'Class to derive in order to interface with executor-type systems
like Celery, Mesos, Yarn and the likes.
:param parallelism: how many jobs should run at one time. Set to
``0`` for infinity
:type parallelism: int'

def __init__(self, parallelism=PARALLELISM):

    self.parallelism = parallelism
    self.queued_tasks = {}
    self.running = {}
    self.event_buffer = {}
