Requirements checklist

• Four modes: run-at, run-after, run-every, run-after-tasks. Executor is a GIVEN pool — say the boundary back.
• Inject the clock — every claim testable with a fake clock (the question interviewers listen for).
• Recurring overlap (run takes 6, interval is 5): skip / queue-one / concurrent — choose and defend.
• Dependencies: all parents SUCCEED; parent failure → descendants skipped, visibly; cycles rejected at submission.
• Missed fires: skip, run the next future one. Cancel by id in scope.

The model

min-heap on next-fire  ->  ONE dispatcher thread
  sleep until top's deadline OR an insert wakes it
  on wake: RE-CHECK top vs clock (never trust the wakeup)
  due -> pop -> submit to the given pool (never execute inline)
recurrence: reinsert on completion (skip-overlap falls out free)
dependencies: indegree counters; 0 -> ready (topological, no sorts)
cancel: mark + lazy discard on pop

Invariants (say them)

• Every non-running task lives in exactly one place: heap or dependency-wait set.
• The dispatcher never runs task code.
• A recurring task has at most one heap entry.
• Indegree always equals unfinished parents; skipped/cancelled are terminal and observable.

Complexity facts

• Schedule / reinsert / pop: O(log n); cancel O(1) marked; propagation O(children).
• The claim that matters: wakeups = due fires + earlier inserts — no polling tick.
• Drift: next = scheduled + interval (aligned) vs now + interval (sliding) — pick knowingly.

What earns points, per report dimension

• Requirements & interface — asked the clock question unprompted; surfaced overlap and parent-failure semantics before designing.
• Core design & invariants — found the heap-and-one-dispatcher shape; stated the invariants; refused per-task threads and polling.
• Extension probe — locate the new demand at a seam; narrate what changes and what provably doesn't; volunteer the cost.
• Complexity honesty — wakeup accounting, not just big-O; the drift decision defended.
• Communication — boundaries said back; decisions narrated as choices, not defaults.

Ready? Sit the live mock → — the interviewer will run a twist this chapter deliberately hasn't shown you.