The job controller is responsible for scheduling and executing the message delivery or message submission tasks upon request by various IMTA components. For example, upon receipt of an incoming message from any source, the IMTA channel that is handling the receipt of the message determines the destination, enqueues the message, and sends a request to the job controller to execute the next channel. The job controller schedules only the client tasks for IMTA.
Internally, the job controller maintains the set of channel queues. Requests are placed on specified queues by server processes as messages are processed. Each queue has a job limit that consists of the maximum number of concurrent jobs that can be processed and the maximum number of jobs that can be enqueued. Requests are executed as they are received until the job limit is exceeded, at which point they are queued to run when a currently executing request finishes. If the capacity of a queue is exceeded, requests directed at that queue are ignored by the job controller.