Chained celery tasks with delay

Leverage Celery Chains to execute sequential tasks. But it wasn't clear from the documentation on how to add a delay in-between executions.

The initial (reasonable) attempt:

result = (
  add.s(1,1) |
  mul.s(3) | 
).apply_async(countdown=5)  # 24

resulted in the task completing immediately.

However setting the countdown for each signature* worked.

result = (
  add.s(1,1) | 
  mul.s(3).set(countdown=5) | 
) # 24

*use immutable signatures (shortcut: si) so the next task doesn't require the return value of the previous one who's result is implicitly passed as the first argument