The Job Controller

Each time a message is enqueued to a channel, the Job Controller ensures that there is a job running to deliver the message. This might involve starting a new job process, adding a thread, or simply noting that a job is already running. If a job cannot be started because the job limit for the channel or pool has been reached, the Job Controller waits until another job has exited. When the job limit is no longer exceeded, the Job Controller starts another job.

Channel jobs run inside processing pools within the Job Controller. A pool can be thought of a “place” where the channel jobs are run. The pool provides a computing area where a set of jobs can operate without vying for resources with jobs outside of the pool. For more information on pools see Job Controller File and Processing Pools for Channel Execution Jobs.

Job limits for the channel are determined by the maxjobs channel keyword. Job limits for the pool are determined by the JOB_LIMIT option for the pool.

Messaging Server normally attempts to deliver all messages immediately. If a message cannot be delivered on the first attempt, however, the message is delayed for a period of time determined by the appropriate backoff keyword. As soon as the time specified in the backoff keyword has elapsed, the delayed message is available for delivery, and if necessary, a channel job is started to process the message.

The Job Controller’s in-memory data structure of messages currently being processed and awaiting processing typically reflects the full set of message files stored on disk in the MTA queue area. However, if a backlog of message files on disk builds up enough to exceed the Job Controller’s in-memory data structure size limit, then the Job Controller tracks in memory only a subset of the total number of messages files on disk. The Job Controller processes only those messages it is tracking in memory. After a sufficient number of messages have been delivered to free enough in-memory storage, the Job Controller automatically refreshes its in-memory store by scanning the MTA queue area to update its list of messages. The Job Controller then begins processing the additional message files it just retrieved from disk. The Job Controller performs these scans of the MTA queue area automatically.

If your site routinely experiences heavy message backlogs, you might want to tune the Job Controller by using the MAX_MESSAGES option. By increasing the MAX_MESSAGES option value to allow Job Controller to use more memory, you can reduce the number of occasions when message backlogs overflow the Job Controller’s in-memory cache. This reduces the overhead involved when the Job Controller must scan the MTA queue directory. Keep in mind, however, that when the Job Controller does need to rebuild the in-memory cache, the process will take longer because the cache is larger. Note also that because the Job Controller must scan the MTA queue directory every time it is started or restarted, large message backlogs mean that starts or restarts of the Job Controller will incur more overhead than starts or restarts when no such backlog exists.

For information about pools and configuring the Job Controller, see Job Controller File and Configuring Message Processing and Delivery.

To Start and Stop the Job Controller

To start the Job Controller, execute the command:

start-msg job_controller

To shut down the Job Controller, execute the command:

stop-msg job_controller

To restart the Job Controller, execute the command:

imsimta restart job_controller

Restarting the Job Controller has the effect of shutting down the currently running Job Controller, then immediately starting a new one.