The Job Controller creates and manages channel jobs for delivering messages. Each set of channel jobs is part of an internal queue. The queue provides an area where a set of channel jobs can run without contention for resources with other channel jobs outside of the queue. The number of jobs that can run inside a particular queue is set with the JOB_LIMIT keyword. For example, if you have a CONV queue defined with a JOB_LIMIT of 10, then only 10 processes can run in that queue at any given time.

Usage of the conversion channel for virus scanning or stripping "dangerous" attached files is a common situation where sites would want to create additional queues. Virus scanning a message requires a significant amount of time, and conversion channel jobs can quickly fill every available job slot at a busy site. Delivery channel jobs such as the l (local) and msgstore channels are only allowed to run infrequently, causing large delays in message delivery.

A solution to this problem is to define a queue named CONV in the Job Controller configuration file (job_controller.cnf) that is dedicated to handling conversion channel jobs:

[QUEUE=CONV] JOB_LIMIT=10

The conversion channel is told to run its channel jobs inside the new queue with the queue keyword. For example:

[CHANNEL=conversion] master_command=/pmdf/bin/conversion queue=conv

This allows up to 10 conversion channel jobs to run in parallel, while leaving all of the job slots in the other queues available for other channel jobs.