三言两语 - 关于JOB Queue的文档摘要

关于JOB调度的问题，有朋友问到（虽然JOB在10g开始以及逐渐被Scheduler所取代），翻了一下文档，摘录一些语句，权作参考：
1.JOB 进程

Job queue processes are used for batch processing. They run user jobs. They can be viewed as a scheduler service that can be used to schedule jobs as PL/SQL statements or procedures on an Oracle instance. Given a start date and an interval, the job queue processes try to run the job at the next occurrence of the interval.

Job queue processes are managed dynamically.Dynamic job queue processes can run a large number of jobs concurrently at a given interval. The job queue processes run user jobs as they are assigned by the CJQ process. Here's what happens:

1. The coordinator process, named CJQ0, periodically selects jobs that need to be run from the system JOB$ table. New jobs selected are ordered by time.

2. The CJQ0 process dynamically spawns job queue slave processes (J000...J999) to run the jobs.

3. The job queue process runs one of the jobs that was selected by the CJQ process for execution. The processes run one job at a time.

4. After the process finishes execution of a single job, it polls for more jobs. If no jobs are scheduled for execution, then it enters a sleep state, from which it wakes up at periodic intervals and polls for more jobs. If the process does not find any new jobs, then it aborts after a preset interval.

The initialization parameter JOB_QUEUE_PROCESSES represents the maximum number of job queue processes that can concurrently run on an instance. However, clients should not assume that all job queue processes are available for job execution.

2.RAC环境中的JOB调度进程 （link）

Each RAC instance has its own job coordinator. The database monitoring checks that determine whether or not to start the job coordinator do take the service affinity of jobs into account. For example, if there is only one job scheduled in the near future and the job class to which this job belongs has service affinity for only two out of the four RAC instances, only the job coordinators for those two instances will be started.

3.JOB的实例调度 （link）

Because you can create jobs at the instance level, cluster level, or cluster database level, jobs can run on any available host in the cluster database.

You can administer Enterprise Manager jobs at both the database and instance levels. For example, you can create a job at the cluster database level and the job will run on any active instance of the target Oracle RAC database. Or you can create a job at the instance level and the job will only run on the specific instance for which you created it. In the event of a failure, recurring jobs can run on a surviving instance.