESSION;
ALTER SYSTEM DISABLE RESTRICTED SESSION;

When you ENABLE a restricted session, SNP background processes do not execute jobs; when you DISABLE a restricted session, SNP background processes execute jobs.

See Also: For more information on SNP background processes, see Oracle8i Concepts.

Multiple SNP processes

An instance can have up to 36 SNP processes, named SNP0 to SNP9, and SNPA to SNPZ. If an instance has multiple SNP processes, the task of executing queued jobs can be shared across these processes, thus improving performance. Note, however, that each job is run at any point in time by only one process. A single job cannot be shared simultaneously by multiple SNP processes.

Starting up SNP processes

Job queue initialization parameters enable you to control the operation of the SNP background processes. When you set these parameters in the initialization parameter file for an instance, they take effect the next time you start the instance.

Table 8-1 describes the job queue initialization parameters.

Table 8-1 Job Queue Initialization Parameters

Managing Job Queues

This section describes the various aspects of managing job queues, and includes the following topics:

• DBMS_JOB Package

• Submitting a Job to the Job Queue

• How Jobs Execute

• Removing a Job from the Job Queue

• Altering a Job

• Broken Jobs

• Forcing a Job to Execute

• Terminating a Job

DBMS_JOB Package

To schedule and manage jobs in the job queue, use the procedures in the DBMS_JOB package. There are no database privileges associated with using job queues. Any user who can execute the job queue procedures can use the job queue. Table 8-2 lists the job queue procedures in the DBMS_JOB package.

Table 8-2 Procedures in the DBMS_JOB Package

Submitting a Job to the Job Queue

To submit a new job to the job queue, use the SUBMIT procedure in the DBMS_JOB package:

DBMS_JOB.SUBMIT(  job             OUT    BINARY_INTEGER,
                  what             IN    VARCHAR2,
                  next_date        IN    DATE DEFAULT SYSDATE,
                  interval         IN    VARCHAR2 DEFAULT 'null',
                  no_parse         IN    BOOLEAN DEFAULT FALSE)

The SUBMIT procedure returns the number of the job you submitted. Table 8-3 describes the procedure's parameters.

Table 8-3 Parameters for DBMS_JOB.SUBMIT

job

what

next_date

interval

no_parse

As an example, let's submit a new job to the job queue. The job calls the procedure DBMS_DDL.ANALYZE_OBJECT to generate optimizer statistics for the table DQUON.ACCOUNTS. The statistics are based on a sample of half the rows of the ACCOUNTS table. The job is run every 24 hours:

VARIABLE jobno number;
begin
          2>           DBMS_JOB.SUBMIT(:jobno, 
          3>                  'dbms_ddl.analyze_object(''TABLE'',
          4>                   ''DQUON'', ''ACCOUNTS'', 
          5>                   ''ESTIMATE'', NULL, 50);' 
          6>                   SYSDATE, 'SYSDATE + 1');
          7>            commit;
          8>  end;
          9>  /
Statement processed.
print jobno
JOBNO
----------
14144

Job Environment

When you submit a job to the job queue or alter a job's definition, Oracle records the following environment characteristics:

• the current user

• the user submitting or altering a job

• the current schema

• MAC privileges (if appropriate)

Oracle also records the following NLS parameters:

• NLS_LANGUAGE

• NLS_TERRITORY

• NLS_CURRENCY

• NLS_ISO_CURRENCY

• NLS_NUMERIC_CHARACTERS

• NLS_DATE_FORMAT

• NLS_DATE_LANGUAGE

• NLS_SORT

Oracle restores these environment characteristics every time a job is executed. NLS_LANGUAGE and NLS_TERRITORY parameters are defaults for unspecified NLS parameters.

You can change a job's environment by using the DBMS_SQL package and the ALTER SESSION command.

