Celery production settings: acks_late, time limits and queue routing

# ======================================================================
# Q: Which Celery settings do you always change in production?
#
# Separate queues and workers for heavy tasks (video), for integrations and for
# fast tasks (emails), so one doesn't block the others. Celery beat runs on a
# single instance, otherwise periodic tasks run twice.
# ======================================================================

CELERY_TASK_ACKS_LATE = True               # ack after execution
CELERY_TASK_REJECT_ON_WORKER_LOST = True
CELERY_WORKER_PREFETCH_MULTIPLIER = 1       # fair distribution for large tasks
CELERY_TASK_TIME_LIMIT = 300                # hard kill
CELERY_TASK_SOFT_TIME_LIMIT = 270           # SoftTimeLimitExceeded for cleanup
CELERY_WORKER_MAX_TASKS_PER_CHILD = 1000    # safety net for memory leaks
CELERY_TASK_ROUTES = {
    'media.tasks.transcode': {'queue': 'heavy'},
    'integrations.tasks.*': {'queue': 'integrations'},
}
# Redis broker: visibility_timeout > the slowest task, otherwise double execution
CELERY_BROKER_TRANSPORT_OPTIONS = {'visibility_timeout': 3600}