Generally, jobs run immediately upon being enabled, or when we call the run_job procedure of the dbms_scheduler package. Many jobs are time-based; they are controlled by a schedule based on some kind of calendar.
However, not everything in real life can be controlled by a calendar. Many things need an action on an ad hoc basis, depending on the occurrence of some other thing. This is called event-based scheduling. Events also exist as the outcome of a job. We can define a job to raise an event in several ways—when it ends, or when it ends in an error, or when it does not end within the expected runtime. Let’s start with creating job events in order to make job monitoring a lot easier for you.
Monitoring job events
Most of the time when jobs just do their work as expected, there is not much to monitor. In most cases, the job controller has to fix application-specific problems (for example, sometimes file systems or table spaces get filled up). To make this easier, we can incorporate events. We can make jobs raise events when something unexpected happens, and we can have the Scheduler generate events when a job runs for too long. This gives us tremendous power. We can also use this to make chains a little easier to maintain.
Events in chains
A chain consists of steps that depend on each other. In many cases, it does not make sense to continue to step 2 when step 1 fails. For example, when a create table fails, why try to load data into the nonexistent table? So it is logical to terminate the job if no other independent steps can be performed.
One of the ways to handle this is implementing error steps in the chain. This might be a good idea, but the disadvantage is that this quickly doubles the steps involved in the chain, where most of the steps—hopefully—will not be executed. Another disadvantage is that the chain becomes less maintainable. It’s a lot of extra code, and more code (mostly) gives us less oversight.
If a job chain has to be terminated because of a failure, using the option of creating an event handler to raise a Scheduler event is recommended instead of adding extra steps that try to tell which error possibly happened. This makes event notification a lot simpler because it’s all in separate code and not mixed up with the application code.
Another situation is when the application logic has to take care of steps that fail, and has well-defined countermeasures to be executed that make the total outcome of the job a success.
An example is a situation that starts with a test for the existence of a fi le. If the test fails, get it by FTP; and if this succeeds, load it into the database. In this case, the first step can fail and go to the step that gets the file. As there is no other action possible when the FTP action fails, this should raise a Scheduler event that triggers—for example—a notification action. The same should happen when the load fails.
In other third-party scheduling packages, I have seen these notification actions implemented as part of the chain definitions because they lack a Scheduler event queue. In such packages, messages are sent by mail in extra chain steps. In the Oracle Scheduler, this queue is present and is very useful for us. Compared to 10g, nothing has changed in 11g. An event monitoring package can de-queue from the SCHEDULER$_EVENT_QUEUE variable into a sys.scheduler$_event_info type variable. The definition is shown in the following screenshot:
What you can do with an event handler is up to your imagination. The following DB Console screenshot shows the interface that can be used to specify which events to raise:
It is easy to generate an event for every possible event listed above and have the handler decide (by the rules defined in tables) what to do. This does sound a little creepy, but it is not very different from having a table that can be used as a lookup for the job found in the event message where—most of the time—a notification mail is sent, or not sent. Sometimes, a user wants to get a message when a job starts running; and most of the time, they want a message when a job ends.
In a chain, it is especially important to be able to tell in which step the event happened and what that step was supposed to do. In the event message, only the job name is present and so you have to search a bit to find the name of the step that failed.
For this, we can use the LOG_ID to find the step name in the SCHEDULER_JOB_LOGS (user/dba/all_SCHEDULER_JOB_LOG) view, where the step name is listed as JOB_SUBNAME. The following query can be used to find the step_name from the dba all_scheduler_log view, assuming that the event message is received in msg:
select job_subname from all_scheduler_job_log where
log_id = msg.log_id;
To enable the delivery of all the events a job can generate, we can set the raise_events attribute to a value of:
dbms_scheduler.job_started + dbms_scheduler.job_succeeded +
dbms_scheduler.job_failed + dbms_scheduler.job_broken +
dbms_scheduler.job_completed + dbms_scheduler.job_stopped +
dbms_scheduler.job_sch_lim_reached + dbms_scheduler.job_disabled +
Or in short, we can set it to: dbms_scheduler.job_all_events.
There are many things that can be called events. In the job system, there are basically two types of events: events caused by jobs (which we already discussed) and events that makes a job execute.