Jobs and Import/Export

Jobs can be exported and imported. Thus, if you define a job in one database, you can transfer it to another database. When exporting and importing jobs, the job's number, environment, and definition remain unchanged.

Job Owners

When you submit a job to the job queue, Oracle identifies you as the owner of the job. Only a job's owner can alter the job, force the job to run, or remove the job from the queue.

Job Numbers

A queued job is identified by its job number. When you submit a job, its job number is automatically generated from the sequence SYS.JOBSEQ.

Once a job is assigned a job number, that number does not change. Even if the job is exported and imported, its job number remains the same.

Job Definitions

The job definition is the PL/SQL code specified in the WHAT parameter of the SUBMIT procedure.

Normally the job definition is a single call to a procedure. The procedure call can have any number of parameters.

There are special parameter values that Oracle recognizes in a job definition. Table 8-4 lists these parameters.

Table 8-4 Special Parameter Values for Job Definitions

job

next_date

broken

The following are examples of valid job definitions:

'myproc(''10-JAN-82'', next_date, broken);'
'scott.emppackage.give_raise(''JFEE'', 3000.00);'
'dbms_job.remove(job);'

Job Execution Interval

The INTERVAL date function is evaluated immediately before a job is executed. If the job completes successfully, the date calculated from INTERVAL becomes the new NEXT_DATE. If the INTERVAL date function evaluates to NULL and the job completes successfully, the job is deleted from the queue.

If a job should be executed periodically at a set interval, use a date expression similar to 'SYSDATE + 7' in the INTERVAL parameter. For example, if you set the execution interval to 'SYSDATE + 7' on Monday, but for some reason (such as a network failure) the job is not executed until Thursday, 'SYSDATE + 7' then executes every Thursday, not Monday.

If you always want to automatically execute a job at a specific time, regardless of the last execution (for example, every Monday), the INTERVAL and NEXT_DATE parameters should specify a date expression similar to 'NEXT_DAY(TRUNC(SYSDATE), "MONDAY")'.

Table 8-5 lists some common date expressions used for job execution intervals.

Table 8-5 Common Job Execution Intervals

'SYSDATE + 7'

'SYSDATE + 1/48'

'NEXT_DAY(TRUNC(SYSDATE), 
''MONDAY'') + 15/24'

'NEXT_DAY(ADD_MONTHS

(TRUNC(SYSDATE, ''Q''), 3), 
''THURSDAY'')'

Database Links and Jobs

If you submit a job that uses a database link, the link must include a username and password. Anonymous database links will not succeed.

See Also: For more information on the DBMS_SQL package, see the Oracle8i Supplied Packages Reference.

How Jobs Execute

SNP background processes execute jobs. To execute a job, the process creates a session to run the job.

When an SNP process runs a job, the job is run in the same environment in which it was submitted and with the owner's default privileges.

When you force a job to run using the procedure DBMS_JOB.RUN, the job is run by your user process. When your user process runs a job, it is run with your default privileges only. Privileges granted to you through roles are unavailable.

Job Queue Locks

Oracle uses job queue locks to ensure that a job is executed one session at a time. When a job is being run, its session acquires a job queue (JQ) lock for that job.

Interpreting Information about JQ Locks

You can use the Enterprise Manager Lock Monitor or the locking views in the data dictionary to examine information about locks currently held by sessions.

The following query lists the session identifier, lock type, and lock identifiers for all sessions holding JQ locks:

SELECT sid, type, id1, id2
   FROM v$lock
   WHERE type = 'JQ';

SID         TY    ID1        ID2
----------   --   ---------- ----------
        12   JQ   0          14144
1 row selected.

In the query above, the identifier for the session holding the lock is 12. The ID1 lock identifier is always 0 for JQ locks. The ID2 lock identifier is the job number of the job the session is running.

Job Execution Errors

When a job fails, information about the failure is recorded in a trace file and the alert log. Oracle writes message number ORA-12012 and includes the job number of the failed job.

