Oracle Job Scheduling - Events - Part 1

So far, we have mostly used jobs that ran immediately upon being enabled, or when we called 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.

In this chapter, we will see how events that are generated by a job or a chain step can be intercepted to enable the monitoring of jobs. After that, we will see how we can use events to start a job that is waiting for an event.

Oracle Job Scheduling - 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.

Oracle Job Scheduling  - 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 file. 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.