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Exploring Common Administration Tools
The Oracle Scheduler is implemented through a PL/SQL package named DBMS_SCHEDULER. This package offers a collection of procedures that are used to create and manage Scheduler objects (jobs, programs, schedules, windows, job classes, window groups, and chains). Each of these object types will be covered thoroughly in this chapter.
Most of the procedures in the DBMS_SCHEDULER package are specific to a certain object type. The object type can be derived from the name of the procedure. For example, the CREATE_PROGRAM procedure is obviously specific to program objects.
However, because all Scheduler objects share some common attributes, there are also procedures that work with any Scheduler object type. These procedures play an important role in the management of Scheduler objects, so they warrant thorough coverage. However, due to their “global” nature, they will be covered in the following sections, separate from any specific object type.
You will learn about the following DBMS_SCHEDULER procedures:
NN ENABLE
NN DISABLE
NN SET_ATTRIBUTE
NN SET_ATTRIBUTE_NULL
You will also learn about any special cases that may exist within the different Scheduler object types.
Using the ENABLE Procedure
With the exception of schedules, all Scheduler objects have a common attribute named ENABLED. The attribute is a Boolean (TRUE or FALSE) value that identifies whether the object is eligible for use by the Scheduler.
Because schedule objects do not have an ENABLED attribute, they cannot be enabled or disabled. They are always enabled by default.
Therefore, to be eligible for use in the Scheduler, the ENABLED attribute must be set to TRUE. By default, only schedule objects are enabled at creation time, because they cannot be disabled. All other objects will be disabled by default when they are created.
To enable an object, the DBMS_SCHEDULER.ENABLE procedure is used. The procedure accepts only one argument, NAME, which designates one of the following:
NN |
The name of one specific object |
NN |
A comma-separated list of objects |
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To disable an object, the DBMS_SCHEDULER.DISABLE procedure is used. This procedure accepts two parameters: NAME and FORCE. The NAME parameter designates one of the following:
NN |
The name of one specific object |
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NN |
A comma-separated list of objects |
The FORCE parameter is a Boolean (TRUE or FALSE) value that tells the procedure how to handle the request if dependencies exist. The default value is FALSE.
There are two situations that could be classified as dependencies:
NN |
A job object that references a program object is considered to be dependent on that object. |
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NNIf an instance of an object is currently running (for example, a window is open or a job is running), there may be a dependency issue.
If any dependencies are found, the value of the FORCE parameter will determine the ultimate outcome of the DISABLE procedure.
The purpose of the FORCE parameter is not to cascade the changes to dependent objects. The purpose is to make you aware of dependencies. No changes will be made to dependent objects.
The effect of the FORCE option varies between object types. The differences are listed in Table 12.1.
Ta b l e 12 .1 Effects of DISABLE with the FORCE Option
Object Type |
Effect |
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Job |
If the FORCE attribute is FALSE: If an instance of the job is currently run- |
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ning, the procedure will fail. If the FORCE attribute is TRUE: The job is dis- |
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abled, but the currently running instance is allowed to finish. |
Schedule |
N/A |
Program |
If the FORCE attribute is FALSE: If the program is referenced by any job, |
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the procedure will fail. If the FORCE attribute is TRUE: The program will be |
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disabled. Jobs that reference the program will not be disabled but will |
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fail at runtime if the program is still disabled. |
Window |
If the FORCE attribute is FALSE: If the window is open or referenced by |
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any job, the procedure will fail. If the FORCE attribute is TRUE: The proce- |
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dure will succeed in disabling the window. If that window is open at the |
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time the DISABLE procedure is called, it will not be affected. Jobs that |
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reference the window will not be disabled. |
Window group |
If the FORCE attribute is FALSE: If any member windows are open or if any |
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member windows are referenced by a job object, the DISABLE procedure |
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will fail. If the FORCE attribute is TRUE: The window group will be disabled. |
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Any open window that is a member of the group will continue to its end. |
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Jobs that reference the window group as their schedule will not be disabled. |
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make a change to an object requires setting its attributes. Therefore, to alter a Scheduler object, the DBMS_SCHEDULER.SET_ATTRIBUTE and DBMS_SCHEDULER.SET_ATTRIBUTE_NULL procedures are used.
In the following sections, you will learn to use these procedures with all types of Scheduler objects.
The SET_ATTRIBUTE procedure sets an attribute for any type of Scheduler object. The SET_ATTRIBUTE_NULL procedure, on the other hand, sets any attribute to NULL for any type of Scheduler object. This is useful for “unsetting” an attribute.
The only attribute that cannot be altered (for any type of Scheduler object) is the name of the object.
When the attributes on an object are changed, Oracle will attempt to disable the object before making the changes. When the attribute has been successfully altered, Oracle will reenable the object automatically. If the SET_ATTRIBUTE procedure fails, the object will remain disabled (and an error message is returned).
Using the SET_ATTRIBUTE procedure does not affect instances of the object that are currently executing. Changes made will affect only future instantiations of the object.
The SET_ATTRIBUTE procedure accepts three parameters: NAME The name of the Scheduler object.
ATTRIBUTE The name of the attribute to be changed.
VALUE The new value for the attribute. The procedure is overloaded to accept a value of any applicable datatype, so no conversion is necessary when setting values for different datatypes.
The SET_ATTRIBUTE_NULL procedure accepts only two parameters: NAME The name of the Scheduler object.
ATTRIBUTE The name of the attribute, which should be set to NULL.
In the preceding section, you learned that an object was considered enabled when the ENABLED attribute was set to a value of TRUE. Therefore, you can enable or disable an object by using the SET_ATTRIBUTE procedure, as shown here:
SQL> begin
2 dbms_scheduler.set_attribute (
3name => ‘TEST_JOB’,
4 attribute => ‘ENABLED’,
5 value => TRUE);
6 end;
7 /
PL/SQL procedure successfully completed.