Worker Controller Threads
Thread running an infinite loop which for every iteration calls its on_iteration() method.
This also implements graceful shutdown of the thread by providing the stop() method.
This is the body of the thread.
To start the thread use start() instead.
Thread continuously sending tasks in the queue to the pool.
A thread that continuously checks if there are celery.task.PeriodicTask tasks waiting for execution, and executes them. It also finds tasks in the hold queue that is ready for execution and moves them to the bucket queue.
(Tasks in the hold queue are tasks waiting for retry, or with an eta/countdown.)