The following can prevent the successful execution of queued jobs:

• not having any SNP background processes to run the job

• a network or instance failure

• an exception when executing the job

Job Failure and Execution Times

If a job returns an error while Oracle is attempting to execute it, Oracle tries to execute it again. The first attempt is made after one minute, the second attempt after two minutes, the third after four minutes, and so on, with the interval doubling between each attempt. When the retry interval exceeds the execution interval, Oracle continues to retry the job at the normal execution interval. However, if the job fails 16 times, Oracle automatically marks the job as broken and no longer tries to execute it.

Thus, if you can correct the problem that is preventing a job from running before the job has failed sixteen times, Oracle will eventually run that job again.

See Also: For more information about the locking views, see the Oracle8i Reference.

For more information about locking, see Oracle8i Concepts.

Removing a Job from the Job Queue

To remove a job from the job queue, use the REMOVE procedure in the DBMS_JOB package:

DBMS_JOB.REMOVE(job IN BINARY_INTEGER)

The following statement removes job number 14144 from the job queue:

DBMS_JOB.REMOVE(14144);

Restrictions

You can remove currently executing jobs from the job queue. However, the job will not be interrupted, and the current execution will be completed.

You can remove only jobs you own. If you try to remove a job that you do not own, you receive a message that states the job is not in the job queue.

Altering a Job

To alter a job that has been submitted to the job queue, use the procedures CHANGE, WHAT, NEXT_DATE, or INTERVAL in the DBMS_JOB package.

In this example, the job identified as 14144 is now executed every three days:

DBMS_JOB.CHANGE(14144, null, null, 'SYSDATE + 3');

Restrictions

You can alter only jobs that you own. If you try to alter a job that you do not own, you receive a message that states the job is not in the job queue.

Syntax for CHANGE

You can alter any of the user-definable parameters associated with a job by calling the DBMS_JOB.CHANGE procedure. Table 8-3 describes the procedure's parameters.

DBMS_JOB.CHANGE( job                     IN  BINARY_INTEGER,
          what                           IN  VARCHAR2,
          next_date                      IN  DATE,
          interval                       IN  VARCHAR2)

If you specify NULL for WHAT, NEXT_DATE, or INTERVAL when you call the procedure CHANGE, the current value remains unchanged.

Syntax for WHAT

You can alter the definition of a job by calling the DBMS_JOB.WHAT procedure. Table 8-3 describes the procedure's parameters.

DBMS_JOB.WHAT( job                      IN  BINARY_INTEGER,
         what                           IN  VARCHAR2)

Syntax for NEXT_DATE

You can alter the next date that Oracle executes a job by calling the DBMS_JOB.NEXT_DATE procedure. Table 8-3 describes the procedure's parameters.

DBMS_JOB.NEXT_DATE( job        IN  BINARY_INTEGER,
next_date                      IN  DATE)

Syntax for INTERVAL

You can alter the execution interval of a job by calling the DBMS_JOB.INTERVAL procedure. Table 8-3 describes the procedure's parameters.

DBMS_JOB.INTERVAL( job                  IN BINARY_INTEGER,
         interval                       IN VARCHAR2)
 

Broken Jobs

A job is labeled as either broken or not broken. Oracle does not attempt to run broken jobs. However, you can force a broken job to run by calling the procedure DBMS_JOB.RUN.

When you submit a job it is considered not broken.

There are two ways a job can break:

• Oracle has failed to successfully execute the job after 16 attempts.

• You have marked the job as broken, using the procedure DBMS_JOB.BROKEN.

To mark a job as broken or not broken, use the procedure BROKEN in the DBMS_JOB package. Table 8-4 describes the procedure's parameters:

DBMS_JOB.BROKEN( job               IN BINARY_INTEGER,
         broken                    IN BOOLEAN,
         next_date                 IN DATE DEFAULT SYSDATE)

The following example marks job 14144 as not broken and sets its next execution date to the following Monday:

DBMS_JOB.BROKEN(14144, FALSE, NEXT_DAY(SYSDATE, 'MONDAY'));

Once a job has been marked as broken, Oracle will not attempt to execute the job until you either mark the job as not broken, or force the job to be executed by calling the procedure DBMS_JOB.RUN.

