System Design — Delayed Job Queue

Jobs that must run at or after a given time: retries with backoff, reminders, email
digests. A worker periodically asks "what is due?".

solve(args, ops) builds new DelayedQueue() and replays ops:

• schedule(id, runAt) — add job id to run at runAt. If id is already pending, its

time is replaced and it counts as newly scheduled. Returns nothing.

• cancel(id) — remove a pending job. Returns true if it was pending, else false.
• poll(now) — remove and return the ids of every pending job with runAt <= now, ordered

by runAt ascending, then by scheduling order (earlier schedule call first).

A sorted array makes poll linear. The scalable structure is a **min-heap keyed by
(runAt, sequence)** with lazy deletion for cancel; any approach that gives the right
answers is accepted here.

Example

schedule("a",10) schedule("b",5) schedule("c",10)
poll(4) → []    poll(5) → ["b"]    poll(10) → ["a","c"]