def __init__(self, parallelism=PARALLELISM):
self.parallelism = parallelism<EOL>self.queued_tasks = OrderedDict()<EOL>self.running = {}<EOL>self.event_buffer = {}<EOL>
Class to derive in order to interface with executor-type systems
like Celery, Yarn and the likes.

:param parallelism: how many jobs should run at one time. Set to
    ``0`` for infinity
:type parallelism: int