Restrictions

You can mark as broken only jobs that you own. If you try to mark a job you do not own, you receive a message stating that the job is not in the job queue.

Running Broken Jobs

If a problem has caused a job to fail 16 times, Oracle marks the job as broken. Once you have fixed this problem, you can run the job by either:

• forcing the job to run by calling DBMS_JOB.RUN

• marking the job as not broken by calling DBMS_JOB.BROKEN and waiting for Oracle to execute the job

If you force the job to run by calling the procedure DBMS_JOB.RUN, Oracle runs the job immediately. If the job succeeds, then Oracle labels the job as not broken and resets its count of the number of failed executions for the job.

Once you reset a job's broken flag (by calling either RUN or BROKEN), job execution resumes according to the scheduled execution intervals set for the job.

Forcing a Job to Execute

There may be times when you would like to manually execute a job. For example, if you have fixed a broken job, you may want to test the job immediately by forcing it to execute.

To force a job to be executed immediately, use the procedure RUN in the DBMS_JOB package. Oracle attempts to run the job, even if the job is marked as broken:

DBMS_JOB.RUN( job  IN BINARY_INTEGER)

When you run a job using DBMS_JOB.RUN, Oracle recomputes the next execution date. For example, if you create a job on a Monday with a NEXT_DATE value of 'SYSDATE' and an INTERVAL value of 'SYSDATE + 7', the job is run every 7 days starting on Monday. However, if you execute RUN on Wednesday, the next execution date will be the next Wednesday.

Restrictions

You can only run jobs that you own. If you try to run a job that you do not own, you receive a message that states the job is not in the job queue.

The following statement runs job 14144 in your session and recomputes the next execution date:

DBMS_JOB.RUN(14144);

The procedure RUN contains an implicit commit. Once you execute a job using RUN, you cannot roll back.

Terminating a Job

You can terminate a running job by marking the job as broken, identifying the session running the job, and disconnecting that session. You should mark the job as broken, so that Oracle does not attempt to run the job again.

After you have identified the session running the job (via V$SESSION), you can disconnect the session using the SQL statement ALTER SYSTEM.

See Also: For examples of viewing information about jobs and sessions, see the following section, "Viewing Job Queue Information".

For more information on V$SESSION, see the Oracle8i Reference.

Viewing Job Queue Information

You can view information about jobs in the job queue via the data dictionary views in Table 8-6:

Table 8-6 Views for Job Queue Information

For example, you can display information about a job's status and failed executions. The following sample query creates a listing of the job number, next execution time, failures, and broken status for each job you have submitted:

SELECT job, next_date, next_sec, failures, broken
   FROM user_jobs;

JOB        NEXT_DATE NEXT_SEC FAILURES    B
---------- --------- -------- ----------  -
  9125    01-NOV-94 00:00:00      4       N
 14144    24-OCT-94 16:35:35      0       N
 41762    01-JAN-00 00:00:00     16       Y
3 rows selected.

You can also display information about jobs currently running. The following sample query lists the session identifier, job number, user who submitted the job, and the start times for all currently running jobs:

SELECT sid, r.job, log_user, r.this_date, r.this_sec
  FROM dba_jobs_running r, dba_jobs j
  WHERE r.job = j.job;



SID        JOB        LOG_USER            THIS_DATE  THIS_SEC
---------- ---------- -------------------- ---------   --------
 12        14144     JFEE                 24-OCT-94  17:21:24
 25         8536     SCOTT                24-OCT-94  16:45:12
2 rows selected.

See Also: For more information on data dictionary views, see the Oracle8i Reference.