Week 8 · Thu Sep 17, 2026 · Sep 14-20, 2026

Day 4: Job Scheduler CORE Deep Dives

A Staff scheduler answer with time buckets, near-term delay queues, leases, zombie fencing, retries, backoff, poison queues, idempotent handlers, and exactly-once illusion.

How do 2-second precision, 10k jobs/sec, and at-least-once execution fit together?Frontier · leases · retries · dedupe · recovery3 Heap/Priority Queue slots

Exact Time Blocks

Follow the Week 4 delivery pattern: blank-page first, source reading second, DDIA decision third, spoken close, contiguous algorithm practice, evening recall.

Time	Block	Required output
08:30	Wake + setup	Open this page, start the board, write the day question: How do 2-second precision, 10k jobs/sec, and at-least-once execution fit together?
08:50—09:00	Board set	List entities, queues, leases, retry states, dedupe keys, metrics before reading.
09:00—10:05	Blank-page attempt + Hello reading	Rehearse the three Hello deep dives. For each one, write the bottleneck, mechanism, failure window, recovery, and metric: 2s precision, 10k jobs/sec, at-least-once execution.
10:05—10:45	Deep dive + DDIA decision	Build the lease story: scanner moves due executions into a near-term queue; worker claims with token and deadline; heartbeat extends; completion uses conditional token match; expired leases reappear; retries use exponential backoff plus max attempts; poison messages stop infinite harm.
10:45—10:55	Spoken close	Say the exact-once line cleanly: the scheduler guarantees at-least-once delivery and externally idempotent effects, not magic exactly-once execution.
14:30—16:15	NeetCode Heap/Priority Queue	Complete contiguous slots W8-H10—W8-H12; capture invariant and bug risk for each.
20:30—21:15	Recall / Q&A / mock	Use Staff Q&A, lecture assets, or mock script for the day's system.

Canonical Heading Anchors

Read only the listed Hello Interview headings for today's assignment. Every link is a direct day-page anchor.

Potential Deep DivesDay 4

CORE deep-dive index

1) How can we ensure the system executes jobs within 2s of their scheduled time?Day 4

CORE two-layer scheduling and timing precision

2) How can we ensure the system is scalable to support up to 10k jobs per second?Day 4

CORE throughput partitioning

3) How can we ensure at-least-once execution of jobs?Day 4

CORE leases, retries, poison messages, idempotence

What is Expected at Each Level?Day 6

Mock rubric calibration

Mid-levelDay 6

Rubric contrast

SeniorDay 6

Senior+ bar

Staff+Day 6

Staff bar

Decision Notes

CORE means you must answer requirement, HLD, deep dive, failure, recovery, metric, and Staff-level tradeoff.
TRANSFER means blind attempt first, then delta against source, invalid assumption, new bottleneck, and follow-up pressure question.
Do not add open-ended AI radar unless it changes a concrete Week 8 design decision; this week it does not.

DDIA Decision Map

Use exact sections only from Ch5, Ch9, and Ch12.

Chapter	Exact DDIA decision section	Concrete Week 8 use
Ch9	分布式锁和租约	Workers lease an execution; they never own it forever.
Ch9	进程暂停	Zombie workers are normal; lease expiry and fencing decide whose completion counts.
Ch12	确认应答与重新传递	Redelivery is expected, so handlers and completion writes must be idempotent.
Ch12	幂等性	Exactly-once is an illusion built from idempotency keys, conditional writes, and dedupe windows.

Heap/Priority Queue Slots

Three contiguous slots today. Keep one invariant and one failure mode per problem.

Slot	Problem	Pattern	Invariant / bug risk	Time · Space
W8-H10	Reorganize String	max-heap with cooldown	A character cannot re-enter the heap until one different char is placed.	O(n log a) · O(a)
W8-H11	Meeting Rooms II	min-heap of end times	Room pressure is the count of active intervals.	O(n log n) · O(n)
W8-H12	Single Threaded CPU	available-work heap	Push all work whose enqueue time is <= current clock before selecting.	O(n log n) · O(n)

Evidence To Save

One compact artifact is enough if it proves mechanism depth.

Invariant

State safety

Name the state that survives restart and the write that makes retry safe.

Failure

Concrete break

Use a worker crash, zombie lease, redelivery, poison message, stale robots decision, or duplicate price event.

Metric

Operational proof

Attach lag, retry rate, duplicate suppression, stale lease reject count, host politeness violation, or alert freshness.

Local study materials
Detailed lecture notes, audio, recall scripts, PDFs, Staff Q&A, and mock packs are archived locally and are intentionally not published on this site.