PMDF uses processing jobs to run all master channel programs. On OpenVMS systems, these jobs take the form of either batch jobs or execution jobs depending upon whether a batch queue or Process Symbiont queue is used for the queue to which the processing jobs are submitted; unless you have configured PMDF otherwise, the use of batch queues is the norm. On UNIX and NT systems, processing jobs are managed by the PMDF Job Controller, discussed in detail in Chapter 10.

PMDF employs two kinds of processing jobs --- periodic and immediate. Immediate jobs are used to process messages as they are submitted. Periodic jobs are jobs that run at fixed time intervals and process deferred requests of one sort or another. Periodic jobs resubmit themselves; immediate jobs do not. Periodic jobs typically handle the messages which immediate jobs were unable to deliver. On OpenVMS systems, periodic jobs, once initially submitted to an OpenVMS batch queue, reschedule (resubmit) themselves as needed. On UNIX systems, periodic jobs are scheduled by the cron daemon. On NT systems, periodic jobs are scheduled by the Task Scheduler at the times specified with the at command.